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Firefox CVE-2026-6748

| EUVDEUVD-2026-24089 CRITICAL
Use of Uninitialized Variable (CWE-457)
2026-04-21 mozilla GHSA-75xc-fv5m-pprr
Information Disclosure Mozilla
9.8
CVSS 3.1 · Vendor: mozilla
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Attack ChainAIDerived

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

  1. Recon
    Navigate to malicious website
  2. Delivery
    Invoke Web Codecs API with crafted media
  3. Exploit
    Trigger uninitialized memory access
  4. Install
    Corrupt memory structures
  5. C2
    Execute arbitrary code
  6. Execute
    Escape browser sandbox
  7. Impact
    Achieve system-level compromise

Vulnerability Assessment

Weakness
CWE
CWE-457
ASVS 5.0
V15 - Secure Coding and Architecture Variable initialization checks
Exploitation Conditions

No special conditions - remote unauthenticated exploitation against default configurations of Firefox and Firefox ESR. The CVSS vector (AV:N/AC:L/PR:N/UI:N) confirms attackers can trigger the vulnerability through network-accessible vectors without requiring authentication, user interaction, or non-default settings. Any Firefox installation with JavaScript enabled (default state) allows web content to invoke the Web Codecs API. Exploitation success depends on browser sandbox effectiveness and platform-specific memory layout, but the core vulnerability is triggerable in all affected versions. No specialized deployment mode, feature flag, or configuration setting is required - the vulnerable code path is accessible to any web content through standard browser APIs.
Impact

This represents critical real-world risk based on converging evidence from multiple frameworks. The CVSS 9.8 score reflects maximum severity across all dimensions: network attack vector (AV:N), low complexity (AC:L), no privileges required (PR:N), no user interaction (UI:N), and complete impact to confidentiality, integrity, and availability (C:H/I:H/A:H). SSVC framework assessment corroborates this with 'Automatable: yes' and 'Technical Impact: total' ratings, indicating attackers can develop reliable exploits requiring minimal human intervention. However, SSVC reports 'Exploitation: none', meaning no active exploitation confirmed at time of analysis - this is a critical mitigating factor. The vulnerability's location in Web Codecs API provides broad attack surface since any website can invoke these APIs, but Firefox's sandboxing architecture may limit post-exploitation impact. With patches already released in Firefox 150 and ESR 140.10, organizations have clear remediation paths. Priority should be immediate for internet-facing systems and users processing untrusted multimedia content, but absence of active exploitation (per SSVC) provides a brief window for controlled patch deployment rather than emergency response.
Exploit ScenarioDerived

An attacker creates a malicious website hosting specially crafted JavaScript that invokes the Web Codecs API to process a manipulated audio or video stream, triggering access to uninitialized memory buffers. The attacker carefully structures the media data to control the contents of uninitialized memory regions, allowing them to corrupt function pointers, bypass security checks, or inject executable code into Firefox's renderer process. Because the attack requires only that a victim browse to the malicious page with no additional user interaction (UI:N), it can be delivered through malvertising, compromised legitimate websites, or phishing campaigns, achieving remote code execution with the privileges of the Firefox process and potentially escaping the browser sandbox to compromise the underlying system.
Act Now Prioritize within 24-48h. Why: CRITICAL severity.
Patch available 0.0% exploitation probability (EPSS)

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AnalysisAIv2 – Apr 22, 2026

Uninitialized memory access in Firefox's Web Codecs API enables remote code execution without authentication. Attackers can exploit this CWE-457 (Use of Uninitialized Variable) flaw through network-accessible vectors with low complexity (AV:N/AC:L/PR:N/UI:N) to achieve complete system compromise including data exfiltration, arbitrary code execution, and denial of service. CVSS 9.8 severity is supported by SSVC assessment indicating automatable exploitation with total technical impact. Vendor-released patches available in Firefox 150 and Firefox ESR 140.10. CISA SSVC reports no active exploitation at time of analysis, though the vulnerability is classified as automatable with total technical impact.

Earlier versions of this analysis (1)
v1 Apr 21, 2026 - 19:22 CVSS 9.8

Uninitialized memory access in Firefox's Web Codecs API allows remote code execution with no user interaction required. Affects Firefox versions prior to 150 and Firefox ESR prior to 140.10. The vulnerability carries a critical CVSS 9.8 score due to network-based exploitation requiring no authentication or user interaction, enabling attackers to achieve arbitrary code execution, information disclosure, and denial of service. Mozilla has released patches in Firefox 150 and Firefox ESR 140.10 per advisories MFSA2026-30 and MFSA2026-32. No active exploitation (CISA KEV) or public exploit code has been identified at time of analysis.

RemediationAI

Upgrade immediately to Firefox 150 or later for standard channel users, or Firefox ESR 140.10 or later for Extended Support Release users. Mozilla Security Advisories MFSA2026-30 (https://www.mozilla.org/security/advisories/mfsa2026-30/) and MFSA2026-32 (https://www.mozilla.org/security/advisories/mfsa2026-32/) contain complete upgrade instructions and release notes. For environments unable to patch immediately, implement compensating controls by disabling JavaScript-accessible media features through Firefox enterprise policies: set 'media.webcodecs.enabled' to false in about:config or via Group Policy Object, which prevents web content from accessing the vulnerable Web Codecs API. Note this workaround breaks legitimate functionality on sites using modern video conferencing, media editing, or streaming features. Network-level controls such as deploying web filtering to block known-malicious media exploitation sites provide additional defense-in-depth but cannot prevent zero-day exploitation attempts. Organizations managing Firefox at scale should prioritize ESR channel deployments for more predictable patching cycles while maintaining rapid update capabilities for critical vulnerabilities like this.

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Technical ContextAI · show details

This vulnerability affects Mozilla Firefox's implementation of the Web Codecs API, a modern browser interface for encoding and decoding audio and video streams. The flaw stems from CWE-457 (Use of Uninitialized Variable), where memory allocated for audio/video processing operations is accessed before being properly initialized. In memory-unsafe languages like C/C++ commonly used in browser media components, uninitialized memory can contain residual data from previous operations or random values. When this memory is used in calculations, buffer operations, or control flow decisions, it creates undefined behavior that attackers can manipulate. The CPE identifier cpe:2.3:a:mozilla:firefox confirms both standard Firefox and Extended Support Release (ESR) channels are affected. The Web Codecs API processes untrusted media data from web content, making it an attractive attack surface for remote exploitation through malicious websites or multimedia content.

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Severity by source

Vendor (mozilla) PRIMARY 9.8 CRITICAL
AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A: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
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

Lifecycle Timeline

4
Patch released
Apr 29, 2026 - 02:30 nvd
Patch available
Analysis Updated
Apr 22, 2026 - 00:29 vuln.today
v2 - CVSS changed
CVSS changed
Apr 21, 2026 - 19:22 NVD
9.8 (CRITICAL)
CVE Published
Apr 21, 2026 - 12:40 nvd
CRITICAL 9.8

DescriptionCVE.org

Uninitialized memory in the Audio/Video: Web Codecs component. This vulnerability was fixed in Firefox 150 and Firefox ESR 140.10.

Executive ViewAI

Why It Matters

CVE-2026-6748 is a critical remote code execution vulnerability in Firefox's Web Codecs API that allows unauthenticated attackers to execute arbitrary code and exfiltrate data through network access with no user interaction required. All Firefox users are at risk of complete system compromise, making immediate browser updates essential for any organization where Firefox is deployed.

Business Impact: Severe
Compensating Controls

Vendor-released patch: Firefox 150 and Firefox ESR 140.10. Apply immediately upon release availability. Until patching is complete: restrict Firefox to air-gapped or sandboxed environments; disable JavaScript execution in Firefox where operationally feasible; block Web Codecs API traffic at network perimeter if supported by security infrastructure.

Threat Intelligence Overview

EPSS Score
0.0%
Top 86% exploitable
Data Quality
7 sources
Confidence: High
EUVD 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 0.0%
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
Out-of-Cycle
Exploitation
None
System Exposure· derived
Open
Automatable
Yes
Human Impact
Very High
Technical Impact
Total

Priority Score

49
Low Medium High Critical
KEV: 0
EPSS: 0
CVSS: +49
PoC: 0
Weaponized: 0
Threat intel: 0
Gov advisory: 0

Attack Technique

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-457 – Use of Uninitialized Variable

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Vendor StatusVendor

SUSE

Severity: Critical
Product Status
SUSE Liberty Linux 10 Fixed
SUSE Liberty Linux 8 Fixed
SUSE Liberty Linux 9 Fixed
SUSE Linux Enterprise Server 16.0 SUSE Linux Enterprise Server for SAP applications 16.0 Fixed
openSUSE Leap 16.0 Fixed

References

Advisories & patches
ADVISORY https://access.redhat.com/errata/RHSA-2026:10757 ADVISORY https://access.redhat.com/errata/RHSA-2026:10766 ADVISORY https://access.redhat.com/errata/RHSA-2026:10767 ADVISORY https://nvd.nist.gov/vuln/detail/CVE-2026-6748 ADVISORY https://vuldb.com/vuln/358445 REF https://access.redhat.com/errata/RHSA-2026:12285 REF https://access.redhat.com/errata/RHSA-2026:13537 REF https://access.redhat.com/errata/RHSA-2026:15892 REF https://access.redhat.com/errata/RHSA-2026:17477 REF https://access.redhat.com/errata/RHSA-2026:17687 REF https://access.redhat.com/errata/RHSA-2026:17688 REF https://access.redhat.com/errata/RHSA-2026:17689 REF https://access.redhat.com/errata/RHSA-2026:17690 REF https://access.redhat.com/errata/RHSA-2026:19041 REF https://access.redhat.com/errata/RHSA-2026:19131 REF https://access.redhat.com/errata/RHSA-2026:19201 REF https://access.redhat.com/errata/RHSA-2026:19348 REF https://access.redhat.com/errata/RHSA-2026:19461 REF https://access.redhat.com/errata/RHSA-2026:19462 REF https://access.redhat.com/errata/RHSA-2026:19463 REF https://access.redhat.com/errata/RHSA-2026:19464 REF https://access.redhat.com/errata/RHSA-2026:19465 REF https://access.redhat.com/errata/RHSA-2026:19466 REF https://access.redhat.com/errata/RHSA-2026:19467 REF https://access.redhat.com/errata/RHSA-2026:19468 REF https://access.redhat.com/errata/RHSA-2026:19469 REF https://access.redhat.com/errata/RHSA-2026:19542 REF https://access.redhat.com/errata/RHSA-2026:19655 REF https://access.redhat.com/errata/RHSA-2026:19704 REF https://access.redhat.com/security/cve/CVE-2026-6748 REF https://bugzilla.mozilla.org/show_bug.cgi?id=2022604 REF https://bugzilla.redhat.com/show_bug.cgi?id=2460103 REF https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-6748.json REF https://www.mozilla.org/security/advisories/mfsa2026-30/ REF https://www.mozilla.org/security/advisories/mfsa2026-32/ REF https://www.mozilla.org/security/advisories/mfsa2026-33/ REF https://www.mozilla.org/security/advisories/mfsa2026-34/

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