Security readout for executives and security teams
Plain-English summary
This Mozilla flaw let a web-delivered Alt-Svc header use an overflowing port value to bypass Firefox or Thunderbird blocked-port protections. The practical concern is policy bypass inside affected clients, not confirmed remote code execution. The source bundle provides no CVSS score and no evidence of active exploitation.
Executive priority
Treat this as a routine but important legacy-client cleanup item. It is not KEV-listed and has no provided exploitation evidence, but browser policy bypass issues can matter in controlled environments and should not remain unpatched.
Technical view
CVE-2021-29946 is an integer overflow issue in handling port values above 16-bit bounds in Alt-Svc headers. Affected versions are Firefox before 88, Firefox ESR before 78.10, and Thunderbird before 78.10. The bundle does not provide CWE, CVSS, exploit maturity, or detailed impact beyond blocked-port bypass.
Likely exposure
Exposure is likely limited to users running outdated Mozilla Firefox, Firefox ESR, or Thunderbird versions below the fixed release thresholds. Organizations with managed browsers or long-lived ESR deployments should verify fleet versions, especially on legacy endpoints.
Exploitation context
CISA KEV status is false in the bundle, and no provided source states active exploitation. The evidence supports a blocked-port restriction bypass through Alt-Svc handling, but not weaponized exploitation, public exploit availability, or broader compromise outcomes.
Researcher notes
The public bundle is sparse: no CVSS, CWE, or exploit detail is provided. Analysis should stay focused on Alt-Svc port parsing and blocked-port policy behavior. Avoid assuming code execution, data theft, or network reachability beyond what Mozilla and CVE records state.
Mitigation direction
- Upgrade Firefox to 88 or later.
- Upgrade Firefox ESR to 78.10 or later.
- Upgrade Thunderbird to 78.10 or later.
- Check Mozilla advisory guidance for any environment-specific notes.
- Prioritize managed endpoints still running legacy browser or mail clients.
Validation and detection
- Inventory installed Firefox, Firefox ESR, and Thunderbird versions.
- Confirm no clients remain below the affected version thresholds.
- Review browser management reports for unmanaged or stale ESR deployments.
- Verify update policy coverage for remote and legacy endpoints.
- Check Mozilla advisories for any revised affected-version details.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
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-2021-29946 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Unknown
- CVSS
- Not scored
- Known Exploited
- No
- Published
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.
CVSS and timeline data
No CVSS vectors or timeline events were available in the normalized CVE source material.
Source materials
- CVE List V5 sourceCVE List V5
- https://www.mozilla.org/security/advisories/mfsa2021-14/CVE reference · x_refsource_MISC
- https://www.mozilla.org/security/advisories/mfsa2021-16/CVE reference · x_refsource_MISC
- https://www.mozilla.org/security/advisories/mfsa2021-15/CVE reference · x_refsource_MISC
- https://bugzilla.mozilla.org/show_bug.cgi?id=1698503CVE reference · x_refsource_MISC
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.