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 want to sanitize a simple text field with a person's name, to protect from XSS and such. Stackoverflow pretty much says I must whitelist. I don't understand this. If I simply remove all < and > from the input value, or replace them with > and &ls;, does not that rule out code injection? Or am I missing something? Perhaps you only need to whitelist in more complex scenarios where you have to put up with angular brackets?
Sorry if it's a silly question, it's important to get this right.
Whether to whitelist or encode depends on how you want to use the text.
If you intend to treat the input as plain text, then encoding special characters is enough, and any HTML code entered will display as text only as long as you are careful not to allow unencoded text to end up anywhere in your HTML output. (This includes making sure any other systems you interface with don’t inappropriately use the unencoded text.)
If you want to allow some markup in the input, such as text styling or links, then you must whitelist the tags that you allow and get rid of all others.
No, it's not sufficient because if you were to include the person's name in an html attribute, you would also need to escape any double-quotes contained therein.
I have started to code a multi-language feature for a medium-sized website with a lot of hardcoded text. As the website is supposed to be translated into Japanese and Korean (multibyte character set) I am considering the following:
If I use string externalization, do the strings for Japanese or Korean need to be in unicode form within the locale file (i.e. 台北 instead of 台北 as string value)?
Would it make more sense to store the localization in a DB (i.e. MySQL) and retrieve the respective values via a localization function in PHP?
Your thought input is much appreciated.
Best regards
$0.02 from someone who has some experience with i18n...
Keep your translations in human-readable form, as it will likely be translators and not coders managing these resources.
If this text (hard-coded, you say) is not subject to frequent change, then you may wish to store these resources as files that you read in at runtime.
If this text is subject to frequent change, then you may wish to explore other alternatives for storing resources, such as databases or in-memory key-value stores.
Depending upon your requirements, you may want to consider a mixture of the above.
But I strongly suggest that you avoid mixing code (the HTML character entities) with your translation resources. Most translators will not understand what they mean and may break them when they are translating. And on the flip-side, a programmer may not understand how to insert code or formatting into the translation resources properly, unless they actually understand that language.
tl;dr
- use UTF-8
- don't mix any code/formatting into the translations themselves
- how you store the translations depends upon your requirements
I doubt that string externalization would be your biggest problem. But let me give you some advise.
String externalization
Of course you would need to separate translatable strings from the code. I would recommend storing translation in plain text, UTF-8 encoded file containing key-value pairs:
some.key=some translation
Of course you would need to write a helper script to resolve this at runtime. The script would need to detect end-user's language.
Language detection
Web browsers are so nice to send AcceptLanguage header each time they send a request. What you need to do, is to read the content of this header and check if you support any of the language user has listed. If so, read the resource file (as defined above) and return strings for given language, return your default language otherwise. The code example below will give you the most desired language (which is not necessary the one you support):
<?php
$locale = Locale::acceptFromHttp($_SERVER['HTTP_ACCEPT_LANGUAGE']);
echo $locale;
?>
This is still, not the biggest of your challenges.
Styles and style sheets
The real problem with multilingual web sites or web applications are styles. People tend to put style definitions in-line, which is problematic to say the least. Also, designers tend to think that Arial is the best font for entire Universe, as well as emphasis always have to come with bolded font. The only problem is, the font might be unreadable under some circumstances.
I must admit, I don't know why it happens, but most of the times web browsers tend to ignore bold attribute for Asian scripts (which is good), but sometimes they do not and it could became a major challenge for end users if your font definition is say font-family:Arial; font-size:10px;.
The other problem could be colors. Depending on your web site design, some colors used might be inappropriate for target customers. That is because we all tend to assign meaning to colors based on our cultural background.
Images containing localizable text could also give you a headache, you would need to either externalize such texts (and write them down just like any other HTML element), or prepare multilingual resources structure (i.e. put all images to directories named after language code ("en", "ja", "ko")).
The real challenge however, are hard-coded formatting tags like <b>, <i>, <u>, <strong>, etc. Nobody should use them nowadays, style classes should be used instead but the common practice is different. You would probably need to replace them with style classes; each element could have more than one style class, which to my surprise is not common knowledge (for example <p class="main boldText">).
OK, once you have your styles externalized, you would probably be forced to implement some sort of CSS Localization Mechanism. This is needed in the lights of what I wrote above. The easiest way to do that is to create directory structure similar to the one I mentioned before - "en" for English base CSS files, "ja" for Japanese and "ko" for Korean, so each language would have their own, separate set of CSS files. This is similar to UI skins, only in that case user won't be able to choose the skin, you will decide on which CSS to present them - you would detect language anyway.
As for in-line style definitions (<p style="whatever">), after you define CSS L10n Mechanism, you could override any style by forcing it with !important keyword. That is, unless somebody in his very wrong mind put this keyword to in-line style definition.
Concatenations
Well, this is your biggest challenge. Even people who understand the need of string externalization tend to concatenate the strings like this:
$result = $label + ": " + $product;
$message = "$your_basket_is + $basket_status + ".";
This poses serious problem for Internationalization (and if it is not resolved for Localization as well). That is because, the order of the sentence tend to be different after translating text into different language (this especially regards to Korean). Also, I showed you hard-coded punctuations, which are not necessary correct for Asian languages. That is what I have to go through on a daily basis :/
What you would probably need to do, is to remove such concatenations, or use some means of message formatting. The PHP example (taken directly from web page I am referencing) would be:
<?php
$fmt = new MessageFormatter("en_US", "{0,number,integer} monkeys on {1,number,integer} trees make {2,number} monkeys per tree");
echo $fmt->format(array(4560, 123, 4560/123));
$fmt = new MessageFormatter("de", "{0,number,integer} Affen auf {1,number,integer} Bäumen sind {2,number} Affen pro Baum");
echo $fmt->format(array(4560, 123, 4560/123));
?>
As you can see in this example, numbers are also formatted to much locale style. This leads us to:
Locale aware formatting
Dates, times, numbers and currencies or other similar information need to be formatted according to user-detected Locale. There is a slight difference here: you should attempt to do that, even if you do not support related language resources (do not have translations). Of course for currency symbol, you would use whatever is your real currency, not the user's default, but the format should respect end user's cultural background.
Summary
I have just presented you with a short introduction to multilingual web site design with focus on Japanese and Korean target markets. If at some point you would need to support Chinese Simplified as well, support for GB18030 encoding would be probably needed as well. This would be very challenging...
You do not want to store all your text as HTML entities. It'll drive you mad. The only reason to do this is if you need to serve your document in an ASCII encoding and cannot embed the characters directly. But in this day and age there's no reason for that; serve your document as UTF-8 and write and store your contents in UTF-8 and be done with it.
Whether or not to store translations in the database depends on many factors, including performance, caching, whether you need to be able to search for the text, whether the text should be editable by non-programmers etc. Usually .mo/.po translation files with gettext are a good way to go unless proven otherwise.