Security readout for executives and security teams
Plain-English summary
CVE-2026-4701 is a critical use-after-free flaw in Mozilla’s JavaScript Engine. If a vulnerable Firefox or Thunderbird build processes attacker-controlled JavaScript content, the impact could include data theft, tampering, or crash-level disruption. Mozilla states it is fixed in Firefox 149, Firefox ESR 140.9, Thunderbird 149, and Thunderbird 140.9.
Executive priority
Prioritize remediation immediately for Mozilla Firefox and Thunderbird deployments. The severity and memory-safety class justify urgent patching, even though the supplied evidence does not confirm active exploitation.
Technical view
The issue is classified as CWE-416, use-after-free, in the JavaScript Engine component. The supplied CVSS 3.1 score is 9.8 with network attack vector, low complexity, no privileges, no user interaction, unchanged scope, and high confidentiality, integrity, and availability impact.
Likely exposure
Exposure is likely where Firefox or Thunderbird installations are not confirmed at the fixed Mozilla versions: Firefox 149, Firefox ESR 140.9, Thunderbird 149, or Thunderbird 140.9. The source bundle does not provide CPEs, vulnerable version ranges, or deployment-specific conditions.
Exploitation context
The supplied sources do not show CISA KEV listing or cited evidence of active exploitation. Treat exploitation status as unconfirmed, not absent. The CVSS vector indicates a remotely reachable, low-complexity issue, which makes patch latency the main risk driver.
Researcher notes
Evidence identifies the bug class, component, fixed releases, CVSS vector, and Mozilla references. The bundle does not include exploit details, affected version ranges, CPEs, or proof of in-the-wild exploitation. Avoid assuming products beyond Firefox and Thunderbird.
Mitigation direction
- Update Firefox to version 149 or later per Mozilla guidance.
- Update Firefox ESR to version 140.9 or later.
- Update Thunderbird to version 149 or Thunderbird 140.9 or later.
- Prioritize managed endpoints, VDI images, and packaged browser/email builds.
- Check Mozilla advisories for any product-specific follow-up guidance.
Validation and detection
- Inventory Firefox and Thunderbird versions across endpoints and images.
- Confirm deployed versions meet or exceed Mozilla’s fixed releases.
- Check software management reports for failed or deferred updates.
- Review exception groups where browser or Thunderbird updates are paused.
- Monitor vendor 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.
CWE-416: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2026-4701 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
- Critical
- CVSS
- 9.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
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.
SSVC decision data
CVSS vector scores
1 official scoreWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9CISA-ADPVulnerability scoring details
Base CVSS 3.1 score
9.8CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- CVE publishedCVE Program
The CVE record was published.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- https://bugzilla.mozilla.org/show_bug.cgi?id=2009303CVE reference, mozilla
- https://www.mozilla.org/security/advisories/mfsa2026-20/CVE reference, mozilla
- https://www.mozilla.org/security/advisories/mfsa2026-22/CVE reference, mozilla
- https://www.mozilla.org/security/advisories/mfsa2026-23/CVE reference, mozilla
- https://www.mozilla.org/security/advisories/mfsa2026-24/CVE reference, mozilla
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.
Use After Free
Use After Free represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.