Public Vulnerability Reports

Exim auth_spa_server() Buffer Overflow Vulnerability



Exim is a message transfer agent developed for use on Unix systems. More
information is available at:


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


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.


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.


iDEFENSE is currently unaware of any effective workarounds for this


The vulnerability has been fixed in Exim release 4.44.


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 (, which standardizes names for
security problems.


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


The discoverer of this vulnerability wishes to remain anonymous.

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Copyright © 2004 Verisign, Inc.

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