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Mozilla Firefox EUVDEUVD-2026-69064

| CVE-2026-84119 CRITICAL
Use After Free (CWE-416)
2026-09-01 mozilla GHSA-jm34-3r5q-pm6j
Information Disclosure Use After Free Memory Corruption Red Hat Mozilla Suse
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
    Lure victim to malicious page
  2. Delivery
    Serve crafted DOM navigation content
  3. Exploit
    Trigger use-after-free in Navigation
  4. Execution
    Reclaim freed object memory
  5. Persist
    Escape content sandbox
  6. Impact
    Execute code on host

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 actively load attacker-controlled web content in Firefox (CVSS UI:R) that triggers the specific navigation sequence freeing and reusing a DOM Navigation object; no authentication or elevated privileges are needed (PR:N). Limiting factors: the bug is not automatable per CISA SSVC and, as a sandbox escape, realistically must be chained to fully compromise the host, so a single memory-corruption primitive alone may not achieve reliable code execution. Only Firefox and its ESR builds below the fixed versions (155 / ESR 115.40 / 140.15 / 153.2) are vulnerable; auto-updated installations at or above those versions are not.
Impact

The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H = 9.6 critical) signals a remotely reachable, low-complexity, unauthenticated bug whose only gate is user interaction (visiting a page) and whose sandbox escape yields total host impact - consistent with SSVC Technical Impact: total. However, SSVC also reports Exploitation: none and Automatable: no, and there is no KEV listing, no POC, and no EPSS score supplied, so real-world exploitation is currently unproven despite the high severity. Use-after-free sandbox escapes are historically hard to weaponize reliably (often chained with a separate content-process bug), which aligns with Automatable: no. This is a genuine high-priority patch item for any Firefox fleet given the critical score and browser ubiquity, but the absence of active exploitation suggests urgency driven by exposure and patch availability rather than confirmed in-the-wild attacks.
Exploit ScenarioDerived

An attacker hosts a malicious web page (or injects malicious content into a site/ad the victim loads) that drives the DOM Navigation component into freeing and then reusing an object. When the victim's browser processes the crafted navigation sequence, the attacker controls the freed memory, corrupting state to escape the content sandbox and run code in the victim's host context. No POC is currently public, and success requires the user to visit the page (UI:R).
Act Now Prioritize within 24-48h. Why: CRITICAL severity.
Patch available 0.3% exploitation probability (EPSS)

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AnalysisAIv1 – Sep 01, 2026

Sandbox escape in Mozilla Firefox stems from a use-after-free in the DOM: Navigation component, letting a malicious web page that a user visits corrupt memory and potentially execute code outside the content sandbox. It affects Firefox before 155 and multiple ESR trains (115.x, 140.x, 153.x), carries a critical 9.6 CVSS with a scope change (browser sandbox to host), and requires the victim to load attacker-controlled content (UI:R). No public exploit identified at time of analysis and CISA SSVC lists exploitation as none.

RemediationAI

Vendor-released patch: upgrade to Firefox 155 or, for extended-support deployments, Firefox ESR 115.40, ESR 140.15, or ESR 153.2 as applicable to your channel. Because most environments run auto-update, verify that updates are actually applied across managed endpoints and re-enable/unblock Mozilla update channels if they were pinned. As an interim compensating control where patching lags, restrict browsing to trusted sites, deploy the update via enterprise policy (ADMX/policies.json) to force the fixed build, and consider network-level filtering of untrusted content; note these controls reduce but do not eliminate exposure since exploitation only requires visiting a crafted page. See the Mozilla advisories (https://www.mozilla.org/security/advisories/mfsa2026-82/ and the -83/-84/-85 entries) for the exact build mapping.

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Technical ContextAI · show details

The flaw is a CWE-416 use-after-free in Firefox's DOM Navigation subsystem, which manages page/session navigation and the associated document lifecycle. A dangling pointer to a freed navigation-related object can be reused, allowing an attacker to control freed heap memory and, because the scope changes (CVSS S:C), pivot out of the content-process sandbox that normally isolates web content from the host. Per the CPE (cpe:2.3:a:mozilla:firefox), only Mozilla Firefox and its ESR builds are affected; the tags (Memory Corruption, Use After Free, Information Disclosure) reflect both the code-execution and potential data-leak consequences of the freed-object reuse.

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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 8.3 HIGH

Network-reachable via a visited page (UI:R, PR:N) with sandbox scope change and full host impact; AC:H reflects heap-grooming needed to reliably exploit a UAF sandbox escape.

3.1 AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H
4.0 AV:N/AC:H/AT:N/PR:N/UI:P/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H
SUSE 8.3 HIGH
AV:N/AC:H/PR:N/UI:R/S:C/C:H/I:H/A:H
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 01, 2026 - 15:22 NVD
9.6 (CRITICAL)
CVE Published
Sep 01, 2026 - 12:18 cve.org
UNKNOWN (no severity yet)
CVE Published
Sep 01, 2026 - 12:18 cve.org
CRITICAL 9.6

DescriptionCVE.org

Sandbox escape due to use-after-free in the DOM: Navigation component. This vulnerability was fixed in Firefox 155, Firefox ESR 115.40, Firefox ESR 140.15, and Firefox ESR 153.2.

Articles & Coverage 2

News 7 New Mozilla Vulnerabilities - 2 Critical, 5 High Severity Sep 02, 2026 News Critical Sandbox Escape via Use-After-Free in Mozilla Firefox - CVE-2026-84119 Sep 01, 2026

Executive ViewAI

Why It Matters

Mozilla Firefox before version 155 (including ESR 115.x, 140.x, and 153.x) contains a critical sandbox escape vulnerability (CVE-2026-84119) that allows attackers to execute arbitrary code at the host level when users visit malicious websites. With no vendor patch currently available, all Firefox users in the organization are at risk of complete system compromise through normal web browsing activity.

Business Impact: Severe
Compensating Controls

Restrict Firefox to allowlisted internal sites only; recommend users employ Chrome, Edge, or Safari for general web browsing; deploy OS-level sandboxing (Windows Sandbox, containerization) if Firefox use is unavoidable; enable endpoint detection and response (EDR) monitoring for suspicious child process execution from browser processes; disable JavaScript in Firefox if business requirements permit; implement strict network content filtering and website allowlists; conduct security awareness training emphasizing avoidance of suspicious links and untrusted websites.

Threat Intelligence Overview

EPSS Score
0.3%
Top 73% exploitable
Prevalence
Ubiquitous
one of the most widely deployed desktop web browsers
Data Quality
5 sources
Confidence: Medium
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 Yes
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: Important
Product Status
openSUSE Tumbleweed Fixed
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

References

Advisories & patches
ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-84119 ADVISORY https://vuldb.com/vuln/397599 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2057817 REF https://www.mozilla.org/security/advisories/mfsa2026-82/ REF https://www.mozilla.org/security/advisories/mfsa2026-83/ REF https://www.mozilla.org/security/advisories/mfsa2026-84/ REF https://www.mozilla.org/security/advisories/mfsa2026-85/ REF https://www.mozilla.org/security/advisories/mfsa2026-86/ REF https://www.mozilla.org/security/advisories/mfsa2026-87/ REF https://www.mozilla.org/security/advisories/mfsa2026-88/

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