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Mozilla Firefox CVE-2026-100786

| EUVDEUVD-2026-88938 CRITICAL
Use After Free (CWE-416)
2026-09-29 mozilla GHSA-vmj2-hchx-7q7v
Information Disclosure Use After Free Memory Corruption Red Hat Mozilla Firefox
9.6
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
    Host crafted graphics web page
  2. Delivery
    Lure victim to visit in Firefox
  3. Exploit
    Trigger use-after-free in Graphics component
  4. Execution
    Groom heap to reuse freed object
  5. Persist
    Escape content sandbox to host process
  6. Impact
    Execute arbitrary code

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

Exploitation requires the victim to load attacker-controlled content that exercises Firefox's Graphics rendering path in an unpatched build (any version prior to Firefox 157 / ESR 153.4 / ESR 140.17 / ESR 115.42). The limiting factor is user interaction (UI:R): the victim must visit or render a malicious page - there is no server-side or zero-click vector. No authentication is required (PR:N), and default browser configurations are affected since graphics rendering is enabled by default. Successful sandbox escape additionally depends on reliable heap grooming of the use-after-free, which is non-trivial and not demonstrated by any public POC in the available data.
Impact

This is a high-priority, RCE-class memory-corruption flaw. The vendor CVSS3.1 vector (AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H = 9.6) is credible for a use-after-free in Firefox's Graphics component that achieves sandbox escape: the scope change (S:C) is justified because exploitation breaks out of the content sandbox to affect the more-privileged host process, which lifts this above the 8.8 non-escape UAF siblings in the same advisory batch (e.g. the DOM/Canvas2D/WASM UAFs). AV:N/AC:L/PR:N/UI:R correctly models a browser drive-by: the victim need only render attacker-controlled graphics content. Against that, exploitation signals are weak - no EPSS score, no CISA KEV listing, and no public POC are present in the input, and Mozilla self-reported this (typical of internally/fuzz-discovered bugs). So while the technical severity is high, there is no evidence of active exploitation; treat it as 'patch promptly' rather than 'emergency because attacked.' The listed tags include 'Information Disclosure,' which understates the issue - a sandbox-escaping UAF is full memory corruption (C/I/A all High), not info-disclosure only.
Exploit ScenarioDerived

An attacker hosts a malicious web page containing crafted graphics content (or injects it via a compromised ad/iframe). When a victim using an unpatched Firefox/ESR build visits and renders the page, the Graphics component frees an object still in use, and the attacker grooms the heap to reuse it - corrupting memory in a way that escapes the content sandbox and executes code in the host process. No public exploit code is identified at time of analysis, and user interaction (visiting the page) is required.
Act Now Prioritize within 24-48h. Why: CRITICAL severity.
No patch listed yet 0.3% exploitation probability (EPSS)

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AnalysisAIv1 – Sep 29, 2026

Use-after-free memory corruption in Mozilla Firefox's Graphics component can be exploited by a remote, unauthenticated attacker to escape the browser's content sandbox and run code at the privilege of the parent/broker process, provided the victim renders attacker-controlled content. The only prerequisite is user interaction - visiting or rendering a crafted page in any unpatched build (Firefox before 157, ESR before 153.4, ESR 140.17, ESR 115.42) - since graphics rendering is enabled by default and no credentials are needed (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H, 9.6). No public exploit code or in-the-wild exploitation has been identified at time of analysis, and the independent assessment notes that reliable heap grooming of the use-after-free is non-trivial, making this a high-priority RCE-class flaw rather than a demonstrated weaponized one.

RemediationAI

Vendor-released patch: upgrade to Mozilla Firefox 157 on the rapid-release channel, or to Firefox ESR 153.4, Firefox ESR 140.17, or Firefox ESR 115.42 on the corresponding extended-support branches, as documented in Mozilla advisories mfsa2026-100, mfsa2026-97, mfsa2026-98 and mfsa2026-99 (engineering details in Bugzilla 2071067). Because the vulnerability is in default-enabled graphics rendering, there is no configuration switch that removes the attack surface without breaking page rendering; partial compensating controls include restricting browsing to trusted sites, enforcing enterprise policy to keep all Firefox installs at the fixed ESR builds, and disabling or blocking the GPU/compositing acceleration path via preferences only as a stopgap since it can degrade rendering performance and stability and does not eliminate the underlying use-after-free. Note that this is a sandbox escape, so browser-process hardening (site isolation, content-process sandboxing) reduces but does not neutralize the impact of an unpatched build; patching is the only complete fix.

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Technical ContextAI · show details

The flaw is a use-after-free (CWE-416) in the Graphics rendering subsystem of Mozilla's Gecko engine, the code path responsible for compositing, rasterization and GPU/software rendering of page content. A stale pointer to a freed graphics object is reused during rendering, which can be turned into controlled memory corruption of heap data. Because the corrupted object lives in the web-content process, the bug is a sandbox-escape primitive: successful exploitation by a renderer-compromised process would cross the browser's process isolation boundary into the more privileged parent process, which is why the assessed vector carries S:C (scope change) alongside C:H/I:H/A:H. The advisory references (mfsa2026-100, mfsa2026-97, mfsa2026-98, mfsa2026-99, Bugzilla 2071067) cover the fixes across the release and ESR channels; CPE data (cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*) confirms all pre-fix Firefox builds are in scope rather than a single version.

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Severity by source

Vendor (mozilla) PRIMARY 9.6 CRITICAL
AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H
vuln.today AI 9.6 CRITICAL

Remote crafted web content (AV:N) needs only a page visit (UI:R, PR:N); the UAF sandbox escape crosses the content/host boundary (S:C) yielding full memory-corruption C/I/A:H.

3.1 AV:N/AC:L/PR:N/UI:R/S:C/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:H/SI:H/SA:H
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
Changed
Confidentiality
High
Integrity
High
Availability
High

Lifecycle Timeline

3
CVSS changed
Sep 29, 2026 - 17:22 NVD
9.6 (CRITICAL)
CVE Published
Sep 29, 2026 - 12:36 cve.org
UNKNOWN (no severity yet)
CVE Published
Sep 29, 2026 - 12:36 cve.org
CRITICAL 9.6

DescriptionCVE.org

Sandbox escape due to use-after-free in the Graphics component. This vulnerability was fixed in Firefox ESR 153.4, Firefox 157, Firefox ESR 115.42, and Firefox ESR 140.17.

Executive ViewAI

Why It Matters

Organizations using Mozilla Firefox are exposed to a critical use-after-free vulnerability (CVE-2026-100786) that allows an unauthenticated remote attacker to escape the browser sandbox and execute code at the parent process privilege when a user visits a malicious page. No public exploit identified at time of analysis, and no vendor-released patch identified at time of analysis, but the flaw requires only user interaction and affects all Firefox versions before 157, as well as ESR versions before 153.4, 140.17, and 115.42. Leadership should prioritize immediate mitigations to prevent potential full endpoint compromise and data loss.

Business Impact: Severe
Compensating Controls

No vendor-released patch identified at time of analysis. Interim mitigations: enforce browser isolation for all web browsing, disable JavaScript or use script-blocking extensions, restrict users to a curated list of trusted websites, and consider switching to an alternative browser until a fix is released.

Threat Intelligence Overview

EPSS Score
0.3%
Top 75% exploitable
Prevalence
Ubiquitous
Firefox is one of the most widely deployed desktop browsers globally
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 CRITICAL
CISA KEV No
Public POC No
EPSS 0.3%
Patch available No
Active threat No
Deciding rule
CRITICAL severity
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
Very High
Technical Impact
Total

Priority Score

48
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +48
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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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-2026-100786 ADVISORY https://vuldb.com/vuln/411355 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2071067 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/mfsa2026-98/ REF https://www.mozilla.org/security/advisories/mfsa2026-99/

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