[Fetchmail-users] The 6.6.7.rc2 release candidate is available (security fix for NTLM reassessed to be DoS only)

[Matthias A. <mat...@gm...>]

The 6.6.7.rc2 release of fetchmail is now available at the usual locations,
including <https://downloads.sourceforge.net/project/fetchmail/branch_6.6/>.

The severity of the NTLM stack buffer overflow has been reassessed
and it is believed that this cannot be used to change fetchmail's 
control flow, but a denial of service was possible in case
compiler hardenings, such as stack or memory protectors, cause
fetchmail to be terminated.

The source archive is available at:
<https://downloads.sourceforge.net/project/fetchmail/branch_6.6/fetchmail-6.6.7.rc2.tar.xz/download>

The detached GnuPG signature is available at:
<https://downloads.sourceforge.net/project/fetchmail/branch_6.6/fetchmail-6.6.7.rc2.tar.xz.asc/download>

The SHA256 hashes for the tarballs are:
SHA2-256(fetchmail-6.6.7.rc2.tar.xz)= 223e2e7c32ed8825900043a5bcdf93b8155d81b5d05ee21e8b993fce59556c8d


Here are the release notes:
--------------------------------------------------------------------------------
fetchmail-6.6.7 (not yet released):

## BUGFIXES:
* Safeguard internal NTLM buffer handling to avoid overrun if server
  sends extremely long fields in the challenge, to avoid stack corruption.
  Reported by "Tristan".

  The code will report the buffer sizing issue and abort the NTLM authentication
  flow properly so that it's clear that message sizes are the issue.
  Fetchmail 6.6.7 currently supports 1 kByte of NTLM payload for each of the
  three messages, plus header.

  Earlier reports of this bug overestimated the impact.  While the bug
  indeed can write beyond the end of a stack-based buffer, it is reaching
  into another stack-based buffer that is at least 2048 bytes large, whereas
  the overflow is a few dozen bytes at most.  The worst impact is that the
  NegotiateFlags value in the final Authenticate Message step of the NTLM
  authentication protocol gets messed up and the authentication fails.

  It MAY happen, depending on hardening, stack protection and other compiler
  flags that these protections terminate fetchmail because one write
  to a data structure crosses into an other variable that is adjacent,
  on normal stack layouts that "victim" would be the challenge message.

  NOTE: NTLM is based on obsolete cryptographic mechanisms
  and should not be used without TLS or SSL security for the transport.

  NOTE: fetchmail 7 will remove support for NTLM and MSN authentication.
  Microsoft (who own the specification) generally advises that applications
  should not use NTLM, and is replacing with with Kerberos, see
  [MS-NLMP]: NT LAN Manager (NTLM) Authentication Protocol,
  https://learn.microsoft.com/en-us/openspecs/windows_protocols/ms-nlmp/
  See Introduction and Security Considerations for Implements
  (In Version 37.0 of the spec, sections 1 and 5.1 on page 84)

  Since this WAS initially reported as a stack smashing vulnerability,
  a security announcement fetchmail-SA-2026-01 will be issued to reduce
  the severity of the impact in public reporting.

* The IMAP protocol exchange was made stricter,
  (1) it will validate tagged responses that we received the right
  response to make sure fetchmail and the IMAP server are still in synch,
  (2) it will no longer accept the response words OK, NO, BAD, BYE if
  there is trailing garbage, and will now reject "OKAY" or "NONE", which
  would previously be accepted as aliases for OK or NO.

* For NTLM: Made protocol exchange more robust and make it track errors
  and SASL cancellation better to avoid hangs if NTLM does not work but other
  authentication schemes do or NTLM gets rejected by the server.

* Handling of escape sequences in the rcfile has been bugfixed to handle
  all ISO C escape sequences and handle octal escapes more strictly.

-------------------------------------------------------------------------------

These parts are from the .rc1 announcement about the NTLM:

Workaround: use the --auth PARAM option, where PARAM is another
mechanism (not NTLM) supported by fetchmail and by the IMAP or POP3 
server.  This may not be viable on shared multi-user systems.

Workaround for distributors and system operators:
Recompile fetchmail with --disable-NTLM option and reinstall it.
To confirm, look at the fetchmail -V | head -1 output, it should NOT 
mention +NTLM.

The immediate minimal fix can be cherry-picked from
https://gitlab.com/fetchmail/fetchmail/-/commit/cb5be5c38471eec19e519ace0bc569176317ea92
with a compiler warning fix in
https://gitlab.com/fetchmail/fetchmail/-/commit/ddf19ffab973bf6a424f878faa8ff6753ddf6246