CVE-2024-3855Public exploitNetwork attack vector

Browser crash via MSubstr loop-hoisting misoptimization

Published Apr 16, 2024 · Updated Nov 12, 2024

Out-of-bounds read in Firefox before 125 allows remote attackers to crash the browser through JIT-compiled JavaScript in a webpage. The JIT hoists MSubstr minimum, maximum, and SubstringKernel operations ahead of a negative-index guard, passing -1 and an oversized span to SubstringKernel. Triggering requires user interaction with attacker-controlled web content containing a loop and negative slice index, and the demonstrated consequence is a wild-pointer crash.

CVSS severity6.5
Medium
EPSS probability0.40%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds read in Firefox before 125 allows remote attackers to crash the browser through JIT-compiled JavaScript in a webpage. The JIT hoists MSubstr minimum, maximum, and SubstringKernel operations ahead of a negative-index guard, passing -1 and an oversized span to SubstringKernel. Triggering requires user interaction with attacker-controlled web content containing a loop and negative slice index, and the demonstrated consequence is a wild-pointer crash.

The record

CVE
CVE-2024-3855
Published
Apr 16, 2024
Updated
Nov 12, 2024
Vendor
Mozilla
Product
Firefox
Classifications
CWE-125, T1203
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Apr 16, 2024CVE publishedPublication date reported by the CVE source.
  2. Nov 12, 2024Record 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 <125

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Mozilla · Firefoxversion=unspecified <125

Published CVSS scores

6.5CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:R/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.

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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-125Out-of-bounds Read
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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