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TUCoPS :: Web :: Apache :: bt336.txt

Mod_gzip Debug Mode Vulnerabilities

Multiple Vulnerabilities in mod_gzip Debugging Routines

I. Synopsis

Affected Systems: mod_gzip and prior
    * Development: High
    * Production: Minimal
Developer URL:
Status: Vendor is not supporting project at this time.

II. Product Description

"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!"

(Quote from developer page)

III. Vulnerability Description

The mod_gzip_printf() procedure has three vulnerabilities that are
exploitable only when the module is compiled in its debug mode.  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
Accept-Encoding: gzip, deflate

The httpd child handling your request will segfault.  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 for a
remote user to submit a specially-crafted HTTP request that causes the child
to segfault:

GET /cgi-bin/ HTTP/1.1
Accept-Encoding: gzip, deflate


GET /cgi-bin/ HTTP/1.1
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:


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.

IV. 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.

V. Vendor Response

After communicating with Christian Kruse and Michael Schroepl, I was told
that the developers weren't currently working on the project, and that the
issues I had raised would be addressed with the next version.  As these
issues have only a minor impact on most production sites, I 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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