FirefoxWeb browser · Mozilla
CVE-2018-5133
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedIf the "app.support.baseURL" preference is changed by a malicious local program to contain HTML and script content, this content is not sanitized. It will be executed if a user loads "chrome://browser/content/preferences/in-content/preferences.xul" directly in a tab and executes a search. This stored preference is also executed whenever an EME video player plugin displays a CDM-disabled message as a notification message. This vulnerability affects Firefox < 59.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceA stored XSS vulnerability in Firefox allows a malicious local program to modify the 'app.support.baseURL' preference with HTML/script content. This unsanitized content executes when users load the preferences.xul page directly in a tab and perform a search, or when EME video player plugins display CDM-disabled notification messages.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
FirefoxWeb browser
< 59.0
Ubuntu LinuxOperating system
= 14.04= 16.04= 17.10CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check installed Firefox versionRun 'firefox --version' or navigate to Firefox > Help > About Firefox. On Ubuntu, also check with 'apt list --installed | grep firefox'Affected if Version is earlier than 59.0 (e.g., 58.x, 57.x, etc.)
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Verify app.support.baseURL preference valueIn Firefox address bar, type 'about:config', search for 'app.support.baseURL', and inspect the string valueAffected if The preference contains HTML tags (like <script>, <iframe>, <img>) or suspicious URLs rather than a plain URL like 'https://support.mozilla.org/'
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Check for unauthorized modifications to Firefox prefs.jsInspect the Firefox profile directory (typically ~/.mozilla/firefox/*.default/) and examine prefs.js for any 'app.support.baseURL' entries with non-standard valuesAffected if The prefs.js file contains a user_pref for app.support.baseURL with HTML/script content or unexpected external URLs
You are affected if Firefox version is below 59.0 AND the app.support.baseURL preference contains unsanitized HTML or script content instead of a standard Mozilla support URL.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor data59.0
Upgrade Firefox to version 59 or later. As a workaround, avoid loading preferences.xul directly in tabs and restrict local program access to Firefox preference files.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation1.0 h
- Testing2.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,696.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2018-5133 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2018-5133 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
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