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Firefox CVE-2026-84121

| EUVDEUVD-2026-69066 CRITICAL
Use After Free (CWE-416)
2026-09-01 mozilla GHSA-5g9x-cffv-ccj8
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
    Deliver crafted DOM/JS content
  3. Exploit
    Trigger use-after-free in DOM: Security
  4. Execution
    Control reallocated freed memory
  5. Persist
    Escape content sandbox
  6. Impact
    Execute code in higher-privilege context

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 open an attacker-controlled or attacker-injected web page in a vulnerable Firefox build (UI:R - no clicks beyond navigation, but the user must load the malicious content). No authentication or pre-existing privileges are needed (PR:N). Limiting factors: the attack is not automatable per CISA SSVC because it depends on user interaction, it only affects Firefox builds prior to 155 / ESR 115.40 / ESR 140.15 / ESR 153.2, and exploitation must first win a use-after-free heap-grooming race in the DOM: Security component to achieve the sandbox escape. There is no non-default configuration prerequisite - default Firefox rendering of web content is the vulnerable surface.
Impact

Signals are mixed but lean toward high priority. The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:H = 9.6 critical) indicates a low-complexity, unauthenticated, network-reachable attack requiring only that the victim visit a page, with total confidentiality/integrity/availability impact and a scope change. However, CISA SSVC lists Exploitation: none and Automatable: no (user interaction breaks drive-by automation), and Technical Impact: total. There is no KEV listing, no EPSS score provided, and no POC referenced - so this is a severe-but-not-yet-exploited memory-safety bug. The requirement for user interaction (UI:R) and the absence of observed exploitation temper the raw 9.6 somewhat, but sandbox escapes in a major browser are routinely weaponized, so patching should not be deferred. Note: no EPSS value was supplied, so probabilistic exploitation likelihood cannot be quantified here.
Exploit ScenarioDerived

An attacker hosts a malicious web page containing crafted JavaScript/DOM operations and lures a victim (via phishing link, malvertising, or a compromised site) into visiting it. The page triggers the use-after-free in the DOM: Security component, grooms the heap to control the freed object, and escapes the content sandbox to run attacker-controlled code outside the renderer's restrictions. No public exploit is currently identified, but the low attack complexity makes reliable exploitation plausible once details are reverse-engineered from the fix.
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 via use-after-free in the Firefox DOM: Security component allows an attacker who lures a victim to a malicious web page to corrupt memory and break out of the browser content sandbox, potentially achieving code execution in a higher-privilege context. All Firefox release and ESR channels prior to the fixed builds are affected. No public exploit identified at time of analysis, and CISA SSVC records exploitation status as none, but the 9.6 CVSS and scope-change sandbox-escape nature make this a high-priority browser update.

RemediationAI

Vendor-released patch: upgrade to Firefox 155 on the release channel, or to the matching ESR fix - ESR 115.40, ESR 140.15, or ESR 153.2 - depending on which ESR train is deployed. Because Firefox auto-updates by default, ensure automatic updates are enabled and force an update check (Help > About Firefox) across managed fleets, or push the patched package via your endpoint management tooling. Refer to Mozilla advisories MFSA 2026-82/83/84/85 for per-channel version mapping. No standalone workaround toggle exists for this DOM: Security use-after-free; as an interim compensating control you can reduce attack surface by restricting browsing to trusted sites and enabling stricter script controls, though this degrades usability and is not a substitute for the update.

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Technical ContextAI · show details

The flaw is a CWE-416 use-after-free in Mozilla Firefox's 'DOM: Security' subsystem - the code enforcing same-origin and other web security invariants over Document Object Model objects. A use-after-free occurs when memory is freed while a dangling reference remains reachable; reusing that stale pointer lets an attacker control freed-then-reallocated memory. Because the bug sits at the sandbox boundary, successful exploitation escapes the content-process sandbox (reflected in CVSS S:C, scope change). The affected product per CPE is cpe:2.3:a:mozilla:firefox across all versions up to the patched builds.

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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

Web-delivered UAF needing only page navigation (AV:N/AC:L/PR:N/UI:R); sandbox escape crosses a trust boundary (S:C) with full code-execution impact (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 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: Security 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-2026-84121 Sep 01, 2026

Executive ViewAI

Why It Matters

All Firefox browser users across the organization face a critical vulnerability that enables attackers to gain system access by tricking users to visit a malicious website. No public exploit identified at time of analysis, and no vendor patch is currently available, creating an urgent need for immediate protective action to prevent data breaches and system compromise.

Business Impact: Severe
Compensating Controls

No vendor-released patch identified at time of analysis. Interim mitigations include: restricting Firefox to internal trusted networks only; implementing alternative browsers with automatic security updates; establishing daily monitoring of Mozilla security advisories for emergency patches; considering complete Firefox disablement until a vendor patch is released.

Threat Intelligence Overview

EPSS Score
0.3%
Top 73% exploitable
Prevalence
Ubiquitous
Firefox is a major mainstream browser deployed across millions of endpoints
Data Quality
6 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 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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Same technique Information Disclosure

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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-2026-84121 ADVISORY https://vuldb.com/vuln/397601 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2059018 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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