Comment Injection Attack

Description
Comments injected into an application through input can be used to compromise a system. As data is parsed, an injected/malformed comment may cause the process to take unexpected actions that result in an attack.

Examples
The attacker may conduct this kind of attack with different programming or scripting languages:

Database:

If the attacker has the ability to manipulate queries, which are sent to the database, then he's able to inject a terminating character too. The aftermath is that the interpretation of the query will be stopped at the terminating character: SELECT body FROM items WHERE id = $ID limit 1; Let's assume that the attacker has sent via the GET method the following data stored in variable $ID: "1 or 1=1; #" In the end the final query form is: SELECT body FROM items WHERE id = 1 or 1=1; # limit 1; After the # character everything will be discarded by the database including "limit 1", so only the last column "body" with all its records will be received as a query response.

Sequences that may be used to comment queries:
 * MySQL:#, --
 * MS SQL: --
 * MS Access: %00 (hack!)
 * Oracle: --

Null byte:

To comment out some parts of the queries, the attacker may use the standard sequences, typical for a given language, or terminate the queries using his own methods being limited only by his imagination. An interesing example is a null byte method used to comment out everything after the current query in MS Access databases. More information about this can be found in Embedding_Null_Code.

Shell:

Shell (bash) also has the character #, which terminates interpretation.

For example:

find.php <? $ =sth $_GET['what]; system("/usr/bin/find -name '$sth' -type f"); ?> Using /find.php?what=*'%20%23 the attacker will bypass limitation "-type f" and this command: /usr/bin/find -name '*' -type f will become: /usr/bin/find -name '*' #-type f So the final form of the command is: /usr/bin/find -name '*'

HTML (injection):

If there are no restrictions about who is able to insert comments, then using the start comment tag:  After: GET /invisible.php?user=<!-- There result will be: hello!:

References:


 * http://dev.mysql.com/doc/refman/5.0/en/comments.html
 * http://www.webapptest.org/ms-access-sql-injection-cheat-sheet-EN.html

Related Threats

 * Category:Information Disclosure

Related Attacks

 * Embedding_Null_Code
 * Unicode Encoding

Related Vulnerabilities

 * Category:Input Validation Vulnerability

Related Countermeasures

 * Category: Input Validation

Developers should anticipate that comments will be injected/removed/manipulated in the input vectors of their software system. Use an appropriate combination of black lists and white lists to ensure only valid, expected and appropriate input is processed by the system.