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CVE Record

CVE-2021-29974: When network partitioning was enabled, e.g.

When network partitioning was enabled, e.g. as a result of Enhanced Tracking Protection settings, a TLS error page would allow the user to override an error on a domain which had specified HTTP Strict Transport Security (which implies that the error should not be override-able.) This issue did not affect the network connections, and they were correctly upgraded to HTTPS automatically. This vulnerability affects Firefox < 90.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

Older Firefox versions could let a user bypass a certificate error on an HSTS-protected site when network partitioning was enabled. HSTS is meant to prevent that override. The issue did not stop Firefox from automatically using HTTPS, but it weakened the warning and refusal behavior users rely on during TLS failures.

Executive priority

Treat this as a standard browser hygiene issue, not an emergency, unless legacy Firefox is common in the environment. Patch promptly because browser TLS protections are core user-safety controls, but there is no cited evidence of active exploitation.

Technical view

CVE-2021-29974 affects Firefox before version 90. With network partitioning enabled, including through Enhanced Tracking Protection settings, the TLS error page could allow override of an HSTS domain’s certificate error. Mozilla states network connections were still upgraded to HTTPS correctly, so the weakness was in error-page policy enforcement.

Likely exposure

Exposure is limited to systems using Mozilla Firefox versions earlier than 90. Organizations with unmanaged browsers, legacy desktops, or delayed Linux distribution packages should verify deployed versions. The source bundle does not identify other affected Mozilla products.

Exploitation context

No source in the bundle reports active exploitation, and the CVE is not listed as KEV. Practical abuse would require a TLS error condition on an HSTS site and user interaction with the error page; the sources do not provide evidence of exploitation in the wild.

Researcher notes

The key behavior is HSTS override availability on the TLS error page when network partitioning is enabled. Mozilla’s description narrows impact by stating HTTPS upgrade behavior remained correct. Severity, CVSS, CWE, and exploit details are not provided in the source bundle.

Mitigation direction

  • Upgrade Firefox to version 90 or later.
  • Apply distribution vendor updates, including relevant Gentoo GLSA guidance where applicable.
  • Review Mozilla MFSA2021-28 for product-specific remediation details.
  • Prioritize removal of unsupported Firefox versions from managed endpoints.

Validation and detection

  • Inventory Firefox versions and flag any instance below 90.
  • Confirm browser update controls successfully deploy current Firefox releases.
  • Check Linux package management records for Mozilla Firefox security updates.
  • Review endpoint telemetry for legacy Firefox usage after remediation.
Prepared
Confidence
medium
Sources
5

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

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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
4Source 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
MozillaFirefoxunspecifiedListed
Weakness

CWE details

No CWE listed

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