CVE-2026-4691Public exploitNetwork attack vector

Memory corruption via desynchronized stylesheet adopter tracking

Published Mar 24, 2026 · Updated Mar 24, 2026

Use-after-free in Mozilla Firefox before 149 and Thunderbird before 149 allows remote attackers to corrupt content-process memory via crafted JavaScript. The ObservableArray backing adoptedStyleSheets honors Array prototype setters, desynchronizing it from StyleSheet::mAdopters and leaving a dangling ShadowRoot pointer that later stylesheet mutations dereference. Firefox ESR before 115.34 or 140.9 and Thunderbird before 140.9 are also affected; no user interaction is required in browser contexts, while Thunderbird mail reading disables scripting.

CVSS severity9.8
Critical
EPSS probability0.48%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Use-after-free in Mozilla Firefox before 149 and Thunderbird before 149 allows remote attackers to corrupt content-process memory via crafted JavaScript. The ObservableArray backing adoptedStyleSheets honors Array prototype setters, desynchronizing it from StyleSheet::mAdopters and leaving a dangling ShadowRoot pointer that later stylesheet mutations dereference. Firefox ESR before 115.34 or 140.9 and Thunderbird before 140.9 are also affected; no user interaction is required in browser contexts, while Thunderbird mail reading disables scripting.

The record

CVE
CVE-2026-4691
Published
Mar 24, 2026
Updated
Mar 24, 2026
Vendor
Mozilla
Product
Firefox
Classifications
CWE-825, CWE-416, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Mar 24, 2026CVE publishedPublication date reported by the CVE source.
  2. Mar 24, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=unspecified <149

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <149
Mozilla · Thunderbirdversion=unspecified <140.9; version=unspecified <149
Mozilla · Firefox ESRversion=unspecified <115.34; version=unspecified <140.9

Published CVSS scores

7.5redhat-SADPCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:H
9.8CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-825Expired Pointer Dereference
CWE-416Use After Free
T1203Exploitation for Client Execution

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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