---
id: "CVE-2026-126135"
page: "https://infrastack.ai/live/CVE-2026-126135"
kind: "vulnerability"
aliases: ["CVE-2026-126135"]
title: "If an attacker was able to perform a man-in-the-middle attack on an SMTP or POP3 server connection using STARTTLS,\u2026"
severity: "unknown"
cvss: null
epss: null
exploited: false
status: "open"
category: null
products: []
ecosystems: []
tags: []
not_affected_if: []
published_at: "2026-11-10T19:21:14+00:00"
first_seen_at: "2026-11-10T19:25:15+00:00"
updated_at: "2026-11-10T19:39:26+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-126135", "first_seen_at": "2026-11-10T19:25:15+00:00"}]
exploit_sources: []
---

# If an attacker was able to perform a man-in-the-middle attack on an SMTP or POP3 server connection using STARTTLS,…

Link for people: https://infrastack.ai/live/CVE-2026-126135 (this .md is for agents)

## Timeline

- 2026-11-10T19:25:15+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-126135

## From the sources

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.

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_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_