CVE-2026-4699Public exploitNetwork attack vector

Content-process crash via crafted web font

Published Mar 24, 2026 · Updated Mar 24, 2026

Heap out-of-bounds write in Firefox before 149, Firefox ESR before 115.34 or 140.9, and Thunderbird before 149 or 140.9 allows attackers to crash. HarfBuzz's apply_stch() lets a 32-bit glyph-count accumulator wrap while shaping a crafted OpenType font, under-allocating the glyph buffer before the CUT pass writes past its end. Loading attacker-controlled content that renders the font and Arabic text is sufficient; a crash is demonstrated, while reliable code execution has not been shown.

CVSS severity7.5
High
EPSS probability0.69%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Heap out-of-bounds write in Firefox before 149, Firefox ESR before 115.34 or 140.9, and Thunderbird before 149 or 140.9 allows attackers to crash. HarfBuzz's apply_stch() lets a 32-bit glyph-count accumulator wrap while shaping a crafted OpenType font, under-allocating the glyph buffer before the CUT pass writes past its end. Loading attacker-controlled content that renders the font and Arabic text is sufficient; a crash is demonstrated, while reliable code execution has not been shown.

The record

CVE
CVE-2026-4699
Published
Mar 24, 2026
Updated
Mar 24, 2026
Vendor
Mozilla
Product
Firefox
Classifications
CWE-754, CWE-787, 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

7.5Red Hat SADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
7.5CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/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-754Improper Check for Unusual or Exceptional Conditions
CWE-787Out-of-bounds Write
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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