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Mozilla Firefox CVE-2026-100776

| EUVDEUVD-2026-88928 HIGH
Use After Free (CWE-416)
2026-09-29 mozilla GHSA-qvvj-7hwq-q5h2
Information Disclosure Use After Free Memory Corruption Red Hat Mozilla Firefox
8.8
CVSS 3.1 · Vendor: mozilla
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Attack ChainAIDerived

Hypothetical attack flow derived from CVE metadata – not a confirmed exploit chain

  1. Access
    Lure victim to malicious page
  2. Delivery
    Serve crafted WebAssembly module
  3. Exploit
    Trigger use-after-free in WASM engine
  4. Execution
    Reclaim freed object and corrupt memory
  5. Impact
    Execute attacker code in content process

Vulnerability Assessment

Weakness
CWE
CWE-416
ASVS 5.0
V15 - Secure Coding and Architecture Memory lifecycle management
MITRE ATT&CK

2 techniques across 2 tactics: Execution, Privilege Escalation

The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up.

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Exploitation Conditions

Requires the victim to visit or render an attacker-controlled web page containing crafted WebAssembly in Firefox (UI:R). No authentication or privileges are needed (PR:N), and it works against default configurations since WebAssembly is enabled by default. Exploitation is LIMITED by the need for user interaction and by browser process sandboxing; only versions prior to Firefox 157, ESR 153.4, and ESR 140.17 are affected. No special non-default configuration is required beyond running an unpatched build.
Impact

CVSS 3.1 rates this 8.8 (High) with vector AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H - network-reachable, low-complexity, no privileges, but requiring user interaction (victim must visit a page). The C:H/I:H/A:H triad reflects that a use-after-free in the WebAssembly engine is a memory-corruption primitive typically exploitable toward arbitrary code execution in the content process, not merely an information leak. No EPSS score, no CISA KEV listing, and no public PoC were provided, so there is no evidence of active exploitation or weaponization - this is a Mozilla self-reported/internally-found class of bug fixed proactively. Signals are internally consistent (network vector matches a browser-delivered WASM payload; UI:R correctly captures the 'user browses to a page' requirement). Real-world priority is MODERATE: extremely broad attack surface (Firefox is ubiquitous) offset by the need for user interaction, sandbox mitigation, and absence of any exploitation signal. Patch quickly given browser ubiquity, but no emergency indicator (KEV/PoC/EPSS) is present.
Exploit ScenarioDerived

An attacker hosts or injects a malicious web page containing crafted WebAssembly that triggers a use-after-free in Firefox's JS/WebAssembly engine. A victim running an unpatched Firefox visits the page, the freed object is reclaimed and manipulated to corrupt memory, potentially yielding code execution in the content process. No public exploit code was identified at time of analysis.
This Week Schedule remediation this week. Why: HIGH severity (CVSS 7.0-8.9).
No patch listed yet 0.3% exploitation probability (EPSS)

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AnalysisAIv1 – Sep 29, 2026

A use-after-free in Mozilla Firefox's WebAssembly component allows remote attackers to potentially execute arbitrary code by enticing a victim to visit a malicious web page. The vulnerability affects Firefox versions before 157, Firefox ESR before 153.4, and Firefox ESR before 140.17; no authentication or non-default configuration is required, but exploitation requires user interaction (UI:R). No public exploit code or active exploitation has been identified at time of analysis, and browser process sandboxing limits the impact.

RemediationAI

Upgrade to a patched version: Firefox 157 or later, Firefox ESR 153.4 or later, or Firefox ESR 140.17 or later. These versions contain the fix for the use-after-free in WebAssembly. The advisories do not list a workaround; however, as a temporary mitigation, users can disable WebAssembly in about:config by setting javascript.options.wasm to false, though this will break WebAssembly-dependent web applications and is not recommended for general browsing. Additionally, users should exercise caution when visiting untrusted websites, as exploitation requires user interaction. See Mozilla advisories mfsa2026-100, mfsa2026-97, and mfsa2026-99 for details.

Affects your stack

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Technical ContextAI · show details

This is a use-after-free (CWE-416) vulnerability in the WebAssembly (Wasm) implementation of Mozilla Firefox's JavaScript engine. WebAssembly is a binary instruction format that runs in a sandboxed environment within the browser, often used for performance-intensive tasks. The use-after-free occurs when the browser processes crafted WebAssembly code, leading to memory corruption that could allow an attacker to execute arbitrary code in the context of the browser. The CPE cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:* indicates that all versions of Firefox are potentially affected prior to the patched releases. The vulnerability requires user interaction (UI:R) because the victim must load a malicious page, and it is unauthenticated (PR:N). The browser's process sandboxing and the need for user interaction reduce the overall risk, but the CVSS score of 8.8 reflects high confidentiality, integrity, and availability impacts.

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Severity by source

Vendor (mozilla) PRIMARY 8.8 HIGH
AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
vuln.today AI 8.8 HIGH

Browser-delivered WASM UAF is network-reachable with no privileges but needs the victim to load the page (UI:R); memory corruption yields full C/I/A impact within the unchanged content-process scope.

3.1 AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
SUSE CRITICAL
Red Hat 7.5 HIGH

Primary rating from Vendor (mozilla).

CVSS VectorVendor: mozilla

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

Lifecycle Timeline

3
CVSS changed
Sep 29, 2026 - 15:22 NVD
8.8 (HIGH)
CVE Published
Sep 29, 2026 - 12:36 cve.org
HIGH 8.8
CVE Published
Sep 29, 2026 - 12:36 cve.org
UNKNOWN (no severity yet)

DescriptionCVE.org

Use-after-free in the JavaScript: WebAssembly component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, and Firefox ESR 140.17.

Executive ViewAI

Why It Matters

Mozilla Firefox users across the organization face a high-severity use-after-free vulnerability in the WebAssembly component, potentially enabling remote code execution if a user visits a malicious webpage. With no public exploit identified at time of analysis and no vendor patch available, leadership should prioritize interim mitigations to protect endpoint integrity and data confidentiality. The required user interaction and existing browser sandboxing reduce but do not eliminate the risk, making proactive measures essential until an official fix is released.

Business Impact: High
Compensating Controls

No vendor-released patch identified at time of analysis. Interim mitigations include disabling WebAssembly, ensuring browser sandboxing is enabled, limiting user privileges, and educating users about phishing and malicious websites.

Threat Intelligence Overview

EPSS Score
0.3%
Top 76% exploitable
Prevalence
Ubiquitous
Firefox is one of the most widely deployed web browsers worldwide
Data Quality
7 sources
Confidence: High
EUVD 🇯🇵JVN NVD CVE.org EPSS Vulnrichment Vendor VulDB

How this decision was reached

The engine reads each signal below and applies a fixed list of rules from top to bottom. The first rule that matches sets the recommended action – like a statement, every line is shown.

SignalValue
Severity HIGH
CISA KEV No
Public POC No
EPSS 0.3%
Patch available No
Active threat No
Deciding rule
HIGH severity (CVSS 7.0-8.9)
What every action level means
Emergency Immediate action required. Criteria: CRITICAL + in CISA KEV (actively exploited) without patch, OR CRITICAL + EPSS > 30% without patch, OR CRITICAL + active threat signals without patch.
Act Now Prioritize within 24-48h. Criteria: in KEV with patch available, OR CRITICAL + public POC, OR HIGH + EPSS > 10%, OR CRITICAL + active threat, OR any CRITICAL severity.
This Week Schedule remediation this week. Criteria: HIGH severity (CVSS 7.0-8.9). Patch within normal change window.
This Month Plan within 30 days. Criteria: MEDIUM severity (CVSS 4.0-6.9), lower exploitation probability. Include in regular patch cycle.
Monitor Track and reassess. Criteria: LOW severity (CVSS 0.1-3.9), minimal exploitation risk. No immediate action needed.

SSVC AssessmentENISA

SSVC Decision
Scheduled
Exploitation
None
System Exposure· derived
Open
Automatable
No
Human Impact
High
Technical Impact
Total

Priority Score

44
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +44
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

Use After Free HIGH

Use-after-free vulnerabilities occur when a program continues to access memory through a pointer after that memory has been deallocated.

Memory Corruption HIGH

Memory corruption occurs when a program writes data beyond the boundaries of allocated memory regions or accesses memory in unintended ways, violating the integrity of the process's address space.

Information Disclosure MEDIUM

Information disclosure occurs when an application unintentionally exposes sensitive data that aids attackers in reconnaissance or directly compromises security.

Affected Products (CPE)CVE.org

cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*

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Same weakness CWE-416 – Use After Free

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CVE-2017-5638 CRITICAL POC
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Vendor StatusVendor

SUSE

Severity: Critical
Product Status
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise Desktop 15 SP7 Affected
SUSE Linux Enterprise Module for Desktop Applications 15 SP7 Affected
SUSE Linux Enterprise Module for Package Hub 15 SP7 Affected
SUSE Linux Enterprise Server 15 SP7 Affected

References

Advisories & patches
ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-100776 ADVISORY https://vuldb.com/vuln/411347 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2068374 REF https://www.mozilla.org/security/advisories/mfsa2026-100/ REF https://www.mozilla.org/security/advisories/mfsa2026-101/ REF https://www.mozilla.org/security/advisories/mfsa2026-102/ REF https://www.mozilla.org/security/advisories/mfsa2026-103/ REF https://www.mozilla.org/security/advisories/mfsa2026-97/ REF https://www.mozilla.org/security/advisories/mfsa2026-99/

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