CVE-2021-4126: mozilla thunderbird
Vulnerability summary
CVE-2021-4126 is a medium (CVSS 6.5) vulnerability in mozilla thunderbird. EPSS estimates a 0.44% chance of exploitation in the next 30 days, higher than 37.2% of scored vulnerabilities.
Advisory lineage
Upstream 0 · Downstream 7 · Related 4Timeline
- 22-12-2022PublishedVulnerability first disclosed.
- 16-04-2025Last modifiedVulnerability information updated.
Description
When receiving an OpenPGP/MIME signed email message that contains an additional outer MIME message layer, for example a message footer added by a mailing list gateway, Thunderbird only considered the inner signed message for the signature validity. This gave the false impression that the additional contents were also covered by the digital signature. Starting with Thunderbird version 91.4.1, only the signature that belongs to the top level MIME part will be considered for the displayed status. This vulnerability affects Thunderbird < 91.4.1.
CVSS metrics
4 scores · 4 sources · 1 version- OSV · UbuntuEcosystem advisory · calculated6.5MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
- OSV · DebianEcosystem advisory · calculated6.5MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
- NVDNIST6.5MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N
EPSS trend
Probability
0.44%
Chance of exploitation in the next 30 days
Percentile
37%
Versus all other CVEs
30-day change
+0.01 pp
From 0.43% on 19-09-2026
Known exploited
Compare KEV catalogsNot in any of the 3 tracked catalogs.
Public exploits
0 foundNo public exploits indexed.
Weaknesses
CWE · root causeNo weakness data has been published for this record.
Affected systems
Debian1 product
- thunderbird< 1:91.4.1-1
Mozilla1 product
- thunderbird≥ unspecified, < 91.4.1 | < 91.4.1
Ubuntu1 product
- thunderbird< 1:91.5.0+build1-0ubuntu0.18.04.1 | < 1:91.5.0+build1-0ubuntu0.20.04.1 | < 1:91.5.0+build1-0ubuntu1
3 vendors · 3 products
References
8 links- www.mozilla.org/security/advisories/mfsa2021-55/Vendor Advisory
