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Vulnerability Note VU#398025Remote Buffer Overflow in SendmailOverviewThere is a vulnerability in sendmail that may allow remote attackers to gain the privileges of the sendmail daemon, typically root.I. DescriptionResearchers at Internet Security Systems (ISS) have discovered a remotely exploitable vulnerability in sendmail. This vulnerability could allow an intruder to gain control of a vulnerable sendmail server.Most organizations have a variety of mail transfer agents (MTAs) at various locations within their network, with at least one exposed to the Internet. Since sendmail is the most popular MTA, most medium-sized to large organizations are likely to have at least one vulnerable sendmail server. In addition, many UNIX and Linux workstations provide a sendmail implementation that is enabled and running by default. This vulnerability is message-oriented as opposed to connection-oriented. That means that the vulnerability is triggered by the contents of a specially-crafted email message rather than by lower-level network traffic. This is important because an MTA that does not contain the vulnerability will pass the malicious message along to other MTAs that may be protected at the network level. In other words, vulnerable sendmail servers on the interior of a network are still at risk, even if the site's border MTA uses software other than sendmail. Also, messages capable of exploiting this vulnerability may pass undetected through many common packet filters or firewalls. Sendmail has indicated to the CERT/CC that this vulnerability has been successfully exploited in a laboratory environment. We do not believe that this exploit is available to the public. However, this vulnerability is likely to draw significant attention from the intruder community, so the probability of a public exploit is high. A successful attack against an unpatched sendmail system will not leave any messages in the system log. However, on a patched system, an attempt to exploit this vulnerability will leave the following log message: Dropped invalid comments from header address A patched sendmail server will drop invalid headers, thus preventing downstream servers from receiving them. II. ImpactSuccessful exploitation of this vulnerability may allow an attacker to gain the privileges of the sendmail daemon, typically root. Even vulnerable sendmail servers on the interior of a given network may be at risk since the vulnerability is triggered from the contents of a malicious email message.III. SolutionApply a patch from SendmailSendmail has produced patches for versions 8.9, 8.10, 8.11, and 8.12. However, the vulnerability also exists in earlier versions of the code; therefore, site administrators using an earlier version are encouraged to upgrade to 8.12.8. These patches are located at ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.12.security.cr.patch ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.11.6.security.cr.patch ftp://ftp.sendmail.org/pub/sendmail/sendmail.8.9.3.security.cr.patch Many vendors include vulnerable sendmail servers as part of their software distributions. We have notified vendors of this vulnerability and recorded their responses in the systems affected section of this document. Enable the RunAsUser option There is no known workaround for this vulnerability. Until a patch can be applied, you may wish to set the RunAsUser option to reduce the impact of this vulnerability. As a good general practice, the CERT/CC recommends limiting the privileges of an application or service whenever possible. Systems Affected
References
Our thanks to Internet Security Systems, Inc. for discovering this problem, and to Eric Allman, Claus Assmann, and Greg Shapiro of Sendmail for notifying us of this problem. We thank both groups for their assistance in coordinating the response to this problem. This document was written by Jeffrey P. Lanza and Shawn V. Hernan.
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