CVE-2026-126135 SMTP and POP3 response injection when using STARTTLS
- Exploited in the wild
- Not known
- Fix
- Available
What is CVE-2026-126135?
CVE-2026-126135 is a vulnerability in Mozilla Thunderbird, published on 10 Nov 2026.
Official description
If an attacker was able to perform a man-in-the-middle attack on an SMTP or POP3 server connection using STARTTLS, the attacker could inject server responses before the TLS handshake completed. Thunderbird would process these responses after the handshake, as if they had been received from the server over the encrypted connection. This could be used to make Thunderbird report a different outcome for sending or fetching mail, and to show attacker-controlled text in error messages. The encrypted connection itself was not compromised, and credentials were not exposed. Connections using SSL/TLS instead of STARTTLS were not affected. This vulnerability was fixed in Thunderbird 160 and Thunderbird 153.7.
Which versions are affected by CVE-2026-126135?
How do I fix CVE-2026-126135?
Upgrade to a fixed version.
Is CVE-2026-126135 being exploited?
There is no known exploitation of CVE-2026-126135 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-126135, GHSA-p47m-32pf-24wg, UBUNTU-CVE-2026-126135.