Which versions of Mozilla are affected by CVE-2026-33347?

CVE-2026-33347 presents moderate security risk, a cross-site scripting vulnerability rated CVSS 6.3. Attackers could inject scripts to steal sessions or perform actions as authenticated users.. A patch is available.

How to fix or mitigate CVE-2026-33347?

Within 30 days: Identify affected systems and apply vendor patches as part of regular patch cycle. Verify Content-Security-Policy and output encoding.

Mozilla CVE-2026-33347

Q: What is the CVSS score of CVE-2026-33347?

CVE-2026-33347 has a CVSS 4.0 base score of 6.3 (Medium). CVSS vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X. EPSS exploitation probability: 0.0%.

MEDIUM

Cross-site Scripting (XSS) (CWE-79)

2026-03-19 https://github.com/thephpleague/commonmark

GHSA-hh8v-hgvp-g3f5

XSS SSRF Mozilla

6.3

CVSS 4.0 · GitHub Advisory

Severity by source

GitHub Advisory PRIMARY

6.3 MEDIUM

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Primary rating from GitHub Advisory · only source for this CVE.

CVSS VectorGitHub Advisory

CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:L/SI:L/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

Attack Vector

Network

Attack Complexity

Low

Privileges Required

None

User Interaction

None

Scope

Lifecycle Timeline

Patch released

Mar 31, 2026 - 21:13 nvd

Patch available

Analysis Generated

Mar 19, 2026 - 20:00 vuln.today

CVE Published

Mar 19, 2026 - 19:04 nvd

MEDIUM 6.3

DescriptionGitHub Advisory

Impact

The DomainFilteringAdapter in the Embed extension is vulnerable to an allowlist bypass due to a missing hostname boundary assertion in the domain-matching regex. An attacker-controlled domain like youtube.com.evil passes the allowlist check when youtube.com is an allowed domain.

This enables two attack vectors:

SSRF: The OscaroteroEmbedAdapter makes server-side HTTP requests to the embed URL via the embed/embed library. A bypassed domain filter causes the server to make outbound requests to an attacker-controlled host, potentially probing internal services or exfiltrating request metadata.
XSS: EmbedRenderer outputs the oEmbed response HTML directly into the page with no sanitization. An attacker controlling the bypassed domain can return arbitrary HTML/JavaScript in their oEmbed response, which is rendered verbatim.

Any application using the Embed extension and relying on allowed_domains to restrict domains when processing untrusted Markdown input is affected.

Patches

This has been patched in version 2.8.2. The fix replaces the regex-based domain check with explicit hostname parsing using parse_url(), ensuring exact domain and subdomain matching only.

Workarounds

Disable the Embed extension, or restrict its use to trusted users
Provide your own domain-filtering implementation of EmbedAdapterInterface
Enable a Content Security Policy (CSP) and outbound firewall restrictions

AnalysisAI

Mozilla's Embed extension contains a domain allowlist bypass in the DomainFilteringAdapter due to insufficient hostname boundary validation in its regex pattern, allowing attacker-controlled domains like youtube.com.evil to pass validation checks for youtube.com. This vulnerability enables Server-Side Request Forgery attacks via the OscaroteroEmbedAdapter to probe internal services, and Cross-Site Scripting attacks through unsanitized oEmbed HTML responses returned by compromised domains. No patch is currently available for this medium-severity flaw.