iDefense Security Advisory 04.11.07: Apache HTTPD suEXEC Multiple Vulnerabilities
Apache HTTPD suEXEC Multiple Vulnerabilities
iDefense Security Advisory 04.11.07
http://labs.idefense.com/intelligence/vulnerabilities/
Apr 11, 2007
I. BACKGROUND
The suexec binary is a helper application which is part of the Apache
HTTP server package. It is designed to allow a script to run with the
privileges of the owner of the script instead the privileges of the
server. More information about the suexec utility can be found at the
following URL.
http://httpd.apache.org/docs/2.0/suexec.html
II. DESCRIPTION
Local exploitation of multiple vulnerabilities within Apache Software
Foundation's suexec utility could allow an attacker to execute
arbitrary code as another user.
1) Path Checking Race Condition Vulnerabilities
One race condition occurs between the obtaining the current directory
and changing to that directory. Another race condition occurs between
changing to a directory and checking that the directory is not a link.
The directory structure may change between each of these operations,
which can lead to the lstat() being performed on an arbitrary directory
chosen by an attacker. These may be exploited with by renaming a parent
directory, or by using symbolic links.
A third race condition occurs between the final symbolic link check and
executing the target binary. The directory structure may change between
these calls, rendering the symbolic link check ineffective.
2) Path Checking Design Error Vulnerabilities
The suexec utility uses a strncmp() to check whether the current
directory is a sub-directory of the document root directory. This check
will succeed in situations where there exists a directory which begins
with the same sequence, but contains extra content. For example, if the
document root is "/var/www/html", the test will also succeed for
"/var/www/html_backup" and "/var/www/htmleditor". A correct test would
also perform a check that the next character is a trailing
null-terminator or directory separator.
A check performed does not verify whether a path to the CGI script (cmd)
is a regular file or not. If the path is pointing at a sub-directory
owned by the appropriate user and group, and the parent directory is
owned by the appropriate user and group, it will be accepted.
3) Arbitrary Group Id Input Validation Vulnerability
Due to a design error, the suexec binary permits any combination of
user/group values taken from command line parameters even if the user
is not a member of the specified group. This may be exploited in
combination with other vulnerabilities if the /proc file system is
mounted. Each time suexec drops its privileges and changes its UID and
GID, all files and directories under /proc/{PID} change their owner to
the corresponding values. As the suexec process changes its UID and GID
unconditionally, creating arbitrary UID and GID owned files is trivial.
III. ANALYSIS
Exploitation of these vulnerabilities would allow a local attacker to
execute arbitrary code with the privileges of another user.
In order to exploit this vulnerability, the user must already have
access to execute the suexec binary. The suexec binary is only able to
be executed by the same user as the web server, typically user 'httpd',
'apache' or 'nobody'. It may be possible to gain access to this user by
exploiting a CGI program, PHP script or other program on the server.
The binary also limits the users it will execute code as to those which
have user and group IDs greater than or equal to AP_UID_MIN and
AP_GID_MIN values respectively. These values are compiled into the
executable.
These factors somewhat mitigate the severity of the problem.
IV. DETECTION
iDefense has confirmed the existence of these vulnerabilities in the
suexec binary distributed with the version 2.2.3 of the Apache httpd in
Red Hat Inc.'s Fedora Core 4. This distribution is not vulnerable in the
default configuration, as exploitation requires additional, but common,
configuration changes to be made to the system.
It is suspected that all previous versions of suexec are vulnerable,
including the 1.3.x versions.
V. WORKAROUND
If the suexec binary is not required for normal operation, remove the
set-uid bit from the file as shown below.
# chmod -s /path/to/suexec
VI. VENDOR RESPONSE
The Apache Software Foundation HTTPD team declined to address the
vulnerabilities and instead provided the following vendor statement.
"The attacks described rely on an insecure server configuration - that
the unprivileged user the server runs as has write access to the
document root. The suexec tool cannot detect all possible insecure
configurations, nor can it protect against privilege "escalation" in
all such cases.
It is important to note that to be able to invoke suexec, the attacker
must also first gain the ability to execute arbitrary code as the
unprivileged server user."
VII. CVE INFORMATION
The Common Vulnerabilities and Exposures (CVE) project has assigned the
name CVE-2007-1741 to this issue. This is a candidate for inclusion in
the CVE list (http://cve.mitre.org/), which standardizes names for
security problems.
VIII. DISCLOSURE TIMELINE
02/08/2006 Initial vendor notification
10/06/2006 Second vendor notification
03/28/2007 Third vendor notification
03/28/2007 Initial vendor response
04/11/2007 Public disclosure
IX. CREDIT
The discoverer of this vulnerability wishes to remain anonymous.
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X. LEGAL NOTICES
Copyright © 2007 iDefense, Inc.
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