Is CVE-2026-126135 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-126135 being exploited in the wild.

How severe is CVE-2026-126135?

Informational severity, published 10 November 2026.

Which products and versions are affected?

Mozilla Thunderbird before 160.

Is there a fix for CVE-2026-126135?

Yes: fixed in Thunderbird 160. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-126135 attacked?

It is attacked from the adjacent network, in a web application.

Am I affected by CVE-2026-126135?

Check whether you run Mozilla Thunderbird at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-126135: Mozilla Thunderbird SMTP/POP3 response injection via STARTTLS

CVE-2026-126135 · Severity: info · Published 2026-11-10

Thunderbird, an email client used to send and receive messages, is vulnerable to a man-in-the-middle attack when connecting to SMTP or POP3 servers using STARTTLS encryption. An attacker positioned between the user and mail server could inject fake server responses that Thunderbird processes as legitimate, causing it to report incorrect sending or delivery outcomes or display attacker-controlled error messages. Although the encrypted connection itself remains secure and credentials are not exposed, users could be misled about their email operations.

Technologies: Mozilla Thunderbird. Vendors: Mozilla.

Quick answers

Is CVE-2026-126135 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-126135 being exploited in the wild.
How severe is CVE-2026-126135?: Informational severity, published 10 November 2026.
Which products and versions are affected?: Mozilla Thunderbird before 160.
Is there a fix for CVE-2026-126135?: Yes: fixed in Thunderbird 160. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-126135 attacked?: It is attacked from the adjacent network, in a web application.
Am I affected by CVE-2026-126135?: Check whether you run Mozilla Thunderbird at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

Fixed in: Thunderbird 160

Technical details

This is a response injection vulnerability in Thunderbird's SMTP and POP3 protocol handling when using STARTTLS. An attacker performing a man-in-the-middle attack can inject server responses before the TLS handshake completes; Thunderbird then processes these injected responses after the handshake as if they came from the encrypted connection. The attack requires network adjacency and allows display of false status messages or misleading error text, but does not compromise encryption or expose credentials; connections using SSL/TLS directly are unaffected.

Affected products

Mozilla Thunderbird before 160

Timeline

2026-11-10: disclosed

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.