FirefoxWeb browser · Mozilla
CVE-2025-5271
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 · uneditedPreviewing a response in Devtools ignored CSP headers, which could have allowed content injection attacks. This vulnerability was fixed in Firefox 139 and Thunderbird 139.
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 confidenceIn Firefox and Thunderbird DevTools, the response preview feature ignored Content Security Policy (CSP) headers when rendering content. This meant that even if a server response included CSP headers designed to prevent content injection (like XSS), the DevTools preview would not enforce them, potentially allowing malicious content to execute.
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
< 139.0CVSS 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
- None
- Scope
- Unchanged
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/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 Firefox versionOpen Firefox, click the menu button (three horizontal lines), select Help, then About Firefox. The version number is displayed at the top of the window.Affected if The version shown is below 139.0 (for example, 138.0, 138.0.1, etc.)
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Check Thunderbird versionOpen Thunderbird, click the menu button, select Help, then About Thunderbird. The version number is displayed at the top of the window.Affected if The version shown is below 139.0 (for example, 138.x series) and you use Thunderbird's DevTools
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Determine if DevTools response preview is in useOpen DevTools (F12 or Ctrl+Shift+I), go to the Network tab, make a network request, click on a response, and view the Preview tab. This is the response preview feature.Affected if You actively use the Network panel preview feature to inspect responses from servers, particularly untrusted ones
You are affected if you run Firefox or Thunderbird version 138.x or lower AND use DevTools to preview network responses, because the preview will not enforce CSP headers from the server.
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.
dbcve · scoped139.0
Upgrade Firefox and Thunderbird to version 139 or later. Until upgraded, avoid previewing untrusted responses in DevTools.
Firefox 139.0
- Open Firefox and navigate to Menu > Help > About Firefox (or go to about:support)
- Check your current Firefox version - if it's below 139.0, an update is needed
- In the About Firefox window, click 'Check for updates' or wait for automatic update check
- Once Firefox 139.0 or later is available, click 'Update to latest version'
- Restart Firefox after the update completes
- Verify the update by returning to Menu > Help > About Firefox and confirming version 139.0 or higher
Generated from the published advisory — verify against the referenced sources before acting.
Fix this in Firefox
Scoped from the published advisory
- Consultation2.0 h
- Implementation4.0 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 $2,560.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2025-5271 — 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-2025-5271 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
- No weaponised exploit code, or anything meant to cause harm
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