CVE-2026-8959Network attack vector

Sandbox escape via undisclosed Win32 boundary error

Published May 19, 2026 · Updated May 19, 2026

Incorrect boundary conditions in Mozilla Firefox and Thunderbird before 151 allow remote attackers to escape the Windows content sandbox. Mozilla has not disclosed which Widget: Win32 routine mischecks which attacker-controlled value or whether the error reads or writes beyond a boundary. Exploitation requires user interaction with attacker-controlled content; ESR releases before 140.11 are affected, while Thunderbird mail reading disables scripting.

CVSS severity9.6
Critical
EPSS probability0.42%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Incorrect boundary conditions in Mozilla Firefox and Thunderbird before 151 allow remote attackers to escape the Windows content sandbox. Mozilla has not disclosed which Widget: Win32 routine mischecks which attacker-controlled value or whether the error reads or writes beyond a boundary. Exploitation requires user interaction with attacker-controlled content; ESR releases before 140.11 are affected, while Thunderbird mail reading disables scripting.

The record

CVE
CVE-2026-8959
Published
May 19, 2026
Updated
May 19, 2026
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-693, CWE-119, CWE-20, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 19, 2026CVE publishedPublication date reported by the CVE source.
  2. May 19, 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

9.6CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-693Protection Mechanism Failure
CWE-119Improper Restriction of Operations within the Bounds of a Memory Buffer
CWE-20Improper Input Validation
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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