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Mozilla Firefox CVE-2026-100818

| EUVDEUVD-2026-88797 CRITICAL
Use After Free (CWE-416)
2026-09-29 mozilla GHSA-5pwm-rr4f-fh76
Information Disclosure Use After Free Memory Corruption Red Hat Mozilla Firefox
Critical
Disputed · 9.6 Vendor: mozilla
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Attack ChainAIDerived

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

  1. Access
    Host crafted web page
  2. Delivery
    Lure victim to load page (UI:R)
  3. Exploit
    Trigger UAF in Widget:Gtk
  4. Execution
    Corrupt freed object in content process
  5. Persist
    Escape sandbox to host (S:C)
  6. Impact
    Execute code with browser privileges

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 web content in Firefox (UI:R), triggering the use-after-free in the Widget:Gtk rendering component; the GTK widget code path implies GTK-based (Linux/Unix) Firefox builds are the primary target surface. No authentication or privileges are needed (PR:N), and no non-default configuration is required - only that a user visits or renders a malicious page. The principal limiting factor is the required user interaction and the difficulty of reliably grooming a UAF into a working sandbox escape (SSVC Automatable: no).
Impact

This is a genuinely high-severity, RCE-class memory-corruption flaw: a use-after-free (CWE-416) in Firefox's Widget:Gtk component that enables a sandbox escape. The CVSS 9.6 rating is driven by the scope change (S:C) - the content-process compromise crosses a trust boundary to affect the host - combined with total technical impact (C:H/I:H/A:H) confirmed by the SSVC 'Technical Impact: total' rating. However, the real-world urgency signals temper the raw score: SSVC reports Exploitation: none and Automatable: no, meaning no observed exploitation and no reliable auto-exploitation. There is no CISA KEV listing, no EPSS score provided, and no public POC identified. The CVSS vector requires UI:R (victim must load a crafted page), which is the standard delivery constraint for browser bugs. Net: this is a real priority for patching due to sandbox-escape potential and ubiquitous deployment, but it is not evidence of active or imminent widespread exploitation. It is a self-reported Mozilla bug fixed on release, consistent with the rest of this MFSA batch - high-CVSS with currently low real-world exploitation pressure.
Exploit ScenarioDerived

An attacker hosts a crafted web page that, when rendered by a victim's Firefox, triggers a use-after-free in the GTK widget code inside the content process. By grooming the freed object, the attacker corrupts memory and escapes the content sandbox to execute code with the privileges of the browser host process. No public exploit code has been identified, and SSVC rates exploitation as none/not-automatable.
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

A use-after-free in Mozilla Firefox's Widget:Gtk component enables a sandbox escape: an unauthenticated remote attacker who persuades a user to load attacker-controlled web content can corrupt freed heap memory in the GTK widget rendering path and cross the browser's security boundary (CVSS 9.6, AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H), turning a rendering bug into code-execution-class impact outside the sandbox. GTK-based (Linux/Unix) Firefox builds are the primary target surface, and the flaw affects Firefox releases prior to 157 as well as the ESR branches prior to 153.4 and 140.17, where Mozilla has shipped the fix. No public exploit code has been identified at time of analysis, and CISA's SSVC decision framework rates exploitation as 'none' with automation rated 'no', although the assessed technical impact is total if the bug is successfully triggered; the principal hurdles are the required user interaction and the difficulty of reliably grooming a UAF into a working sandbox escape.

RemediationAI

Vendor-released patch: Mozilla fixed this issue in Firefox 157, Firefox ESR 153.4, and Firefox ESR 140.17, so the primary action is to upgrade every affected install to the appropriate fixed release for its channel and to verify the running version via about:support or the browser's version dialog - paying particular attention to ESR fleets, which are often frozen for months and require an explicit channel update. Patch details and per-release notes are in the Mozilla advisories at https://www.mozilla.org/security/advisories/mfsa2026-100/, https://www.mozilla.org/security/advisories/mfsa2026-99/, and https://www.mozilla.org/security/advisories/mfsa2026-97/, and the underlying bug is tracked at https://bugzilla.mozilla.org/show_bug.cgi?id=2069399. Because the flaw is a memory-corruption sandbox escape reachable simply by rendering a malicious page, there is no meaningful configuration toggle that removes the attack surface: disabling JavaScript wholesale would break most of the web and does not reliably prevent native widget code paths from being reached; switching Linux users to a non-GTK build is generally not an available or supported option and would only shift the widget surface, not eliminate the risk. Practical compensating controls while patching is in progress are therefore containment-oriented - enforce automatic or MDM/enterprise-policy-driven updates so the fixed build propagates without user action, block known-bad or untrusted destinations at the network/DNS layer where a campaign is identifiable, and reduce the window of exposure by prioritizing the update ahead of routine change freezes; each of these lowers exposure but none eliminates the vulnerability, and each carries a trade-off (update-driven compatibility regressions, false positives from domain blocking, and operational disruption from emergency change windows). If an exploit is later observed in the wild, treat any host that rendered hostile content before patching as potentially compromised and rebuild rather than clean.

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Technical ContextAI · show details

The root cause is a use-after-free (CWE-416): memory associated with an object in the Widget:Gtk layer is released while a reference to it remains live, so subsequent dereferencing reads or writes attacker-influenced heap data. Widget:Gtk is Mozilla's platform widget implementation that binds Gecko's layout and painting abstractions to native GTK widgets on Linux and Unix - it sits on the boundary between browser content and native UI/compositing code, which is precisely why a memory-safety failure there can be leveraged to escape the content sandbox rather than merely crash a renderer. Because the CVSS vector carries S:C (scope changed) alongside C:H/I:H/A:H, the assessed impact is a boundary crossing with high confidentiality, integrity, and availability loss, consistent with a sandbox escape reachable through an unprivileged web page (PR:N, UI:R). The affected product space in the supplied CPE data is a wildcarded Mozilla Firefox entry (cpe:2.3:a:mozilla:firefox:*:*:*:*:*:*:*:*), which combined with the 'Widget: Gtk' component confirms that all GTK-backed Firefox builds in the vulnerable version ranges are in scope, while the intelligence tags - Use After Free, Memory Corruption, Information Disclosure - align with the CWE-416 classification and with a memory-disclosure-to-code-execution exploitation chain. Secondary development in the vulnerability is tracked in Mozilla Bugzilla issue 2069399, with fixes documented across the Mozilla security advisories mfsa2026-97, mfsa2026-99, and mfsa2026-100, which correspond to the separate release trains patched by Mozilla (mainline and the two ESR channels).

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Severity by source

Sources disagree (Medium–Critical)
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

Network-delivered crafted page needs victim interaction (UI:R) but no privileges; sandbox escape crosses trust boundary (S:C) with total impact, matching vendor 9.6.

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 6.1 MEDIUM

vuln.today treats the vendor’s rating as authoritative. A higher third-party CVSS (e.g. CISA-ADP) is shown for transparency but does not drive the headline severity.

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

2
CVE Published
Sep 29, 2026 - 12:36 cve.org
CRITICAL 9.6
CVE Published
Sep 29, 2026 - 12:36 cve.org
UNKNOWN (no severity yet)

DescriptionCVE.org

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

Executive ViewAI

Why It Matters

Mozilla Firefox on Linux and Unix systems is exposed to a critical sandbox escape vulnerability (CVE-2026-100818) that could let an unauthenticated remote attacker execute code outside the browser's security boundary if a user loads malicious web content. No public exploit code has been identified at time of analysis, but the potential for full endpoint compromise, data theft, or lateral movement makes this a high priority for organizations with Linux/Unix users. Vendor-released patch: Firefox 157 and ESR 153.4/140.17.

Business Impact: Severe
Compensating Controls

Apply vendor patch.

Threat Intelligence Overview

EPSS Score
0.3%
Top 82% exploitable
Prevalence
Ubiquitous
Firefox is one of the most widely deployed desktop browsers worldwide
Social Media
1
Sightings
Data Quality
7 sources
Confidence: High
EUVD 🇯🇵JVN NVD CVE.org EPSS Vulnrichment Vendor CIRCL 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-2026-100818 ADVISORY https://vuldb.com/vuln/411390 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2069399 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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