I realize that parameterized SQL queries is the optimal way to sanitize user input when building queries that contain user input, but I'm wondering what is wrong with taking user input and escaping any single quotes and surrounding the whole string with single quotes. Here's the code:
sSanitizedInput = "'" & Replace(sInput, "'", "''") & "'"
Any single-quote the user enters is replaced with double single-quotes, which eliminates the users ability to end the string, so anything else they may type, such as semicolons, percent signs, etc., will all be part of the string and not actually executed as part of the command.
We are using Microsoft SQL Server 2000, for which I believe the single-quote is the only string delimiter and the only way to escape the string delimiter, so there is no way to execute anything the user types in.
I don't see any way to launch an SQL injection attack against this, but I realize that if this were as bulletproof as it seems to me someone else would have thought of it already and it would be common practice.
What's wrong with this code? Is there a way to get an SQL injection attack past this sanitization technique? Sample user input that exploits this technique would be very helpful.
UPDATE:
I still don't know of any way to effectively launch a SQL injection attack against this code. A few people suggested that a backslash would escape one single-quote and leave the other to end the string so that the rest of the string would be executed as part of the SQL command, and I realize that this method would work to inject SQL into a MySQL database, but in SQL Server 2000 the only way (that I've been able to find) to escape a single-quote is with another single-quote; backslashes won't do it.
And unless there is a way to stop the escaping of the single-quote, none of the rest of the user input will be executed because it will all be taken as one contiguous string.
I understand that there are better ways to sanitize input, but I'm really more interested in learning why the method I provided above won't work. If anyone knows of any specific way to mount a SQL injection attack against this sanitization method I would love to see it.
First of all, it's just bad practice. Input validation is always necessary, but it's also always iffy.
Worse yet, blacklist validation is always problematic, it's much better to explicitly and strictly define what values/formats you accept. Admittedly, this is not always possible - but to some extent it must always be done.
Some research papers on the subject:
http://www.imperva.com/docs/WP_SQL_Injection_Protection_LK.pdf
http://www.it-docs.net/ddata/4954.pdf (Disclosure, this last one was mine ;) )
https://www.owasp.org/images/d/d4/OWASP_IL_2007_SQL_Smuggling.pdf (based on the previous paper, which is no longer available)
Point is, any blacklist you do (and too-permissive whitelists) can be bypassed. The last link to my paper shows situations where even quote escaping can be bypassed.
Even if these situations do not apply to you, it's still a bad idea. Moreover, unless your app is trivially small, you're going to have to deal with maintenance, and maybe a certain amount of governance: how do you ensure that its done right, everywhere all the time?
The proper way to do it:
Whitelist validation: type, length, format or accepted values
If you want to blacklist, go right ahead. Quote escaping is good, but within context of the other mitigations.
Use Command and Parameter objects, to preparse and validate
Call parameterized queries only.
Better yet, use Stored Procedures exclusively.
Avoid using dynamic SQL, and dont use string concatenation to build queries.
If using SPs, you can also limit permissions in the database to executing the needed SPs only, and not access tables directly.
you can also easily verify that the entire codebase only accesses the DB through SPs...
Okay, this response will relate to the update of the question:
"If anyone knows of any specific way to mount a SQL injection attack against this sanitization method I would love to see it."
Now, besides the MySQL backslash escaping - and taking into account that we're actually talking about MSSQL, there are actually 3 possible ways of still SQL injecting your code
sSanitizedInput = "'" & Replace(sInput, "'", "''") & "'"
Take into account that these will not all be valid at all times, and are very dependant on your actual code around it:
Second-order SQL Injection - if an SQL query is rebuilt based upon data retrieved from the database after escaping, the data is concatenated unescaped and may be indirectly SQL-injected. See
String truncation - (a bit more complicated) - Scenario is you have two fields, say a username and password, and the SQL concatenates both of them. And both fields (or just the first) has a hard limit on length. For instance, the username is limited to 20 characters. Say you have this code:
username = left(Replace(sInput, "'", "''"), 20)
Then what you get - is the username, escaped, and then trimmed to 20 characters. The problem here - I'll stick my quote in the 20th character (e.g. after 19 a's), and your escaping quote will be trimmed (in the 21st character). Then the SQL
sSQL = "select * from USERS where username = '" + username + "' and password = '" + password + "'"
combined with the aforementioned malformed username will result in the password already being outside the quotes, and will just contain the payload directly.
3. Unicode Smuggling - In certain situations, it is possible to pass a high-level unicode character that looks like a quote, but isn't - until it gets to the database, where suddenly it is. Since it isn't a quote when you validate it, it will go through easy... See my previous response for more details, and link to original research.
In a nutshell: Never do query escaping yourself. You're bound to get something wrong. Instead, use parameterized queries, or if you can't do that for some reason, use an existing library that does this for you. There's no reason to be doing it yourself.
I realize this is a long time after the question was asked, but ..
One way to launch an attack on the 'quote the argument' procedure is with string truncation.
According to MSDN, in SQL Server 2000 SP4 (and SQL Server 2005 SP1), a too long string will be quietly truncated.
When you quote a string, the string increases in size. Every apostrophe is repeated.
This can then be used to push parts of the SQL outside the buffer. So you could effectively trim away parts of a where clause.
This would probably be mostly useful in a 'user admin' page scenario where you could abuse the 'update' statement to not do all the checks it was supposed to do.
So if you decide to quote all the arguments, make sure you know what goes on with the string sizes and see to it that you don't run into truncation.
I would recommend going with parameters. Always. Just wish I could enforce that in the database. And as a side effect, you are more likely to get better cache hits because more of the statements look the same. (This was certainly true on Oracle 8)
I've used this technique when dealing with 'advanced search' functionality, where building a query from scratch was the only viable answer. (Example: allow the user to search for products based on an unlimited set of constraints on product attributes, displaying columns and their permitted values as GUI controls to reduce the learning threshold for users.)
In itself it is safe AFAIK. As another answerer pointed out, however, you may also need to deal with backspace escaping (albeit not when passing the query to SQL Server using ADO or ADO.NET, at least -- can't vouch for all databases or technologies).
The snag is that you really have to be certain which strings contain user input (always potentially malicious), and which strings are valid SQL queries. One of the traps is if you use values from the database -- were those values originally user-supplied? If so, they must also be escaped. My answer is to try to sanitize as late as possible (but no later!), when constructing the SQL query.
However, in most cases, parameter binding is the way to go -- it's just simpler.
Input sanitation is not something you want to half-ass. Use your whole ass. Use regular expressions on text fields. TryCast your numerics to the proper numeric type, and report a validation error if it doesn't work. It is very easy to search for attack patterns in your input, such as ' --. Assume all input from the user is hostile.
It's a bad idea anyway as you seem to know.
What about something like escaping the quote in string like this: \'
Your replace would result in: \''
If the backslash escapes the first quote, then the second quote has ended the string.
Simple answer: It will work sometimes, but not all the time.
You want to use white-list validation on everything you do, but I realize that's not always possible, so you're forced to go with the best guess blacklist. Likewise, you want to use parametrized stored procs in everything, but once again, that's not always possible, so you're forced to use sp_execute with parameters.
There are ways around any usable blacklist you can come up with (and some whitelists too).
A decent writeup is here: http://www.owasp.org/index.php/Top_10_2007-A2
If you need to do this as a quick fix to give you time to get a real one in place, do it. But don't think you're safe.
There are two ways to do it, no exceptions, to be safe from SQL-injections; prepared statements or prameterized stored procedures.
If you have parameterised queries available you should be using them at all times. All it takes is for one query to slip through the net and your DB is at risk.
Patrick, are you adding single quotes around ALL input, even numeric input? If you have numeric input, but are not putting the single quotes around it, then you have an exposure.
Yeah, that should work right up until someone runs SET QUOTED_IDENTIFIER OFF and uses a double quote on you.
Edit: It isn't as simple as not allowing the malicious user to turn off quoted identifiers:
The SQL Server Native Client ODBC driver and SQL Server Native Client OLE DB Provider for SQL Server automatically set QUOTED_IDENTIFIER to ON when connecting. This can be configured in ODBC data sources, in ODBC connection attributes, or OLE DB connection properties. The default for SET QUOTED_IDENTIFIER is OFF for connections from DB-Library applications.
When a stored procedure is created, the SET QUOTED_IDENTIFIER and SET ANSI_NULLS settings are captured and used for subsequent invocations of that stored procedure.
SET QUOTED_IDENTIFIER also corresponds to the QUOTED_IDENTIFER setting of ALTER DATABASE.
SET QUOTED_IDENTIFIER is set at parse time. Setting at parse time means that if the SET statement is present in the batch or stored procedure, it takes effect, regardless of whether code execution actually reaches that point; and the SET statement takes effect before any statements are executed.
There's a lot of ways QUOTED_IDENTIFIER could be off without you necessarily knowing it. Admittedly - this isn't the smoking gun exploit you're looking for, but it's a pretty big attack surface. Of course, if you also escaped double quotes - then we're back where we started. ;)
Your defence would fail if:
the query is expecting a number rather than a string
there were any other way to represent a single quotation mark, including:
an escape sequence such as \039
a unicode character
(in the latter case, it would have to be something which were expanded only after you've done your replace)
What ugly code all that sanitisation of user input would be! Then the clunky StringBuilder for the SQL statement. The prepared statement method results in much cleaner code, and the SQL Injection benefits are a really nice addition.
Also why reinvent the wheel?
Rather than changing a single quote to (what looks like) two single quotes, why not just change it to an apostrophe, a quote, or remove it entirely?
Either way, it's a bit of a kludge... especially when you legitimately have things (like names) which may use single quotes...
NOTE: Your method also assumes everyone working on your app always remembers to sanitize input before it hits the database, which probably isn't realistic most of the time.
I'm not sure about your case, but I just encountered a case in Mysql that Replace(value, "'", "''") not only can't prevent SQL injection, but also causes the injection.
if an input ended with \', it's OK without replace, but when replacing the trailing ', the \ before end of string quote causes the SQL error.
While you might find a solution that works for strings, for numerical predicates you need to also make sure they're only passing in numbers (simple check is can it be parsed as int/double/decimal?).
It's a lot of extra work.
It might work, but it seems a little hokey to me. I'd recommend verifing that each string is valid by testing it against a regular expression instead.
Yes, you can, if...
After studying the topic, I think input sanitized as you suggested is safe, but only under these rules:
you never allow string values coming from users to become anything else than string literals (i.e. avoid giving configuration option: "Enter additional SQL column names/expressions here:"). Value types other than strings (numbers, dates, ...): convert them to their native data types and provide a routine for SQL literal from each data type.
SQL statements are problematic to validate
you either use nvarchar/nchar columns (and prefix string literals with N) OR limit values going into varchar/char columns to ASCII characters only (e.g. throw exception when creating SQL statement)
this way you will be avoiding automatic apostrophe conversion from CHAR(700) to CHAR(39) (and maybe other similar Unicode hacks)
you always validate value length to fit actual column length (throw exception if longer)
there was a known defect in SQL Server allowing to bypass SQL error thrown on truncation (leading to silent truncation)
you ensure that SET QUOTED_IDENTIFIER is always ON
beware, it is taken into effect in parse-time, i.e. even in inaccessible sections of code
Complying with these 4 points, you should be safe. If you violate any of them, a way for SQL injection opens.
I am using php, mysql with smarty and I places where users can put comments and etc. I've already escaped characters before inserting into database for SQL Injection. What else do I need to do?
XSS is mostly about the HTML-escaping(*). Any time you take a string of plain text and put it into an HTML page, whether that text is from the database, directly from user input, from a file, or from somewhere else entirely, you need to escape it.
The minimal HTML escape is to convert all the & symbols to & and all the < symbols to <. When you're putting something into an attribute value you would also need to escape the quote character being used to delimit the attribute, usually " to ". It does no harm to always escape both quotes (" and the single quote apostrophe '), and some people also escape > to >, though this is only necessary for one corner case in XHTML.
Any good web-oriented language should provide a function to do this for you. For example in PHP it's htmlspecialchars():
<p> Hello, <?php htmlspecialchars($name); ?>! </p>
and in Smarty templates it's the escape modifier:
<p> Hello, {$name|escape:'html'}! </p>
really since HTML-escaping is what you want 95% of the time (it's relatively rare to want to allow raw HTML markup to be included), this should have been the default. Newer templating languages have learned that making HTML-escaping opt-in is a huge mistake that causes endless XSS holes, so HTML-escape by default.
You can make Smarty behave like this by changing the default modifiers to html. (Don't use htmlall as they suggest there unless you really know what you're doing, or it'll likely screw up all your non-ASCII characters.)
Whatever you do, don't fall into the common PHP mistake of HTML-escaping or “sanitising” for HTML on the input, before it gets processed or put in the database. This is the wrong place to be performing an output-stage encoding and will give you all sort of problems. If you want to validate your input to make sure it's what the particular application expects, then fine, but weeding out or escaping “special” characters at this stage is inappropriate.
*: Other aspects of XSS are present when (a) you actually want to allow users to post HTML, in which case you have to whittle it down to acceptable elements and attributes, which is a complicated process usually done by a library like HTML Purifier, and even then there have been holes. Alternative, simpler markup schemes may help. And (b) when you allow users to upload files, which is something very difficult to make secure.
In regards to SQL Injection, escaping is not enough - you should use data access libraries where possible and parameterized queries.
For XSS (cross site scripting), start with html encoding outputted data. Again, anti XSS libraries are your friend.
One current approach is to only allow a very limited number of tags in and sanitize those in the process (whitelist + cleanup).
You'll want to make sure people can't post JavaScript code or scary HTML in their comments. I suggest you disallow anything but very basic markup.
If comments are not supposed to contain any markup, doing a
echo htmlspecialchars($commentText);
should suffice, but it's very crude. Better would be to sanitize all input before even putting it in your database. The PHP strip_tags() function could get you started.
If you want to allow HTML comments, but be safe, you could give HTML Purifier a go.
You should not modify data that is entered by the user before putting it into the database. The modification should take place as you're outputting it to the website. You don't want to lose the original data.
As you're spitting it out to the website, you want to escape the special characters into HTML codes using something like htmlspecialchars("my output & stuff", ENT_QUOTES, 'UTF-8') -- make sure to specify the charset you are using. This string will be translated into my output & stuff for the browser to read.
The best way to prevent SQL injection is simply not to use dynamic SQL that accepts user input. Instead, pass the input in as parameters; that way it will be strongly typed and can't inject code.
I'm using WWW::Mechanize to do some standard website traversal, but at one point I have to construct a special POST request and send it off. All this requires session cookies.
In the POST request I'm making, spaces are being encoded to + symbols, but I need them encoded as a %20.
I can't figure out how to alter this behaviour. I realise that they are equivalent, but for reasons that are out of my hands, this is what I have to do.
Thanks for any help.
This is hard-coded in URI::_query::query_form(). It translates the spaces to +.
$val =~ s/ /+/g;
It then calls URI::_query::query with the joined pairs, where the only + signs should be encoded spaces. The easiest thing to do is probably to intercept calls to URI::_query::query with Hook::LexWrap, modify the argument before the call starts so you can turn + into %20, and go on from there.
A little bit more annoying would be to redefine URI::_query::query. It's not that long, and you just need to add some code at the beginning of the subroutine to transform the arguments before it continues.
Or, you can fix the broken parser on the other side. :)
I have a couple chapters on dealing with method overriding and dynamic subroutines in Mastering Perl. The trick is to do it without changing the original source so you don't introduce new problems for everyone else.
This appears to be hardcoded in URI::_query::query_form(). I'd conditionally modify that based on a global as is done with $URI::DEFAULT_QUERY_FORM_DELIMITER and submit your change to the URI maintainer.
Other than that, perhaps you could use a LWP::UserAgent request_prepare callback handler?