CVE-2026-4701Network attack vector

Memory corruption via JavaScript engine use-after-free

Published Mar 24, 2026 · Updated Mar 24, 2026

JavaScript engine use-after-free in Firefox and Thunderbird before 149, or their ESR editions before 140.9, allows remote attackers to corrupt memory. Mozilla has not disclosed the freed object, triggering JavaScript operation, or stale-pointer dereference because the referenced security bug remains restricted. Firefox exposure is browser content; Thunderbird disables scripting while reading mail, limiting reachability to browser-like contexts, while successful corruption can compromise data or interrupt the client.

CVSS severity9.8
Critical
EPSS probability0.46%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

JavaScript engine use-after-free in Firefox and Thunderbird before 149, or their ESR editions before 140.9, allows remote attackers to corrupt memory. Mozilla has not disclosed the freed object, triggering JavaScript operation, or stale-pointer dereference because the referenced security bug remains restricted. Firefox exposure is browser content; Thunderbird disables scripting while reading mail, limiting reachability to browser-like contexts, while successful corruption can compromise data or interrupt the client.

The record

CVE
CVE-2026-4701
Published
Mar 24, 2026
Updated
Mar 24, 2026
Vendor
Mozilla
Product
Firefox
Classifications
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 <140.9

Published CVSS scores

9.8NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/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

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 exploitUnconfirmed

Weakness, pattern, technique

CWE-416Use After Free
T1203Exploitation for Client Execution

Public exploit references

No public exploit references are available in this record.

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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