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IBM AIX bellmail Stack Buffer Overflow Vulnerability



bellmail is a mail user-agent (MUA) and is commonly used for accessing locally stored electronic mail messages. Under AIX, the bellmail program is installed by default and is set-uid root. More information can be found at the URL shown.



Local exploitation of a buffer overflow vulnerability in the bellmail program of IBM Corp.'s AIX operating system allows attackers to execute arbitrary code with root privileges.

The problem specifically exists within sendrmt function. This function is called when a user tries to send mail using the "m" command. Within this function, several sprintf calls are made to concatenate user-supplied input with static strings. No bounds checking is performed to ensure that the resulting string will fit in the destination buffer located on the stack. By supplying a long parameter, an attacker is able to overwrite program control data located on the stack and take control of the affected process.


Exploitation allows an attacker to execute arbitrary code with root privileges. Local access is required to execute and interact with the bellmail program.

It should be noted that the bellmail program does initially set its user (both saved and effective) to that of the calling user. Generally, it would be sufficient to drop these privileges. However, in this case, the bellmail program uses the AIX-specific setpriv functionality to retain the ability chown arbitrary files on the system.


iDefense has confirmed the existence of this vulnerability within AIX version 5.3 (5300-06) and 5.2. Previous versions are suspected to be vulnerable.


Removing the set-uid bit from the bellmail program will protect against exploitation. However, doing so will render the program unusable.


IBM Corp. has addressed this vulnerability by releasing interim fixes. More information can be found via the Bulletins tab of IBM's Subscription Service for UNIX and Linux servers. You can reach this service by clicking the URL shown below.



The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2007-4623 to this issue. This is a candidate for inclusion in the CVE list (http://cve.mitre.org/), which standardizes names for security problems.


08/28/2007 Initial vendor notification
08/28/2007 Initial vendor response
10/30/2007 Coordinated public disclosure


This vulnerability was discovered by Joshua J. Drake of VeriSign iDefense Labs.

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Copyright © 2007 Verisign, Inc.

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