CVE-2026-5731Network attack vector

Code execution via memory corruption

Published Apr 7, 2026 · Updated Apr 7, 2026

Memory safety flaws in affected Firefox and Thunderbird releases allow remote attackers to execute code through browser content. Mozilla groups multiple memory-safety bugs under this CVE and discloses only that some show memory corruption; individual loci and unsafe operations remain undisclosed. Reachability requires browser or browser-like rendering of attacker-controlled content; ordinary Thunderbird email reading disables scripting, reducing that path.

CVSS severity9.8
Critical
EPSS probability0.34%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Memory safety flaws in affected Firefox and Thunderbird releases allow remote attackers to execute code through browser content. Mozilla groups multiple memory-safety bugs under this CVE and discloses only that some show memory corruption; individual loci and unsafe operations remain undisclosed. Reachability requires browser or browser-like rendering of attacker-controlled content; ordinary Thunderbird email reading disables scripting, reducing that path.

The record

CVE
CVE-2026-5731
Published
Apr 7, 2026
Updated
Apr 7, 2026
Vendor
Mozilla
Product
Firefox
Classifications
CWE-119, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 7, 2026CVE publishedPublication date reported by the CVE source.
  2. Apr 7, 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 <149.0.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <149.0.2
Mozilla · Thunderbirdversion=unspecified <140.9.1; version=unspecified <149.0.2
Mozilla · Firefox ESRversion=unspecified <115.34.1; version=unspecified <140.9.1

Published CVSS scores

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-119Improper Restriction of Operations within the Bounds of a Memory Buffer
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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