https://seclists.org/oss-sec/2026/q2/1047: fetchmail's NTLM authentication vulnerable to stack buffer overflow up to lease 6.6.6 (FW: The 6.6.7.rc1 lease candidate is available (security fix for NTLM protocol, possible RCE))

Q: Which fetchmail deployments are affected?

Only builds compiled with NTLM support enabled are affected. NTLM is disabled by default, but distribution packages may enable it for compatibility; builds without NTLM support are unaffected.

Q: How can I determine whether my fetchmail build includes the vulnerable feature?

Run "fetchmail -V | head -1" and inspect the feature list. The presence of "+NTLM" indicates that NTLM was enabled at compile time.

Q: What must an attacker be able to do to exploit this issue?

The issue is in NTLM authentication code and may be triggered remotely during NTLM authentication. The report says that approximately 52 bytes may be overwritten on the stack and that remote code execution cannot be ruled out.

Q: What should I do if I cannot immediately update fetchmail?

Disable NTLM in the fetchmail build. The advisory specifically recommends that distributors either update to 6.6.7.rc1 or disable NTLM.

Published Jun 27, 2026

Updated

Affected Software

1 affected component

fetchmail<=6.6.6, =6.6.7.rc1

Frequently Asked Questions

Which fetchmail deployments are affected?

Only builds compiled with NTLM support enabled are affected. NTLM is disabled by default, but distribution packages may enable it for compatibility; builds without NTLM support are unaffected.

How can I determine whether my fetchmail build includes the vulnerable feature?

Run "fetchmail -V | head -1" and inspect the feature list. The presence of "+NTLM" indicates that NTLM was enabled at compile time.

What must an attacker be able to do to exploit this issue?

The issue is in NTLM authentication code and may be triggered remotely during NTLM authentication. The report says that approximately 52 bytes may be overwritten on the stack and that remote code execution cannot be ruled out.

What should I do if I cannot immediately update fetchmail?

Disable NTLM in the fetchmail build. The advisory specifically recommends that distributors either update to 6.6.7.rc1 or disable NTLM.