BlackTreeCVE Intelligence
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Full vulnerability report · 2021
CVE-2021-29981High confidence

An issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that would lead to a potentially exploitable crash

Mozilla · Firefox

8.8HighCVSS 3.1
Recommended action
Within 7 days

High technical severity; prioritise exposed affected systems while verifying vendor guidance.

Fix not verified
R
Operational reassessment

Published severity in operational context

Open reassessment dashboard →
Published severityHighOperational priority:High, unchanged from published severity.unchanged

Evidence used

  • No CISA KEV confirmation is currently recorded.
  • EPSS is 1.01% for the current model date.

Compensating controls

  • Restrict the affected network interface to trusted sources where business-safe.
  • Monitor vendor guidance and exploitation sources for a material change.

Verification

  1. Confirm that the asset runs Mozilla Firefox and falls inside the recorded affected range.
  2. Recheck the vendor advisory before scheduling a change because no verified fixed version is currently retained.
  3. Validate exposure, authentication requirements and compensating controls in the actual environment.
  4. Reopen this reassessment when CVSS, KEV, EPSS, exploit evidence or remediation changes.
Mitigation target: Within 30 daysRemediation target: Within 180 days

This automated reassessment organises public evidence. It does not know asset exposure, business impact or control effectiveness and does not replace CVSS or a human risk decision.

Distribution package intelligence

Release-specific package status

Alpine, Debian findings are scoped to the named distribution, release and source package. An absent finding does not mean a package is unaffected.

14 package states
Repository candidate not checked

A published vendor fix does not prove that a matching update is enabled and installable on a particular asset. Confirm the local package candidate before scheduling remediation.

Distribution releaseSource packageVendor stateFixed versionEvidence
Alpine v3.23v3.23 · communityfirefoxVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.92.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.23v3.23 · communitylibrewolfVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.92.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.23v3.23 · communitythunderbirdVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.91.3.2-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.22v3.22 · communityfirefoxVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.92.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.22v3.22 · communitylibrewolfVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.92.0.1-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.22v3.22 · communitythunderbirdVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.91.3.2-r0Alpine Security Database ↗Source updated 11 Sep 2026
Alpine v3.21v3.21 · communityfirefoxVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.92.0.1-r0Alpine Security Database ↗Source updated 19 Aug 2026
Alpine v3.21v3.21 · communitylibrewolfVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.92.0.1-r0Alpine Security Database ↗Source updated 19 Aug 2026
Alpine v3.21v3.21 · communitythunderbirdVendor fix publishedAlpine records a security fix at this version. An absent entry does not mean the package is unaffected.91.3.2-r0Alpine Security Database ↗Source updated 19 Aug 2026
Debian trixietrixie · sourcethunderbirdNot affectedDebian marks this release not affected (fixed-version marker 0).Not published in this feedDebian Security Tracker ↗Source updated 5 Oct 2026
Debian bookwormbookworm · sourcethunderbirdNot affectedDebian marks this release not affected (fixed-version marker 0).Not published in this feedDebian Security Tracker ↗Source updated 5 Oct 2026
Debian forkyforky · sourcethunderbirdNot affectedDebian marks this release not affected (fixed-version marker 0).Not published in this feedDebian Security Tracker ↗Source updated 5 Oct 2026
Debian sidsid · sourcefirefoxVendor fix publishedDebian records a fixed source-package version for this release.91.0-1Debian Security Tracker ↗Source updated 5 Oct 2026
Debian sidsid · sourcethunderbirdNot affectedDebian marks this release not affected (fixed-version marker 0).Not published in this feedDebian Security Tracker ↗Source updated 5 Oct 2026
Optional official sources

National CERT insights
?CERT means Computer Emergency Response Team; CSIRT is the closely related term Computer Security Incident Response Team.

Choose official national sources for this report. Each advisory shows its original language. Your selection is remembered on this device and included in shared links.

Official European source

ENISA European Vulnerability Database

Official EUVD identifiers, advisory evidence and known-exploited context. Missing fields are not treated as evidence of low risk.

1 current
ENISA EUVD identifier

EUVD-2021-16440

No EUVD known-exploited evidence

ENISA has published the identifier mapping but no EUVD description has been stored yet.

EUVD state
Present in the current official mapping
Known exploitation
Not present in the current ENISA EUVD known-exploited dataset. This is not proof of no exploitation.
ENISA score
Not supplied in the stored EUVD record
Advisory evidence
No linked advisory details stored yet
Recommended actionWithin 7 days

High technical severity; prioritise exposed affected systems while verifying vendor guidance.

Fix not verified
01

What, why and how

An issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that would lead to a potentially exploitable crash. This vulnerability affects Firefox < 91 and Thunderbird < 91.

What

An issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that would lead to a potentially exploitable crash. This vulnerability affects Firefox < 91 and Thunderbird < 91.

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may disrupt the affected service.

What

An issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that would lead to a potentially exploitable crash. This vulnerability affects Firefox < 91 and Thunderbird < 91.

Why

The current structured CVE record identifies a security weakness, but the root cause requires confirmation in the linked vendor material.

How

An attacker operating through a network path may attempt exploitation without authentication after a user interaction. If successful, the issue may disrupt the affected service.

02

Exploit reality and attack path

CVSS severity, EPSS forecast probability, public exploit material and CISA-confirmed exploitation are separate signals.

Observed exploitation
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
No confirmed evidence

No CISA KEV match was present at the last successful refresh. This means no confirmation from that source, not proof of no exploitation.

Public PoC / exploit material
?Confirmed exploitation and public exploit material are separate signals. Attacks can occur without public proof-of-concept or exploit code.
None recorded

No exploit-tagged reference or CISA SSVC proof-of-concept state is currently recorded. Research may still exist outside the structured feeds.

Likely attack path
a network path → vulnerable operation → disrupt the affected service
Attack surface
Network
Privileges required
None: unauthenticated exploitation is possible
User interaction
Required interaction required
Attack complexity
Low: no specialised conditions are recorded
Security boundary
Unchanged: impact remains within the vulnerable component's security authority
Weakness
?CWE means Common Weakness Enumeration.
CWE not yet assigned
CVSS vector
?CVSS means Common Vulnerability Scoring System. The vector records the metric values used to calculate technical severity.
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Common Vulnerability Scoring System 3.1: the compact vector below is decoded into plain language.

AVNetworkAttack vector: The vulnerable component can be reached over a network.ACLowAttack complexity: No specialised conditions are required beyond attacker-controlled input.PRNonePrivileges required: The attacker does not need an account or existing privileges.UIRequiredUser interaction: Another user must perform an action for exploitation to succeed.SUnchangedScope: The security impact remains within the vulnerable component's authority.CHighConfidentiality impact: A successful attack can cause a major loss.IHighIntegrity impact: A successful attack can cause a major loss.AHighAvailability impact: A successful attack can cause a major loss.
Post-exploitation / living off the land
No specific living-off-the-land technique is confirmed in the structured sources. Monitor normal administration tools for activity inconsistent with the affected service's baseline.
NetworkUnauthenticatedDenial of service
A

Official authority intelligence

Only matched European and national findings are included. Language selectors and unavailable sources are omitted.

Cyber Security Agency of Singapore · English · CSA-SB-20210818Security Bulletin 18 Aug 2021

The Cyber Security Agency of Singapore included this CVE in its official Security Bulletin 18 Aug 2021, published on 18 August 2021. Open the linked bulletin for the product, severity and reference information published in that issue.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-623Multiples vulnérabilités dans Mozilla Thunderbird

De multiples vulnérabilités ont été découvertes dans Mozilla Thunderbird. Elles permettent à un attaquant de provoquer une exécution de code arbitraire à distance, un contournement de la politique de sécurité et une atteinte à la confidentialité des données.

Official advisory ↗
CERT-FR · French · CERTFR-2021-AVI-614Multiples vulnérabilités dans les produits Mozilla

De multiples vulnérabilités ont été découvertes dans les produits Mozilla. Elles permettent à un attaquant de provoquer une exécution de code arbitraire à distance, un contournement de la politique de sécurité et une atteinte à la confidentialité des données.

Official advisory ↗
JVN iPedia · Japanese · JVNDB-2021-010868Mozilla Firefox および Thunderbird における脆弱性

Mozilla Firefox および Thunderbird には、不特定の脆弱性が存在します。

Official advisory ↗
03

Patch and workaround

Operational remediation based on structured source evidence.

Status
?Patch availability is based on structured fixed-version fields and authoritative update references. If no fix is verified, check the vendor advisory before making a change.
Fix not verified
Affected
  • Firefox: unspecified < 91
  • Thunderbird: unspecified < 91
Fixed
No fixed version is explicitly recorded in the structured CVE data.
Action
No verified patch reference is present in the current structured sources. Check the vendor advisory before making a change.
Workaround
No verified workaround is recorded. If business-safe, reduce exposure to the affected interface and allow only trusted sources until authoritative guidance is available.
04

Evidence and provenance

Published 17 Aug 2021 · Last source change 3 Aug 2024, 22:24 UTC · CWE not yet assigned

CVE recordCVE.org · 5.1
CVSS sourceNIST NVD
EPSS source
?The date BlackTree first stored a score for this CVE from the daily FIRST EPSS feed.
FIRST · tracked since 2026-08-14
European sourceENISA EUVD · EUVD-2021-16440
Product sourceCNA
Remediation sourceCVE/CNA references
CWE sourceUnavailable
NVD statusNVD modified after enrichment

Missing structured fields: CWE classification. Missing data is not evidence of low risk; review the primary advisory.

Material change intelligence

What changed after publication

View recent updates ↗

No material field changes have been recorded since change tracking began. Routine source refreshes and cosmetic edits are intentionally excluded.

Material fields only · duplicate refreshes suppressed · history retained for the configured operational retention period
CVE published
ENISA EUVD · EUVD-2021-16440↗CVE authority or vendor reference↗Vendor advisory↗Vendor advisory↗Official CVE record (CNA and ADP data)↗NVD record↗
Technical terms and abbreviations used in this report
CVE
Common Vulnerabilities and Exposures: the public identifier for one disclosed vulnerability.
CVSS
Common Vulnerability Scoring System: a technical severity framework; it is not patching priority by itself.
EPSS
Exploit Prediction Scoring System: FIRST's estimate of the probability that exploitation activity will be observed in the next 30 days; it is a forecast, not confirmation.
CWE
Common Weakness Enumeration: the standard category describing the underlying software or hardware weakness.
CNA
CVE Numbering Authority: an organisation authorised to assign and publish CVE records.
CISA ADP
Cybersecurity and Infrastructure Security Agency Authorized Data Publisher: structured enrichment added to a CVE record.
NVD
National Vulnerability Database: NIST's enrichment service for CVE records.
CERT / CSIRT
A computer security incident response team that publishes warnings or coordinates incident response.
PoC
Proof of concept: public material that demonstrates or helps reproduce exploitation.
CSAF
Common Security Advisory Framework: a machine-readable format for security advisories.
LoTL
Living off the land: abuse of legitimate tools or system functions during an attack.
Free version - for non-commercial use only.CVE-2021-29981 · cve.blacktree.nl