Executive Summary
A critical uninitialized memory vulnerability (CVE-2026-6748) has been disclosed in the Audio/Video Web Codecs component shared by Mozilla Firefox and Thunderbird. The flaw carries a CVSS 3.1 score of 9.8 (Critical) and enables remote code execution through specially crafted audio or video media content. Mozilla has released emergency fixes in Firefox 150, Firefox ESR 140.10, Thunderbird 150, and Thunderbird ESR 140.10.
CVSS Score: 9.8 (Critical)
CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
Vulnerability Overview
| Attribute | Value |
|---|---|
| CVE ID | CVE-2026-6748 |
| CVSS Score | 9.8 (Critical) |
| Type | Uninitialized Memory Use (CWE-908) |
| Attack Vector | Network |
| Attack Complexity | Low |
| Privileges Required | None |
| User Interaction | None |
| Affected Component | Audio/Video Web Codecs |
| Fixed In | Firefox 150, Firefox ESR 140.10, Thunderbird 150, Thunderbird ESR 140.10 |
| Published | 2026-04-21 |
Affected Products
| Product | Vulnerable Versions | Fixed Version |
|---|---|---|
| Mozilla Firefox | < 150 | 150 |
| Mozilla Firefox ESR | < 140.10 | 140.10 |
| Mozilla Thunderbird | < 150 | 150 |
| Mozilla Thunderbird ESR | < 140.10 | 140.10 |
Both applications share a common Web Codecs implementation for processing audio and video streams. The flaw exists in the codec pipeline's handling of media data, making all users of either browser or email client on any platform potentially vulnerable.
Technical Details
Root Cause
The vulnerability stems from uninitialized memory in the Web Codecs API component that processes Audio/Video streams. When the codec pipeline handles certain malformed or specially crafted media frames, memory is allocated and passed to codec routines without being fully initialized. Reading or acting on this uninitialized memory can expose sensitive data from adjacent heap regions or, in exploitation scenarios, be leveraged to achieve controlled memory corruption.
Exploitation Path
1. Attacker hosts or delivers a crafted media resource
(audio stream, video file, or inline media element)
2. Victim opens the page in Firefox or processes a message
containing the media in Thunderbird
3. The Web Codecs pipeline allocates a codec buffer without
zeroing memory — adjacent heap data bleeds into codec context
4. Attacker-controlled codec parameters interact with the
uninitialized region, enabling:
- Information disclosure of heap contents (addresses, secrets)
- Type confusion or use-after-free conditions
- Arbitrary code execution via memory manipulation primitives
5. Full code execution in the context of the browser/email client
process — no user interaction required beyond loading contentWhy CVSS 9.8?
The maximum-severity-adjacent score reflects:
- No authentication required — any unauthenticated network attacker can deliver the malicious payload
- No user interaction — the vulnerability can be triggered automatically on page load or email preview
- Low complexity — no race condition or special timing required
- Full CIA impact — confidentiality, integrity, and availability all rated High
The score is 9.8 rather than 10.0 because the scope is unchanged (attacker is confined to the browser/email process unless additional sandbox escapes are chained).
Impact Assessment
| Impact Area | Description |
|---|---|
| Remote Code Execution | Arbitrary code execution in Firefox or Thunderbird process context |
| Credential Theft | Stored passwords, session cookies, and browser secrets accessible |
| Data Exfiltration | Heap memory disclosure may expose sensitive application or OS data |
| Drive-By Exploitation | No user click required — page load sufficient to trigger |
| Email Client Attack Surface | Thunderbird users at risk via malicious HTML email with embedded media |
| Enterprise Impact | Organizations using Thunderbird for corporate email face elevated exposure |
Affected Use Cases
Browser exploitation (Firefox):
- Malicious web pages embedding crafted video or audio elements via the Web Codecs API
- Advertisements, embedded players, or iframe-delivered media on compromised or attacker-controlled sites
- WebRTC streams with malicious codec parameters
Email client exploitation (Thunderbird):
- HTML email with embedded audio/video triggering the codec path on message preview
- Malicious newsletter content delivered to corporate email lists
- Attachments or inline media in phishing campaigns
Recommendations
Immediate Actions
- Update immediately — upgrade to Firefox 150, Firefox ESR 140.10, Thunderbird 150, or Thunderbird ESR 140.10
- Verify version — check
Help > About FirefoxorHelp > About Thunderbirdto confirm the patched version is installed - Enable automatic updates — ensure auto-update is active so future critical fixes apply promptly
- Disable HTML email preview if Thunderbird patching is delayed in managed environments
- Block media-heavy domains at the perimeter if immediate patching is not possible
Enterprise Deployment
- Use group policy or MDM to force Firefox/Thunderbird update deployment
- Prioritize ESR versions (140.10) for managed enterprise fleets
- Monitor for signs of exploitation: unusual child process spawning
from firefox.exe or thunderbird.exe
- Apply web content filtering to block sites serving codec-abusing media
- Review IDS rules for anomalous outbound connections from browser processesDetection Indicators
| Indicator | Description |
|---|---|
| Unexpected child process from Firefox/Thunderbird | Possible code execution post-exploitation |
| Firefox/Thunderbird crash dumps referencing codec paths | Exploitation attempt (failed or succeeded) |
| Unusual network connections from browser process | Possible C2 callback post-exploitation |
| Heap corruption errors in browser telemetry | Exploitation signature |
Post-Patch Verification
- Confirm installed version — ensure Firefox reports 150+ or Thunderbird 150+ (ESR: 140.10+)
- Clear cached media — purge browser cache post-update to eliminate any cached malicious assets
- Rotate credentials — if exploitation is suspected, rotate passwords, session tokens, and API keys accessible via the browser
- Review browser history — look for unusual sites visited near the time of suspected exploitation
- Audit email logs — if Thunderbird was targeted, review inbound email sources for phishing indicators