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CVE Record

CVE-2021-29976: Mozilla developers reported memory safety bugs present in code shared between Firefox and Thunderbird.

Mozilla developers reported memory safety bugs present in code shared between Firefox and Thunderbird. Some of these bugs showed evidence of memory corruption and we presume that with enough effort some of these could have been exploited to run arbitrary code. This vulnerability affects Thunderbird < 78.12, Firefox ESR < 78.12, and Firefox < 90.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2021-29976 is a Mozilla memory-safety issue affecting older Firefox, Firefox ESR, and Thunderbird releases. Mozilla said some bugs showed memory corruption and could potentially allow arbitrary code execution with enough effort. No active exploitation is identified in the provided sources.

Executive priority

Prioritize remediation for fleets with outdated browsers or mail clients. The sources do not show active exploitation, but potential arbitrary code execution in common user-facing applications justifies prompt patch verification.

Technical view

The CVE covers multiple memory safety bugs in code shared by Firefox and Thunderbird. Affected versions are Thunderbird before 78.12, Firefox ESR before 78.12, and Firefox before 90. The bundle provides no CVSS score or CWE classification.

Likely exposure

Organizations are exposed if they still run Firefox below 90, Firefox ESR below 78.12, or Thunderbird below 78.12. Exposure is most relevant on user workstations and any managed desktop images retaining outdated Mozilla packages.

Exploitation context

The bundle lists KEV as false and provides no cited evidence of active exploitation. Mozilla’s advisory language says memory corruption was observed and arbitrary code execution was presumed possible with enough effort, so treat vulnerable clients as meaningful endpoint risk.

Researcher notes

Bug details are represented as a Mozilla Bugzilla list, but the bundle does not provide exploit primitives, CVSS, or CWE data. Analysis should stay bounded to Mozilla’s memory-corruption statement and the affected-version thresholds.

Mitigation direction

  • Upgrade Firefox to version 90 or later.
  • Upgrade Firefox ESR to version 78.12 or later.
  • Upgrade Thunderbird to version 78.12 or later.
  • Apply relevant distribution security updates, including Gentoo advisories where applicable.
  • Check Mozilla guidance for any product-specific remediation details.

Validation and detection

  • Inventory installed Firefox, Firefox ESR, and Thunderbird versions.
  • Confirm no managed desktop image contains versions below the fixed thresholds.
  • Review package-manager status for Mozilla security updates.
  • Check vulnerability scanners for CVE-2021-29976 findings after updates.
  • Confirm unsupported legacy Mozilla builds are removed or isolated.
Prepared
Confidence
medium
Sources
7

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

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description · low confidence lookup

Execution behavior lookup

The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.

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cve · low confidence lookup

CVE-2021-29976 mapping review

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
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.

0CVSS vectors
0Timeline events
0ADP providers
7Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
MozillaThunderbirdunspecifiedListed
MozillaFirefox ESRunspecifiedListed
MozillaFirefoxunspecifiedListed
Weakness

CWE details

No CWE listed

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