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LSD puts Sendmail bug under the microscope

Proof of concept

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Polish ethical hackers Last Stage of Delirium (LSD) yesterday published proof of concept code for a serious flaw in Sendmail which emerged this week.

In a posting to BugTraq yesterday, LSD provides a detailed analysis of the buffer overflow vulnerability for the first time.

Previous advisories on the flaw, which has been present in Sendmail (undiscovered) for some years, are noticeably lacking in detail.

The vulnerability resides within the crackaddr(char* addr) function of the headers.c file, according to LSD's research. Ironically this function is designed to be a security mechanism.

Based on their preliminary work the Poles tentatively suggest the Sendmail vulnerability is less bad than
it first appeared.

"Due to the nature of the discussed Sendmail vulnerability it seems that it is unexploitable on most of commercially available UNIX systems," LSD writes. "It also doesn't seem to be exploitable on most of the default SMTP installations of x86 based open-source systems.

"This leads to the conclusion that the overall impact of the vulnerability is rather limited and not so significant as it might be thought," the group adds.

By its own admission, LSD's analysis is not exhaustive, testing only for one possible exploit mechanism on Unix and (Slackwave and Red Hat) Linux servers. So don't read too much into the group's conclusions just yet.

"We cannot exclude that there does not exist another execution path in the Sendmail code, that could lead to the program counter overwrite," LSD writes.

In a follow-up post, Sendmail's Eric Allman points out patches still need to be applied.

"Please don't breath a sigh of relief because you are running on one of the 'does not crash' systems," he writes.

"Besides direct execution path exploits, there are other variables that are not pointers that have security implications; finding one of them within range will be more difficult, but probably not impossible." ®

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External Links

CERT advisory: Remote Buffer Overflow in Sendmail
Patch application and testing tool from Sendmail

SANS - Survey on application security programs

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