Sql Injection 2

The Target Intranet This appeared to be an entirely custom application, and we had no prior knowledge of the application nor access to the source code: this was a "blind" attack. A bit of poking showed that this server ran Microsoft's IIS 6 along with ASP.NET, and this suggested that the database was Microsoft's SQL server: we believe that these techniques can apply to nearly any web application backed by any SQL server. The login page had a traditional username-and-password form, but also an email-me-my-password link; the latter proved to be the downfall of the whole system. When entering an email address, the system presumably looked in the user database for that email address, and mailed something to that address. Since my email address is not found, it wasn't going to send me anything. So the first test in any SQL-ish form is to enter a single quote as part of the data: the intention is to see if they construct an SQL string literally without sanitizing. When submitting the form with a quote in the email address, we get a 500 error (server failure), and this suggests that the "broken" input is actually being parsed literally. Bingo. We speculate that the underlying SQL code looks something like this: SELECT f i e l d l i s t FROM tab le WHERE f i e l d= $ ' EMAIL ';

Here, $EMAIL is the address submitted on the form by the user, and the larger query provides the quotation marks that set it off as a literal string. We don't know the specific names of the fields or table involved, but we do know their nature, and we'll make some good guesses later. When we enter [email protected]' - note the closing quote mark this yields constructed SQL: SELECT f i e l d l i s t FROM tab le WHERE f i e l d= 'steve@unixwiz .net ' ';

when this is executed, the SQL parser find the extra quote mark and aborts with a syntax error. How this manifests itself to the user depends on the application's internal error-recovery procedures, but it's usually different from "email address is unknown". This error response is a dead giveaway that user input is not being sanitized properly and that the application is ripe for exploitation.

Since the data we're filling in appears to be in the WHERE clause, let's change the nature of that clause in an SQL legal way and see what happens. By entering anything' OR 'x'='x, the resulting SQL is: SELECT f i e l d l i s t FROM tab le WHERE f i e l d= a ' nything ' OR ' x '= 'x ';

Because the application is not really thinking about the query - merely constructing a string - our use of quotes has turned a singlecomponent WHERE clause into a two-component one, and the 'x'='x' clause is guaranteed to be true no matter what the first clause is (there is a better approach for this "always true" part that we'll touch on later). But unlike the "real" query, which should return only a single item each time, this version will essentially return every item in the members database. The only way to find out what the application will do in this circumstance is to try it. Doing so, we were greeted with: Your login information has been mailed to [email protected]. Our best guess is that it's the first record returned by the query, effectively an entry taken at random. This person really did get this forgotten-password link via email, which will probably come as surprise to him and may raise warning flags somewhere. We now know that we're able to manipulate the query to our own ends, though we still don't know much about the parts of it we cannot see. But we have observed three different responses to our various inputs: •

"Your login information has been mailed to email"

•

"We don't recognize your email address"

•

Server error

The first two are responses to well-formed SQL, while the latter is for bad SQL: this distinction will be very useful when trying to guess the structure of the query.

Schema field mapping The first steps are to guess some field names: we're reasonably sure that the query includes "email address" and "password", and there may be things like "US Mail address" or "userid" or "phone number". We'd dearly love to perform a SHOW TABLE, but in addition to not knowing the name of the table, there is no obvious vehicle to get the output of this command routed to us. So we'll do it in steps. In each case, we'll show the whole query as we know it, with our own snippets shown specially. We know that the tail

end of the query is a comparison with the email address, so let's guess email as the name of the field: SELECT f i e l d l i s t FROM tab le WHERE f i e l d= x ' ' AND emai l IS NULL; -';-

The intent is to use a proposed field name (email) in the constructed query and find out if the SQL is valid or not. We don't care about matching the email address (which is why we use a dummy 'x'), and the -- marks the start of an SQL comment. This is an effective way to "consume" the final quote provided by application and not worry about matching them. If we get a server error, it means our SQL is malformed and a syntax error was thrown: it's most likely due to a bad field name. If we get any kind of valid response, we guessed the name correctly. This is the case whether we get the "email unknown" or "password was sent" response. Note, however, that we use the AND conjunction instead of OR: this is intentional. In the SQL schema mapping phase, we're not really concerned with guessing any particular email addresses, and we do not want random users inundated with "here is your password" emails from the application - this will surely raise suspicions to no good purpose. By using the AND conjunction with an email address that couldn't ever be valid, we're sure that the query will always return zero rows and never generate a password-reminder email. Submitting the above snippet indeed gave us the "email address unknown" response, so now we know that the email address is stored in a field email. If this hadn't worked, we'd have tried email_address or mail or the like. This process will involve quite a lot of guessing. Next we'll guess some other obvious names: password, user ID, name, and the like. These are all done one at a time, and anything other than "server failure" means we guessed the name correctly. SELECT f i e l d l i s t FROM tab le WHERE emai l = 'x ' AND user id IS NULL; -';-

As a result of this process, we found several valid field names: •

email

•

passwd

•

login_id

•

full_name

There are certainly more (and a good source of clues is the names of the fields on forms), but a bit of digging did not discover any. But we

still don't know the name of the table that these fields are found in how to find out?

Finding the table name The application's built-in query already has the table name built into it, but we don't know what that name is: there are several approaches for finding that (and other) table names. The one we took was to rely on a subselect. A standalone query of SELECT COUNT(* ) FROM tabname

Returns the number of records in that table, and of course fails if the table name is unknown. We can build this into our string to probe for the table name: SELECT email, passwd, login_id, full_name FROM tab le WHERE emai l = x ' ' AND 1=(SELECT COUNT(*) FROM --';

tabname );

We don't care how many records are there, of course, only whether the table name is valid or not. By iterating over several guesses, we eventually determined that members was a valid table in the database. But is it the table used in this query? For that we need yet another test using table.field notation: it only works for tables that are actually part of this query, not merely that the table exists. SELECT email, passwd, login_id, full_name FROM members WHERE email = 'x' AND members.email IS NULL; --';

When this returned "Email unknown", it confirmed that our SQL was well formed and that we had properly guessed the table name. This will be important later, but we instead took a different approach in the interim.

Finding some users At this point we have a partial idea of the structure of the members table, but we only know of one username: the random member who got our initial "Here is your password" email. Recall that we never received the message itself, only the address it was sent to. We'd like to get some more names to work with, preferably those likely to have access to more data. The first place to start, of course, is the company's website to find who is who: the "About us" or "Contact" pages often list who's running the place. Many of these contain email addresses, but even those that

don't list them can give us some clues which allow us to find them with our tool. The idea is to submit a query that uses the LIKE clause, allowing us to do partial matches of names or email addresses in the database, each time triggering the "We sent your password" message and email. Warning: though this reveals an email address each time we run it, it also actually sends that email, which may raise suspicions. This suggests that we take it easy. We can do the query on email name or full name (or presumably other information), each time putting in the % wildcards that LIKE supports: SELECT email, passwd, login_id, full_name FROM members WHERE email = 'x ' OR fu l l _name LIKE '%Bob%' ;

Keep in mind that even though there may be more than one "Bob", we only get to see one of them: this suggests refining our LIKE clause narrowly. Ultimately, we may only need one valid email address to leverage our way in.

Brute-force password guessing One can certainly attempt brute-force guessing of passwords at the main login page, but many systems make an effort to detect or even prevent this. There could be logfiles, account lockouts, or other devices that would substantially impede our efforts, but because of the nonsanitized inputs, we have another avenue that is much less likely to be so protected. We'll instead do actual password testing in our snippet by including the email name and password directly. In our example, we'll use our victim, [email protected] and try multiple passwords. SELECT email, passwd, login_id, full_name FROM members WHERE email = '[email protected]' AND passwd = 'he l lo123

';

This is clearly well-formed SQL, so we don't expect to see any server errors, and we'll know we found the password when we receive the "your password has been mailed to you" message. Our mark has now been tipped off, but we do have his password. This procedure can be automated with scripting in perl, and though we were in the process of creating this script, we ended up going down another road before actually trying it.

The database isn't readonly

So far, we have done nothing but query the database, and even though a SELECT is readonly, that doesn't mean that SQL is. SQL uses the semicolon for statement termination, and if the input is not sanitized properly, there may be nothing that prevents us from stringing our own unrelated command at the end of the query. The most drastic example is: SELECT email, passwd, login_id, full_name FROM members WHERE email = 'x ' ; DROP TABLE members; - - ' ;

- - Boom!

The first part provides a dummy email address -- 'x' -- and we don't care what this query returns: we're just getting it out of the way so we can introduce an unrelated SQL command. This one attempts to drop (delete) the entire members table, which really doesn't seem too sporting. This shows that not only can we run separate SQL commands, but we can also modify the database. This is promising.

Adding a new member Given that we know the partial structure of the members table, it seems like a plausible approach to attempt adding a new record to that table: if this works, we'll simply be able to login directly with our newly-inserted credentials. This, not surprisingly, takes a bit more SQL, and we've wrapped it over several lines for ease of presentation, but our part is still one contiguous string: SELECT email, passwd, login_id, full_name FROM members WHERE ema i l = x' ' ; INSERT INTO members ( 'emai l ' , 'passwd' , ' log in_ id ' , ' fu l l _name' ) VALUES ( ' s teve@unixwiz .net ' , 'he l lo ' , ' s teve ' , 'Steve Fr ied l ' ) ;'-;-

Even if we have actually gotten our field and table names right, several things could get in our way of a successful attack: 1. We might not have enough room in the web form to enter this much text directly (though this can be worked around via scripting, it's much less convenient). 2. The web application user might not have INSERT permission on the members table. 3. There are undoubtedly other fields in the members table, and some may require initial values, causing the INSERT to fail.

4. Even if we manage to insert a new record, the application itself might not behave well due to the auto-inserted NULL fields that we didn't provide values for. 5. A valid "member" might require not only a record in the members table, but associated information in other tables (say, "accessrights"), so adding to one table alone might not be sufficient. In the case at hand, we hit a roadblock on either #4 or #5 - we can't really be sure -- because when going to the main login page and entering in the above username + password, a server error was returned. This suggests that fields we did not populate were vital, but nevertheless not handled properly. A possible approach here is attempting to guess the other fields, but this promises to be a long and laborious process: though we may be able to guess other "obvious" fields, it's very hard to imagine the bigger-picture organization of this application. We ended up going down a different road.

Mail me a password We then realized that though we are not able to add a new record to the members database, we can modify an existing one, and this proved to be the approach that gained us entry. From a previous step, we knew that [email protected] had an account on the system, and we used our SQL injection to update his database record with our email address: SELECT email, passwd, login_id, full_name FROM members WHERE ema i l = x' ' ; UPDATE members SET emai l = ' s teve@unixwiz .net ' WHERE emai l = '[email protected] ';

After running this, we of course received the "we didn't know your email address", but this was expected due to the dummy email address provided. The UPDATE wouldn't have registered with the application, so it executed quietly. We then used the regular "I lost my password" link - with the updated email address - and a minute later received this email: From: [email protected] To: [email protected] Subject: Intranet login This email is in response to your request for your Intranet log in information.

Your User ID is: bob Your password is: hello

Now it was now just a matter of following the standard login process to access the system as a high-ranked MIS staffer, and this was far superior to a perhaps-limited user that we might have created with our INSERT approach. We found the intranet site to be quite comprehensive, and it included among other things - a list of all the users. It's a fair bet that many Intranet sites also have accounts on the corporate Windows network, and perhaps some of them have used the same password in both places. Since it's clear that we have an easy way to retrieve any Intranet password, and since we had located an open PPTP VPN port on the corporate firewall, it should be straightforward to attempt this kind of access. We had done a spot check on a few accounts without success, and we can't really know whether it's "bad password" or "the Intranet account name differs from the Windows account name". But we think that automated tools could make some of this easier.

Other Approaches In this particular engagement, we obtained enough access that we did not feel the need to do much more, but other steps could have been taken. We'll touch on the ones that we can think of now, though we are quite certain that this is not comprehensive. We are also aware that not all approaches work with all databases, and we can touch on some of them here. Use xp_cmdshell Microsoft's SQL Server supports a stored procedure xp_cmdshell that permits what amounts to arbitrary command execution, and if this is permitted to the web user, complete compromise of the webserver is inevitable. What we had done so far was limited to the web application and the underlying database, but if we can run commands, the webserver itself cannot help but be compromised. Access to xp_cmdshell is usually limited to administrative accounts, but it's possible to grant it to lesser users. Map out more database structure Though this particular application provided such a rich post-login environment that it didn't really seem necessary to dig further, in other more limited environments this may not have been sufficient.

Being able to systematically map out the available schema, including tables and their field structure, can't help but provide more avenues for compromise of the application. One could probably gather more hints about the structure from other aspects of the website (e.g., is there a "leave a comment" page? Are there "support forums"?). Clearly, this is highly dependent on the application and it relies very much on making good guesses.

Mitigations We believe that web application developers often simply do not think about "surprise inputs", but security people do (including the bad guys), so there are three broad approaches that can be applied here. Sanitize the input It's absolutely vital to sanitize user inputs to insure that they do not contain dangerous codes, whether to the SQL server or to HTML itself. One's first idea is to strip out "bad stuff", such as quotes or semicolons or escapes, but this is a misguided attempt. Though it's easy to point out some dangerous characters, it's harder to point to all of them. The language of the web is full of special characters and strange markup (including alternate ways of representing the same characters), and efforts to authoritatively identify all "bad stuff" are unlikely to be successful. Instead, rather than "remove known bad data", it's better to "remove everything but known good data": this distinction is crucial. Since - in our example - an email address can contain only these characters: abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ 0123456789 @.-_+

There is really no benefit in allowing characters that could not be valid, and rejecting them early - presumably with an error message - not only helps forestall SQL Injection, but also catches mere typos early rather than stores them into the database. Sidebar on email addresses

It's important to note here that email addresses in particular are troublesome to validate programmatically, because everybody seems to have his own idea about what makes one "valid", and

it's a shame to exclude a good email address because it contains a character you didn't think about. The only real authority is RFC 2822 (which encompasses the more familiar RFC822), and it includes a fairly expansive definition of what's allowed. The truly pedantic may well wish to accept email addresses with ampersands and asterisks (among other things) as valid, but others - including this author - are satisfied with a reasonable subset that includes "most" email addresses. Those taking a more restrictive approach ought to be fully aware of the consequences of excluding these addresses, especially considering that better techniques (prepare/execute, stored procedures) obviate the security concerns which those "odd" characters present. Be aware that "sanitizing the input" doesn't mean merely "remove the quotes", because even "regular" characters can be troublesome. In an example where an integer ID value is being compared against the user input (say, a numeric PIN): SELECT f i e l d l i s t FROM tab le WHERE id = 23 OR 1=1 ;

- - Boom! A lways matches !

In practice, however, this approach is highly limited because there are so few fields for which it's possible to outright exclude many of the dangerous characters. For "dates" or "email addresses" or "integers" it may have merit, but for any kind of real application, one simply cannot avoid the other mitigations. Escape/Quotesafe the input Even if one might be able to sanitize a phone number or email address, one cannot take this approach with a "name" field lest one wishes to exclude the likes of Bill O'Reilly from one's application: a quote is simply a valid character for this field. One includes an actual single quote in an SQL string by putting two of them together, so this suggests the obvious - but wrong! technique of preprocessing every string to replicate the single quotes: SELECT f i e l d l i s t FROM customers WHERE name = 'Bi l l O' 'Re i l' ly ; - - works OK

However, this naïve approach can be beaten because most databases support other string escape mechanisms. MySQL, for instance, also permits \' to escape a quote, so after input of \'; DROP TABLE users; -- is "protected" by doubling the quotes, we get:

SELECT f i e l d l i s t FROM customers WHERE name = '\ ' ' ; DROP TABLE users ; '- ;-

- - Boom!

The expression '\'' is a complete string (containing just one single quote), and the usual SQL shenanigans follow. It doesn't stop with backslashes either: there is Unicode, other encodings, and parsing oddities all hiding in the weeds to trip up the application designer. Getting quotes right is notoriously difficult, which is why many database interface languages provide a function that does it for you. When the same internal code is used for "string quoting" and "string parsing", it's much more likely that the process will be done properly and safely. Some examples are the MySQL function mysql_real_escape_string() and perl DBD method $dbh>quote($value). These methods must be used. Use bound parameters (the PREPARE statement) Though quotesafing is a good mechanism, we're still in the area of "considering user input as SQL", and a much better approach exists: bound parameters, which are supported by essentially all database programming interfaces. In this technique, an SQL statement string is created with placeholders - a question mark for each parameter - and it's compiled ("prepared", in SQL parlance) into an internal form. Later, this prepared query is "executed" with a list of parameters: Example in perl $sth = $dbh->prepare("SELECT email, userid FROM members WHERE email = ?; " ) ; $sth->execute($emai l ) ;

Thanks to Stefan Wagner, this demonstrates bound parameters in Java: Insecure version Statement s = connection.createStatement(); ResultSet rs = s.executeQuery("SELECT email FROM member WHERE name = " + formFie ld) ; / / *boom* Secure version

PreparedStatement ps = connection.prepareStatement( "SELECT email FROM member WHERE name = ?" ) ; ps.setString(1, formFie ld) ; ResultSet rs = ps.executeQuery();

Here, $email is the data obtained from the user's form, and it is passed as positional parameter #1 (the first question mark), and at no point do the contents of this variable have anything to do with SQL statement parsing. Quotes, semicolons, backslashes, SQL comment notation - none of this has any impact, because it's "just data". There simply is nothing to subvert, so the application is be largely immune to SQL injection attacks. There also may be some performance benefits if this prepared query is reused multiple times (it only has to be parsed once), but this is minor compared to the enormous security benefits. This is probably the single most important step one can take to secure a web application.

Limit databas and segregate In the case at hand, we observed just two interactions that are made not in the context of a logged-in user: "log in" and "send me password". The web application ought to use a database connection with the most limited rights possible: query-only access to the members table, and no access to any other table. The effect here is that even a "successful" SQL injection attack is going to have much more limited success. Here, we'd not have been able to do the UPDATE request that ultimately granted us access, so we'd have had to resort to other avenues. Once the web application determined that a set of valid credentials had been passed via the login form, it would then switch that session to a database connection with more rights. It should go almost without saying that sa rights should never be used for any web-based application.

Use stored pro When the database server supports them, use stored procedures for performing access on the application's behalf, which can eliminate SQL entirely (assuming the stored procedures themselves are written properly). By encapsulating the rules for a certain action - query, update, delete, etc. - into a single procedure, it can be tested and documented on a standalone basis and business rules enforced (for instance, the "add new order" procedure might reject that order if the customer were over his credit limit).

For simple queries this might be only a minor benefit, but as the operations become more complicated (or are used in more than one place), having a single definition for the operation means it's going to be more robust and easier to maintain. Note: it's always possible to write a stored procedure that itself constructs a query dynamically: this provides no protection against SQL Injection - it's only proper binding with prepare/execute or direct SQL statements with bound variables that provide this protection.

Isolate the we Even having taken all these mitigation steps, it's nevertheless still possible to miss something and leave the server open to compromise. One ought to design the network infrastructure to assume that the bad guy will have full administrator access to the machine, and then attempt to limit how that can be leveraged to compromise other things. For instance, putting the machine in a DMZ with extremely limited pinholes "inside" the network means that even getting complete control of the webserver doesn't automatically grant full access to everything else. This won't stop everything, of course, but it makes it a lot harder.

Configure erro The default error reporting for some frameworks includes developer debugging information, and this cannot be shown to outside users. Imagine how much easier a time it makes for an attacker if the full query is shown, pointing to the syntax error involved. This information is useful to developers, but it should be restricted - if possible - to just internal users. Note that not all databases are configured the same way, and not all even support the same dialect of SQL (the "S" stands for "Structured", not "Standard"). For instance, most versions of MySQL do not support subselects, nor do they usually allow multiple statements: these are substantially complicating factors when attempting to penetrate a network. We'd like to emphasize that though we chose the "Forgotten password" link to attack in this particular case, it wasn't really because this particular web application feature is dangerous. It was simply one of several available features that might have been vulnerable, and it would be a mistake to focus on the "Forgotten password" aspect of the presentation. This Tech Tip has not been intended to provide comprehensive coverage on SQL injection, or even a tutorial: it merely documents the

process that evolved over several hours during a contracted engagement. We've seen other papers on SQL injection discuss the technical background, but still only provide the "money shot" that ultimately gained them access. But that final statement required background knowledge to pull off, and the process of gathering that information has merit too. One doesn't always have access to source code for an application, and the ability to attack a custom application blindly has some value. Thanks to David Litchfield and Randal Schwartz for their technical input to this paper, and to the great Chris Mospaw for graphic design (© 2005 by Chris Mospaw, used with permission).

Other resources •

(more) Advanced SQL Injection, Chris Anley, Next Generation Security Software.

•

SQL Injection walkthrough, SecuriTeam

•

GreenSQL, an open-source database firewall that tries to protect against SQL injection errors

•

"Exploits of a Mom" — Very good xkcd cartoon about SQL injection

•

SQL Injection Cheat Sheet — by Ferruh Mavituna

Last modified: Wed Oct 10 06:28:06 PDT 2007

One of the more devastating attacks on a web application is also one of the most common: SQL injection. This technique allows an attacker to gain access to the database that underlies many web sites and read and potentially modify data that is not meant to be available to users of that site. This article provides an overview of how SQL injection works and what can be done to avoid it.

A classic example of SQL injection starts with a query that looks something like: SELECT id FROM users WHERE name='$name' AND pass='$pass';

This query might be used to authenticate users when they log in to a web site. If it returns a row, the user id returned is considered to be authenticated and the application proceeds to serve the correct page for that user. In this case, the $name and $pass variables would come from a login form that might look something like:

If the login.php program in this example blindly sets the variables to the values that come from the user, a malicious user can bypass the authentication. Consider the following inputs: $user = "' OR 1=1 "; $pass = "' OR 1=1 LIMIT 1";

This results in a query that is completely different from what the web programmer expected:

SELECT id FROM users WHERE name='' OR 1=1 AND pass='' OR 1=1 LIMIT

1;

This query will always return one row (unless the table is empty) and it is likely to be the first entry in the table. For many applications, that entry is the administrative login; the one with the most privileges. This simple example barely scratches the surface of the kinds of attacks that can be made using SQL injection. Depending on the DBMS, it may be possible to do multiple queries via an injection by separating each with a semicolon: SELECT id FROM users WHERE name='' AND pass=''; DROP TABLE users;

which is, of course, a rather destructive injection. MySQL does not allow multiple queries in a statement, but PostgreSQL is susceptible to this technique.

Web site and/or database search functions are particularly dangerous because they display their output; if a malicious user can inject any query they choose, they can capture the entire contents of the database. The UNION keyword can turn a query such as: SELECT city, state FROM users WHERE name LIKE '%$search%';

into:

SELECT city, state FROM users WHERE name LIKE '%%' UNION SELECT name, pass FROM users WHERE name LIKE '%%';

And instead of just printing the city and state of users that match the input string, we are also printing the username and password of every user in the system. A certain amount of guessing column names and types is required if an attacker does not have access to the database schema, but they are often not very hard to guess given some understanding of the application. Some database systems, notably Microsoft SQL Server, seem to deliberately shoot themselves in the foot by providing the schema for all tables in a generally accessible database, thereby removing all the guesswork. Injection also requires a certain amount of imagination to visualize the kinds of queries that might be going on behind the input boxes of a web form. It requires quite a bit of trial and error unless one has access to the source; this is why the majority of reported SQL injections are in free software or open source web applications. Note that it is not only web forms using the POST method that are vulnerable, many web applications that use the GET method are vulnerable to injections via the URL: http://vulnerablewebapp.com/login.php?\ name=%27%20OR%201%3D1%20&pass=%27%20OR%201%3D1%20LIMIT%201

Like many other web vulnerabilities, SQL injection stems from insufficient filtering of user input. Unfortunately, it is sometimes difficult to determine what kinds of input should be accepted (for example the password "' OR 1=1" would not necessarily seem illegal) and using various filtering functions provided by the language may not actually prevent injections. The PHP addslashes() function is often used to sanitize user input because it will put a backslash in front of single quotes which will stop the kinds of injections described above. Unfortunately, there are techniques to circumvent this particular 'fix' as well. Probably the simplest way to protect queries from SQL injection is by using prepared statements with placeholders. Any reasonable database interface will provide a way to use this functionality and in many cases, it is fairly portable between languages and DBM systems. Instead of directly interpolating string values into query strings, a query is prepared using '?' as a placeholder for the variables as shown in the following pseudocode: $sth = prepare("SELECT id FROM users WHERE name=? AND pass=?"); execute($sth, $name, $pass);

This has a number of advantages: the DBMS library is responsible for properly quoting the values and because of the way the variables are bound to the query, they can never be

treated as anything other than data for the particular place they have in the prepared statement. This effectively turns the injection attempt above into a query like: SELECT id FROM users WHERE name='\' OR 1=1 ' AND pass='\' OR 1=1 LIMIT 1';

which is unlikely to authenticate. Another way to defend against injections is by ensuring that all user input is passed through a database specific quoting function before being used in a query: $name = db_quote($name); $pass = db_quote($pass); SELECT id FROM users WHERE name=$name AND pass=$pass;

Depending on the language and database API, this method may also be fairly portable. The final recommended technique is also the most complicated; but it can provide an additional level of security if stored procedures are available for the DBMS. Stored procedures are queries (and more complicated functions) that are created by the database administrator and stored with the database. These procedures are then called by the application code to do any queries that they require. The equivalent of the prepare functionality is done on the procedures at the time they are stored and with proper coding, this will prevent injections. One of the main advantages is that these procedures run with the privileges of the user that stored them, instead of the user invoking them and this allows the application to have a much more limited set of privileges than it would normally require. The upshot is that it can protect the database from reading or writing even if the application is subverted in some way. SQL injections are clearly a serious security problem, but one that can be thwarted relatively easily once one understands the problem and the ways to program around it. (Log in to post comments) SQL injection attacks Posted Mar 30, 2006 2:28 UTC (Thu) by jwb (subscriber, #15467) [Link] Another great article. One of the many horrors of MS-SQL is the incredible amount of functionality available to a SQL injection attacker. MS-SQL can be made to open a connection to any other database, even on other hosts or networks. A SQL injection attack against MS-SQL can allow the attacker to tell your database to connect to any random instance of SQL Server and replicate itself. This obviously takes all the guesswork out of trying to reverse engineer the schema. An attacker can rip off an entire MS-SQL instance with a single HTTP request. Guess work? Posted Mar 30, 2006 23:49 UTC (Thu) by GreyWizard (subscriber, #1026) [Link] s/guess work/security through obscurity/ Database features do not excuse sloppy applications.

Guess work? Posted Mar 31, 2006 6:39 UTC (Fri) by hppnq (guest, #14462) [Link] Basically, you are suggesting that in order to kill a fly, you should use a proper gun, practice at a firing range, isolate the fly in a safe environment, surgically optimize your eye-hand coordination, calculate environmental influences, suppress urges to start shooting at random, before taking a shot at removing the annoying intruder. Sure, if a gun is my only tool, I'd go through all that. But I'd rather whack the bastard with a newspaper. Nonsense Posted Mar 31, 2006 15:07 UTC (Fri) by GreyWizard (subscriber, #1026) [Link] That is a perfectly ridiculous analogy. Whining that the database has too many features that might be useful for someone exploiting SQL injection vulnerabilities in an unrelated application is not so much swatting the fly as cursing the publisher for printing a newspaper that's too hard to swing while the thing is still buzzing around your head. Nonsense Posted Mar 31, 2006 15:36 UTC (Fri) by hppnq (guest, #14462) [Link] So what are you suggesting then? That we should all write perfect code? Yes, that would solve the problem. Is it realistic? Not a chance in hell. Most or all security implementations heavily depend on defining proper interfaces to resources and making sure that access to resources is only possible through these interfaces. It follows quite simply that it's wise to start off with as little resources and interfaces as possible if you care about security. RTFA Posted Mar 31, 2006 22:17 UTC (Fri) by GreyWizard (subscriber, #1026) [Link] Contrary to your raving, filtering user input does not require perfect code. I suggest reading the article to which this thread is attached. There you will find suggestions such as using prepared statements or stored procedures. As stated above, "SQL injections [...] can be thwarted relatively easily once one understands the problem and the ways to program around it." On the other hand, no database can provide protection from gaping security holes in external applications. Mmmmhh Posted Apr 1, 2006 8:53 UTC (Sat) by hppnq (guest, #14462) [Link] It seems to me that we are making a lot of fuss about something that we basically feel the same about. If you take the time to calm down and read the comments as well as the article you might see this too. Now, you were the one that brought up the topics of sloppy programming and "security through obscurity", taking this discussion explicitly to the realm of the real world, where

the perfect solution does not exist. You observed that database features are no excuse for bad programming, while I am of the opinion that they should not be an excuse. In the real world resources are limited. At some point a decision will have to be made: is it good enough? Since security means nothing in the laboratory, and everything in the real world, this is a very important observation. This is also why I mention writing perfect code: it cannot be done, and the only way to avoid having to make suboptimal decisions is to remove the necessity of making those decisions. This is a classic trade-off between security and functionality. Instead of having to protect features one does not need, it is better to not have them available in the first place. That of course leaves more resources available to get the actual job done: defining the correct interfaces to the functionality you want to provide or use and protecting those interfaces properly. On the other hand, no database can provide protection from gaping security holes in external applications. This is the same problem. Do take some time to think about it. Practice What You Preach Posted Apr 3, 2006 15:50 UTC (Mon) by GreyWizard (subscriber, #1026) [Link] You reply to a comment about security through obscurity with an irrelevant analogy to shooting mosquitoes, and now you accuse me of not reading what I reply to? You rant and rave about the impossibility of perfect code, and now you tell me to calm down? Amusing. But your airy hand waving about "protecting features one doesn't need" still misses the point: using the dumbest database available would be a trade-off between security and functionality only if this were an effective substitute for plugging SQL injection holes in the application. As long as there are remote exploits the application cannot meet even the least demanding security requirements with any database. This is really not so complicated. Practice what you preach, especially with regard to taking the time to think about it. Practice What You Preach Posted Apr 3, 2006 22:19 UTC (Mon) by hppnq (guest, #14462) [Link] Well, I just tried to add some more perspective to your rather simplistic "thou shalt not program sloppily" statement. It appears to me that in your enthusiasm to slight me, you seem to miss your own point completely. *plonk* Perspective Indeed Posted Apr 4, 2006 2:58 UTC (Tue) by GreyWizard (subscriber, #1026) [Link] You are confused. "Database features do not excuse sloppy applications" is simple. "Thou shalt not program sloppily" is simplistic. The latter is your contribution, not mine.

Rambling about mosquitoes, whining about perfect code, splitting hairs over "are" and "should" and pretending I don't understand my own point is your idea of adding perspective, is it? Spare me such generosity. SQL injection attacks Posted Mar 31, 2006 11:06 UTC (Fri) by pdc (subscriber, #1353) [Link] To try to avoid this we do all access from a web app to the SQL Server database via stored procedures, with user input passed as parameters. At least then you can reastrict the privileges of the web application to just the procedures it needs to use. Makes the database development rather tedious, however. SQL injection attacks Posted Mar 31, 2006 19:20 UTC (Fri) by dwkunkel (subscriber, #5999) [Link] I use Oracle stored procedures to simplify my web applications. All the business logic is handled by stored procedures and there is no sql in the web pages. The pages just pass parameters to the appropriate stored procedure. An Oracle stored procedure can return multiple reference cursors that can be cast to Java ResultSets and used directly in a web page. The reference cursors can also be converted to Cached RowSets and used in Data Transfer Objects. Performance is quite good because everything is done in a single trip to the database. I don't know about SQL Server, but I find Oracle's PL/SQL to be an easy to use programming language that makes it relatively simple to produce very readable code. CHAR() + mysql for injection? Posted Mar 30, 2006 8:21 UTC (Thu) by wingo (subscriber, #26929) [Link] I hadn't heard of the CHAR() injection strategy briefly mentioned in the article. Google is not being helpful about it. Anyone have more information on that one? It sounds particularly nasty. SQL injection attacks Posted Mar 30, 2006 10:14 UTC (Thu) by NAR (subscriber, #1313) [Link] Great article. I've worked on a project which displayed the results of an SQL query in a HTML page and once I played with inserting HTML code into the database to break the output. However, I didn't try to break the SQL insert commands. I think I've just found an interesting pet project for the afternoon :-) Bye,NAR SQL injection attacks Posted Mar 31, 2006 15:46 UTC (Fri) by hppnq (guest, #14462) [Link] Cancel your dinner reservations, NAR. ;-) quoting Posted Mar 30, 2006 14:20 UTC (Thu) by rfunk (subscriber, #4054) [Link] It's important to note that the details of quoting strings are DBMS-dependent, and PHP's addslashes() is insufficient (or in some cases just plain wrong). Some of the comments on the PHP addslashes() doc page go into the details. It's always better to use a DBMS-

specific quoting function (e.g. mysql_real_escape_string) than to blindly add backslashes. Which is why PHP's "magic quotes" feature is so annoyingly useless. This tendency to SQL injection is one of the reasons people see PHP as an inherently insecure language, or at least one that encourages insecure programming. input filtering Posted Mar 30, 2006 14:51 UTC (Thu) by ccyoung (subscriber, #16340) [Link] the function db_quote() for input filtering is in my experience inadequate. what is needed is one filter function for each data type. this not only formats but does type checking. for example, db_get_string may not allow quotes and punctuation, whereas db_get_text might be more forgiving. db_get_code( $code, $mustexist=false ) db_get_string( $str, $mustexist=false ) db_get_text( $text, $mustexist=false ) ... db_get_integer( $int, $mustexist=false ) a big gotcha in PHP is it's confusion between 0, null, and an empty string. SQL injection attacks Posted Mar 30, 2006 17:55 UTC (Thu) by iabervon (subscriber, #722) [Link] The prepared statement functionality, at least in some databases, is actually even better than this article suggests: it causes the statement with the question marks to be parsed to generate the sequence of database-internal operations which will be performed. Then when the arguments are filled, it doesn't need to escape the strings, because it doesn't unescape them; it doesn't treat any characters specially at this point. Furthermore, since all of the parsing is already done when user input comes in, it can't be induced to perform unexpected operations, because the sequence of operations it will perform is already determined at this point. Furthermore, it's often faster, because it can cache the execution plan for the query, because all of the "SELECT id FROM users WHERE user=? AND pass=?" parts are identical, and the parsing can be a significant portion of the query time, since there's a bunch of effort in figuring out what's going on, and that the useful optimization for this query is the unique index on users.user. SQL injection attacks Posted Mar 30, 2006 19:10 UTC (Thu) by yodermk (subscriber, #3803) [Link] Great article! However, given this: SELECT id FROM users WHERE name='$name' AND pass='$pass'; the shown "modified" query:

SELECT id FROM users WHERE name='' OR 1=1 AND pass='' OR 1=1 LIMIT 1; does not show the end quote (') after $name and $pass. Would it not translate to this: SELECT id FROM users WHERE name='' OR 1=1' AND pass='' OR 1=1 LIMIT 1'; which would be an SQL error? Or am I missing something??? SQL injection attacks Posted Mar 30, 2006 19:54 UTC (Thu) by jake (editor, #205) [Link] > SELECT id FROM users WHERE name='' OR 1=1' AND pass='' OR 1=1 LIMIT 1'; > which would be an SQL error? Or am I missing something??? No, nice catch. $user = "' OR 1=1 OR name='"; $pass = "' OR 1=1 LIMIT 1 --"; should do the trick ... jake SQL injection attacks Posted Mar 31, 2006 16:56 UTC (Fri) by dps (guest, #5725) [Link] There is a simpler fix, which also stops other attacks too... do proper input validation---if something is suppsesd to be a number, make sure it really is. Numbers like "1 OR 1=1" (without the quotes) can do evil things in contexts expecting numbers. Input validation stops that too. Using magic_quote_qpc, SQL syntax randomisation, etc are all useful backstops in case you somehow fail to properly validate something. I have my doutbs about the security of stored procedures when fed evil input, unless handling it safely is a primary design goal. Sadly there is no agreement about parameters in prepared statements. ODBC and MySQL wants ?. Postgresql want $1, $2, etc and oracle accept $. (This sort of thing is one of the "joys" of writng multiple database server SQL.) SQL injection attacks Posted Apr 1, 2006 13:35 UTC (Sat) by holstein (subscriber, #6122) [Link] As other have pointed out, '1 OR 1=1' can be a valid password. Or at least, a validating routine for valid password should accept this; something like '1hotguys = 1' is a valid password... So, input validation is not a silver bullet; it's just a step in the journey.

Using a stored procedure help because the data will be used just as it is: data. You can't turn the input in a variation of the execute query (at least, note easily). Of course, bad data could trigger other kind of problems (like, say, a buffer overflow attack on the RDBMS). A for the syntax for placeholders in prepared statements, with the Perl DBI at least, the syntax is the same for every RDBMS. And having worked with MySQL, Oracle and SQLite with PHP, I don't recall having seen different syntax used; maybe it was because I was not using the 'direct' API (like mysql_xx, oc8_xx, etc.) but instead using abstraction layer, like PEAR DB. But if you are writing multiple database server application, I would guess that this is a 'sine qua none' condition. At least for me! SQL injection attacks Posted Apr 7, 2006 9:33 UTC (Fri) by m.alessandrini (guest, #36991) [Link] A very interesting review of internet application vulnerabilities can be found at http://www.owasp.org/documentation/topten.html (OWASP Top Ten Most Critical Web Application Security Vulnerabilities)