[BreachExchange] Exim Vulnerability Puts More than 500, 000 Email Servers at High Risk

[Destry Winant]

https://staging.gbhackers.com/exim-email-servers-vulnerability/

A critical vulnerability with Exim email servers put’s more than
500,000 servers at risk. The bug affects all the version of Exim from
the first commit to last 3.x release.

The Vulnerability resides with b64decode function in the SMTP
listener. When the provided input is not a valid base64 string it
consumes additional bytes while decoding which causes the one-byte
heap overflow.

Taiwanese security Researcher Meh discovered the vulnerability. He
says generally this bug is not critical, but with Exim, it overwrites
the critical data when the string fits some specific length and also
the extra byte is controllable which make the exploitation more
feasible.

Exim uses base64 encoding to communicate with the clients and the
encodes functions will be stored in a buffer that allocated by
store_get(). To leverage the one-byte heap overflow Meh selected
sender_host_name as a better choice.

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Meh published a blog post describing the vulnerability along with the
steps to exploit it. According to the recent Mail (MX) Server Survey,
around 56 Percent servers running Exim.

The bug was tracked as CVE-2018-6789, an attacker could leverage the
vulnerability by sending a crafted malicious request that causes the
buffer overflow and it can use to execute the remote code.

Exim email servers – Patch

Meh reported the vulnerability to Exim and the Exim released version
4.90.1, which fixes the vulnerability. Users are advised to update
their Exim server version to 4.90.1 or above.