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Fencer Intel
no cvss —

CVE-2026-126135 json

SMTP and POP3 response injection when using STARTTLS

published yesterday modified yesterday also EUVD-2026-122689

cvss
—
not scored
cisa kev
Not listed
not in CISA's catalog
epss
0.2%
EPSS 0.2% chance of exploitation in 30 days, percentile 8%
public exploits
0
none known

Summary

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.

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Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-126135 updated cvss 6.1

thunderbird: SMTP and POP3 response injection when using STARTTLS

  • Red Hat Enterprise Linux 6 thunderbird Red Hat Enterprise Linux 7 thunderbird
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 8 thunderbird Red Hat Enterprise Linux 9 thunderbird Red Hat Enterprise Linux 10 thunderbird, rhel10/thunderbird-flatpak
    affected: no fix available

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

0.19% chance of exploitation in the next 30 days, higher than 8% of all scored CVEs (FIRST EPSS, 2026-11-11).

Affected products

  • Mozilla Thunderbird

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.