Emedding Null Code

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Description

The Embedding NULL Bytes/characters technique exploits applications that don’t properly handle postfix NULL terminators. It is used as a technique to perform other attacks, like directory browsing, path traversal, SQL injection, execution of arbitrary code, among others. It can be found lots of vulnerable applications and exploits available to abuse systems using this technique.

This technique includes several variations to represent the postfix NULL terminator:

PATH%00 PATH[0x00] PATH[alternate representation of NULL character]

%00

Example

Example1 – PHP Script

In the following example, it’s shown the use of this technique to modify an URL and access arbitrary files on a filesystem due a vulnerability on PHP script.

$whatever = addslashes($_REQUEST['whatever']); include("/path/to/program/" . $whatever . "/header.htm");

By manipulating the URL using postfix NULL bytes, one can have access to UNIX password file:

http://vuln.example.com/phpscript.php?whatever=../../../etc/passwd%00

Example2 – Adobe PDF ActiveX Attack

Another know attack, consists in the exploitation of buffer overflow in the ActiveX component (pdf.ocx) that allows remote execution arbitrary code.

The problem happens when a link is requested as:

GET /some_dir/file.pdf.pdf%00[long string] HTTP/1.1

In this case, the request must be made to a web server that truncates the request at the null byte (%00), as Microsoft IIS and Netscape Enterprise web servers. Though the requested URI is truncated for the purposes of locating the file, the long string is still passed to the Adobe ActiveX component. This component triggers a buffers overflow within RTLHeapFree() allowing for an attacker to overwrite an arbitrary word in memory.

This attack can be performed by adding malicious content to the end of any embedded link and referencing any Microsoft IIS or Netscape Enterprise web server. It is not necessary to establish a malicious web site to execute this attack.

For more details about this attack, see External References topics.

External References

http://capec.mitre.org/data/definitions/52.html

http://nvd.nist.gov/nvd.cfm?cvename=CVE-2004-0629