A stack-based buffer overflow flaw was found in fetchmail when built with NTLM support. A malicious or compromised mail server advertising NTLM authentication can send a crafted Type 2 challenge that causes fetchmail to write past a fixed stack buffer while building the NTLM authenticate response. This may lead to remote code execution depending on stack-frame layout, or to authentication failure or process termination under memory hardening.
Rebuild fetchmail without --enable-NTLM (omit NTLM at configure time), or upgrade to fetchmail 6.6.7 or later. Confirm with fetchmail -V that the build does not list +NTLM.