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Mozilla Firefox CVE-2026-100777

| EUVDEUVD-2026-88929 HIGH
Use After Free (CWE-416)
2026-09-29 mozilla GHSA-vwr3-v9qx-vq5r
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
    Host crafted web page
  2. Delivery
    Lure victim to load it
  3. Exploit
    Trigger Canvas2D use-after-free
  4. Execution
    Corrupt heap memory
  5. Impact
    Achieve code execution 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

Exploitation requires the victim to visit or render attacker-controlled web content that exercises the Graphics: Canvas2D component (e.g. a malicious page, ad, or iframe) - this is the UI:R requirement and the primary limiting factor. No authentication or elevated privileges are needed (PR:N), and default Firefox configurations are affected since Canvas2D is a standard always-enabled rendering feature. Exploitation is limited to unpatched builds prior to Firefox 157 / ESR 153.4 / ESR 140.17 / ESR 115.42; users on updated versions are not vulnerable. Reliable code execution beyond a crash further depends on defeating browser hardening (site isolation, sandboxing, ASLR), raising practical difficulty above the raw 8.8 score.
Impact

This is a genuine but conditional priority. The CVSS 3.1 vector (AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H, 8.8) reflects a memory-corruption bug reachable over the network against an unauthenticated user with low complexity, but it hinges entirely on UI:R - the victim must load attacker-controlled web content. There is no EPSS score, no CISA KEV listing, and no public POC in the provided data, so exploitation status is 'no public exploit identified at time of analysis' - this is NOT confirmed active exploitation. The high CVSS is driven by the RCE-class triple-impact (C/I/A all High), which is standard for browser renderer use-after-frees, but the absence of KEV/POC and Mozilla's self-reporting (typical of internally-found or fuzzed bugs) suggests it was fixed before weaponization. Note a minor signal conflict: the 'Information Disclosure' tag understates the actual impact - a Canvas2D UAF is memory corruption enabling potential code execution, not merely info leakage, which the C:H/I:H/A:H vector correctly captures. Net: patch promptly given Firefox's ubiquity and browser bugs' history of being chained into exploit kits, but there is no evidence of imminent mass exploitation.
Exploit ScenarioDerived

An attacker hosts a malicious web page (or injects a malicious ad/iframe into a legitimate site) that drives Firefox's Canvas2D graphics component into a use-after-free state when the victim visits. Triggering the freed-object reuse corrupts heap memory, which a skilled attacker can groom toward arbitrary code execution in the content process. No public exploit code is available at time of analysis, and successful exploitation requires the victim to actively load the crafted page.
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

Use-after-free in the Graphics: Canvas2D component of Mozilla Firefox allows remote, unauthenticated attackers to corrupt browser memory when a victim renders attacker-controlled web content - a malicious page, ad, or iframe provides the required user interaction (UI:R). Successful exploitation can expose or modify memory (high C/I/A impact per the 8.8 CVSS), causing crashes and potentially arbitrary code execution inside the content/renderer process, although turning the primitive into reliable code execution also requires defeating Firefox's site isolation, sandbox, and ASLR hardening, which keeps practical difficulty above the raw score. Only unpatched builds are affected; Firefox 157, Firefox ESR 153.4, ESR 140.17, and ESR 115.42 carry the fix, and no public exploit code or active exploitation was identified at the time of analysis.

RemediationAI

Vendor-released patch: upgrade to Mozilla Firefox 157 (rapid release), Firefox ESR 153.4, Firefox ESR 140.17, or Firefox ESR 115.42 - the exact fixed versions named in the advisory set (MFSA2026-97, MFSA2026-98, MFSA2026-99, MFSA2026-100 at https://www.mozilla.org/security/advisories/mfsa2026-100/ and companions); there is no partial fix, so any build older than these versions remains exposed. Because the flaw is in an always-on rendering feature, there is no clean configuration workaround: disabling or restricting the HTML5 canvas API via prefs such as privacy.resistFingerprinting or canvas-blocking extensions breaks a large amount of legitimate web functionality (maps, charts, document viewers, some web apps) and is only appropriate as an emergency, short-lived measure on high-value endpoints until the browser can be updated. Compensating controls that reduce exposure with fewer functional side effects include keeping site isolation enabled, blocking untrusted script and ads at the network/DNS layer, restricting browsing to a hardened or contained profile, and relying on the renderer sandbox to limit fallout - though all of these lower risk without removing the underlying memory-corruption primitive, since exploitation only requires the victim to render attacker-controlled content.

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Technical ContextAI · show details

The vulnerability is a use-after-free (CWE-416) in Firefox's Graphics: Canvas2D subsystem, the standard always-enabled HTML5 canvas rendering path used by virtually every web page. A dangling pointer to a freed canvas-related object is dereferenced after the underlying memory has been released - typically triggered by object lifecycle races involving canvas resource reallocation, resizing, or garbage collection - allowing an attacker to read or write memory that has been reallocated with attacker-influenced content. Intelligence tags for this CVE include 'Use After Free', 'Memory Corruption', and 'Information Disclosure', consistent with both an out-of-bounds read primitive (information disclosure that can defeat ASLR) and an arbitrary write/control-flow corruption primitive. Because Canvas2D is a default-on feature with no authentication barrier (PR:N in the assessed CVSS vector CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H), the only meaningful access prerequisite is that the victim render attacker-controlled content; the impact is normally contained to the sandboxed content process, and full system compromise would additionally require a separate sandbox escape. Technical details are tracked in Mozilla Bugzilla bug 2068375 with fixes shipped via the MFSA2026-97 through MFSA2026-100 advisories.

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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

Network-delivered via a crafted web page (AV:N) requiring victim to load content (UI:R), no privileges (PR:N); a renderer use-after-free yields memory corruption with RCE-class C/I/A:H.

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 MEDIUM
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 Graphics: Canvas2D 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

Mozilla Firefox's Canvas2D component contains a high-severity use-after-free flaw that can corrupt browser memory when users view attacker-controlled web content, posing a risk of browser crashes, data exposure, and potential code execution within the renderer process. The issue affects everyday browsing and could disrupt operations or expose sensitive information for any employee, contractor, or customer using unpatched Firefox builds. No public exploit identified at time of analysis, but leadership should treat the vulnerability as a priority because exploitation requires only user interaction and the blast radius includes common endpoint and web session data.

Business Impact: High
Compensating Controls

Apply vendor patch.

Threat Intelligence Overview

EPSS Score
0.3%
Top 75% exploitable
Prevalence
Ubiquitous
Firefox is one of the most widely deployed 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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Vendor StatusVendor

SUSE

Severity: Moderate
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-2026-100777 ADVISORY https://vuldb.com/vuln/411348 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2068375 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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