Public Vulnerability Reports

Apache HTTPD suEXEC Multiple Vulnerabilities

04.11.07

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

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.

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.

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.

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 

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."

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.

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

CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

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LEGAL NOTICES

Copyright © 2007 Verisign, Inc.

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