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| mod_gzip is "an Internet Content Acceleration module for the popular Apache Web Server. It compresses the contents delivered to the client. There is no need to install any additional software on the client". The mod_gzip_printf() procedure has three vulnerabilities that are exploitable only when the module is compiled in its debug mode. |
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Credit:
The information has been provided by Matthew Murphy.
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Vulnerable systems:
* mod_gzip version 1.3.26.1a
The vulnerabilities are listed in order of severity:
Stack overflow vulnerability
The log line is superfluously formatted into a 2048-byte buffer before being passed off to Apache and/or file. By requesting a long file name that the GZIP module handles, such as:
GET [overflow] HTTP/1.1
Host: www.apachesite.com
Accept-Encoding: gzip, deflate
The httpd child process handling your request segfaults. Consistent crashing can be seen with a buffer of about 2500 characters. If the saved return address is overwritten, code execution becomes trivial.
Format string vulnerability
Exploitable only when using the Apache log, this vulnerability allows remote users to submit a specially crafted HTTP request that causes the child to segfault:
GET /cgi-bin/printenv.pl?x=%25n%25n%25n%25n%25n HTTP/1.1
Host: www.apachesite.com
Accept-Encoding: gzip, deflate
Alternatively,
GET /cgi-bin/printenv.pl?x=%n%n%n%n%n HTTP/1.1
Host: www.apachesite.com
Accept-Encoding: gzip, deflate
Race condition (/tmp)
mod_gzip insecurely logs debugging information when the Apache log is not used. It generates a predictably named log file and fails to check it for unique naming. The log file naming is as follows:
t<PID>.log
An attacker who knew or guessed the PID of the httpd child servicing the request could overwrite arbitrary files as the superuser. At some instances during mod_gzip's initialization, it logs debug events as root. A well-placed series of symbolic links could cause arbitrary files to be overwritten. For example, linking /tmp/t760.log to /bin/ls would overwrite /bin/ls if mod_gzip logged an event from a process with ID 760.
A similar possibility exists on NTFS file systems on Win32 via NTFS hard links, but the default "Strengthen default permissions of internal system objects" policy prevents this.
Impact:
The impact of these issues on production sites should be minimal. Users running internet-accessible sites should immediately switch from the debug build to the release build of the module.
Vendor Response:
After communicating with Christian Kruse and Michael Schroepl, Matthew was told that the developers were not currently working on the project, and that the issues Matthew had raised would be addressed with the next version. As these issues have only a minor impact on most production sites, Matthew decided to release this advisory to inform those still running the debug build to make the change to release for the security and stability of their sites.
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