LiveActive security incident?Get immediate response
CVE Record

CVE-2021-4126: When receiving an OpenPGP/MIME signed email message that contains an additional outer MIME message layer, f...

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.

MediumCVSS 6.5Not KEV-listedUpdated
Discuss exposureBack to CVEs
Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

Thunderbird could show a valid OpenPGP/MIME signature while extra outer email content was not actually signed. This could make recipients believe added text, such as a mailing-list footer layer, was protected by the sender’s signature. The impact is trust and message integrity, not data theft or service outage.

Executive priority

Treat as a targeted trust-risk issue, not an emergency outage risk. Patch promptly where Thunderbird supports sensitive signed communications, because misleading signature status can undermine approval and authenticity controls.

Technical view

In Thunderbird before 91.4.1, OpenPGP/MIME signed messages with an additional outer MIME message layer were evaluated using only the inner signed message. The displayed signature status could therefore imply coverage of unsigned outer content. Mozilla changed Thunderbird 91.4.1 to consider only the signature belonging to the top-level MIME part for displayed status.

Likely exposure

Exposure is limited to users running Thunderbird versions before 91.4.1 who receive and rely on OpenPGP/MIME signed email. Organizations using signed email for approval, legal, security, or executive communications have higher practical risk.

Exploitation context

The CVE is not listed in KEV, and the provided sources do not state active exploitation. The issue requires user interaction with a crafted or transformed signed email and mainly affects recipient trust decisions about whether all displayed content was signed.

Researcher notes

No CWE is listed in the provided CVE data. Scope is Thunderbird before 91.4.1. Evidence names the display-validation behavior and vendor fix, but does not provide public exploitation evidence or broader affected-product claims.

Mitigation direction

  • Upgrade Thunderbird to version 91.4.1 or later.
  • Prioritize users who rely on OpenPGP/MIME signed mail.
  • Check Mozilla advisory guidance for any deployment-specific notes.
  • Educate users that older clients may misrepresent signature coverage.
  • Review high-trust signed email workflows for Thunderbird dependency.

Validation and detection

  • Inventory Thunderbird versions across managed endpoints.
  • Confirm no active installations remain below 91.4.1.
  • Verify signed-mail workflows use updated Thunderbird builds.
  • Check security tooling for legacy Thunderbird exceptions or unmanaged installs.
  • Document remediation evidence for users handling sensitive signed email.
Prepared
Confidence
high
Sources
4

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.

cve · low confidence lookup

CVE-2021-4126 mapping review

Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.

Open ATT&CK lookup
Vulnerability profileCVE Program record
Severity
Medium
CVSS
6.5 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
3Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
6.5CVSS 3.1MediumCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N2.83.6Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

6.5Medium
CVSS 3.1 vector shape for CVE-2021-4126Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MozillaThunderbirdunspecifiedListed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.