Exim auth_spa_server() Buffer Overflow Vulnerability 

iDEFENSE Security Advisory [IDEF0731]
www.idefense.com/application/poi/display?id=178&type=vulnerabilities
January 07, 2004

I. BACKGROUND

Exim is a message transfer agent developed for use on Unix systems. More

information is available at: 

  http://www.exim.org/

II. DESCRIPTION

Remote exploitation of a buffer overflow vulnerability in Exim 4.41 may
allow execution of arbitrary commands with elevated privileges.

Exim is a message transfer agent developed for use on Unix systems. The 
problem specifically exists in the auth_spa_server function. The 
function fails to check the length of input to spa_base64_to_bits(), 
which decodes a Base64-encoded string into a buffer of a fixed length. 
This string is user-controlled and passed to the program from a remote 
connection.

III. ANALYSIS

Exploitation of this vulnerability will give an attacker remote access 
to the mailer uid. The exim mailer is setuid root, but drops privileges 
before the vulnerable code is reached. A remote attacker may be able to 
use other vulnerabilities to further elevate their privileges.

This vulnerability is only exploitable when the spa authentication 
method has been configured by setting AUTH_SPA=yes in Local/Makefile 
when building it.

IV. DETECTION

Exim versions 4.40 and 4.41 have been confirmed vulnerable. The source 
code for version 4.42 suggests that it is vulnerable. It is suspected 
that previous versions are also vulnerable.

To determine if the Exim version being used is vulnerable, connect to
port 25 of the machine with Exim installed and type:

   EHLO localhost

If AUTH NTLM appears in the output the application may be vulnerable.

V. WORKAROUND

iDEFENSE is currently unaware of any effective workarounds for this 
vulnerability.

VI. VENDOR RESPONSE

A patch for Exim release 4.43 which addresses this vulnerability is
available at:

   http://www.exim.org/mail-archives/exim-announce/2005/msg00000.html

The patch will be incorporated into a future Exim release (4.50).

VII. CVE INFORMATION

The Common Vulnerabilities and Exposures (CVE) project has assigned the
names CAN-2005-0022 to these issues. This is a candidate for inclusion
in the CVE list (http://cve.mitre.org), which standardizes names for
security problems.

VIII. DISCLOSURE TIMELINE

12/23/2004  Initial vendor notification
12/29/2004  Initial vendor response
01/07/2004  Coordinated public disclosure

IX. CREDIT

The discoverer of this vulnerability wishes to remain anonymous.

Get paid for vulnerability research
http://www.idefense.com/poi/teams/vcp.jsp

X. LEGAL NOTICES

Copyright (c) 2004 iDEFENSE, Inc.

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Disclaimer: The information in the advisory is believed to be accurate
at the time of publishing based on currently available information. Use
of the information constitutes acceptance for use in an AS IS condition.
There are no warranties with regard to this information. Neither the
author nor the publisher accepts any liability for any direct, indirect,
or consequential loss or damage arising from use of, or reliance on,
this information.