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CVE Record

CVE-2021-29988: Firefox incorrectly treated an inline list-item element as a block element, resulting in an out of bounds r...

Firefox incorrectly treated an inline list-item element as a block element, resulting in an out of bounds read or memory corruption, and a potentially exploitable crash. This vulnerability affects Thunderbird < 78.13, Thunderbird < 91, Firefox ESR < 78.13, and Firefox < 91.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

This is a Mozilla rendering memory-safety flaw in older Firefox, Firefox ESR, and Thunderbird versions. A specially handled page or message content could cause an out-of-bounds read or memory corruption, leading to a potentially exploitable crash. The main business risk is unmanaged endpoints still running versions older than the fixed releases.

Executive priority

Treat this as a high-priority endpoint patching issue, especially for organizations with outdated browsers or mail clients. No active exploitation is shown in the supplied evidence, but browser memory corruption flaws are commonly risk-relevant and should not remain unpatched.

Technical view

Firefox treated an inline list-item element as a block element, causing an out-of-bounds read or memory corruption. The CVE states this could produce a potentially exploitable crash. Affected versions are Thunderbird before 78.13 and 91, Firefox ESR before 78.13, and Firefox before 91.

Likely exposure

Exposure is most likely on workstations, VDI images, kiosks, or managed Linux fleets running outdated Firefox, Firefox ESR, or Thunderbird. Public servers are generally not the primary exposure unless they run vulnerable client software for browsing or mail rendering.

Exploitation context

The provided bundle does not show CISA KEV listing or any cited evidence of active exploitation. The impact is still material because memory corruption in browser or mail rendering code can become high consequence when triggered through untrusted content.

Researcher notes

The source bundle lacks CVSS, CWE, and detailed exploitability analysis. Bugzilla is cited, but the CVE description is the main technical basis provided. Avoid assuming remote code execution or active exploitation without additional vendor or threat-intelligence evidence.

Mitigation direction

  • Upgrade Firefox to version 91 or later, or a currently supported release.
  • Upgrade Firefox ESR to version 78.13 or later, or a currently supported ESR.
  • Upgrade Thunderbird to version 78.13, 91, or a currently supported release.
  • Prioritize managed endpoints that process untrusted web pages or email content.
  • Check Mozilla and distribution advisories for platform-specific package guidance.

Validation and detection

  • Inventory installed Firefox, Firefox ESR, and Thunderbird versions across managed endpoints.
  • Confirm no Firefox versions earlier than 91 remain in use.
  • Confirm no Firefox ESR versions earlier than 78.13 remain in use.
  • Confirm no Thunderbird versions earlier than 78.13 or 91 remain in use.
  • Review Linux distribution security advisories for backported package fixes.
Prepared
Confidence
medium
Sources
9

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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CVE-2021-29988 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
8Source 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
MozillaThunderbirdunspecified, unspecifiedListed
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.