Mozilla
CVE-2026-33347
MEDIUM
GHSA-hh8v-hgvp-g3f5
Vulnerability Assessment
| Weakness |
CWE
OWASP Top 10:2025
A05:2025 - Injection
ASVS 5.0
V1 - Encoding and Sanitization Output encoding, context-aware escaping
MITRE ATT&CK
2 techniques across 2 tactics: Initial Access, Execution The technique list with ATT&CK links and the kill-chain view is part of the Basic plan and up. |
| Impact | Attackers can steal session cookies, redirect users to malicious sites, deface content, or perform actions on behalf of authenticated users. |
| Exploit ScenarioDerived | An attacker injects a JavaScript payload into a vulnerable input field. When another user views the page, the script executes in their browser, stealing their session token. |
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Alert meAnalysisAIv1 – Mar 19, 2026
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.
Earlier versions of this analysis (1)
RemediationAI
Encode all user-supplied output contextually (HTML, JS, URL). Implement Content Security Policy (CSP) headers. Use HTTPOnly and Secure cookie flags.
Affected Packages
Source: OSV (Open Source Vulnerabilities)
Affects your stack
Affects 1 package in composer
Checking this CVE against your own stacks and servers is part of the Basic plan and up.
Technical ContextAI · show details
Cross-site scripting (XSS) allows injection of client-side scripts into web pages viewed by other users due to insufficient output encoding.
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Severity by source
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
Primary rating from GitHub Advisory · only source for this CVE.
CVSS VectorGitHub Advisory
Lifecycle Timeline
2DescriptionGitHub 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
OscaroteroEmbedAdaptermakes server-side HTTP requests to the embed URL via theembed/embedlibrary. 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:
EmbedRendereroutputs 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
Embedextension, 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
Executive ViewAI
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. Organizations should apply vendor updates when available and monitor for exploitation.
Apply vendor patch. If patching is delayed: implement CSP headers, enable HttpOnly cookies, deploy WAF XSS rules.
Threat Intelligence Overview
NVD
CVE.org
EPSS
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.
What every action level means
SSVC AssessmentENISA
Priority Score
Attack Technique
Cross-Site Scripting occurs when an application accepts untrusted data and sends it to a web browser without proper validation or encoding.
Server-Side Request Forgery exploits applications that fetch remote resources based on user-supplied URLs.
Affected Products (CPE)GitHub Advisory
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