FirefoxWeb browser · Mozilla

CVE-2014-8639

MEDIUM · 6.8 CVSS v2.0 Published 2015-01-14

Fix available

A fix is available. Upgrade to after 34.0.5 or later.

See remediation →

77/100

Remediation priority · High

Remotely reachable No privileges Zero-click

Official description

NVD · unedited

Mozilla Firefox before 35.0, Firefox ESR 31.x before 31.4, Thunderbird before 31.4, and SeaMonkey before 2.32 do not properly interpret Set-Cookie headers within responses that have a 407 (aka Proxy Authentication Required) status code, which allows remote HTTP proxy servers to conduct session fixation attacks by providing a cookie name that corresponds to the session cookie of the origin server.

Technical summary

dbcve analysis · high confidence

Mozilla Firefox, Firefox ESR, Thunderbird, and SeaMonkey incorrectly process Set-Cookie headers from HTTP 407 (Proxy Authentication Required) responses, treating cookies from the proxy as if they originated from the destination server. This enables remote proxy servers to perform session fixation attacks by injecting session cookie names that override legitimate session cookies from the target website.

MitigationUpdate affected Mozilla products to version 35.0 or later for Firefox, 31.4 or later for Firefox ESR and Thunderbird, and 2.32 or later for SeaMonkey. Avoid using untrusted or compromised HTTP proxy servers.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions

NVD · CPE data

FirefoxWeb browser

Affected:<= 34.0.5= 31.0= 31.1.0= 31.1.1= 31.3.0

Firefox EsrWeb browser

Affected:= 31.2

SeamonkeyApplication

Affected:<= 2.31

ThunderbirdApplication

Affected:<= 31.3.0

CVSS breakdown

From the vector

Attack vector: Network
Complexity: M
Authentication: None
Confidentiality: Partial
Integrity: Partial
Availability: Partial

AV:N/AC:M/Au:N/C:P/I:P/A:P

Am I affected?

dbcve checks

Work through these to decide whether this CVE applies to you.

Identify installed Mozilla product

Check for Firefox, Firefox ESR, Thunderbird, or SeaMonkey installation. On Windows, look in Program Files; on macOS, check /Applications; on Linux, check /usr/lib or package manager listings.

Affected if Any of the four affected products (Firefox, Firefox ESR, SeaMonkey, Thunderbird) are installed
Determine product version number

For Firefox: run 'firefox --version' or check Help > About. For Thunderbird: run 'thunderbird --version' or check Help > About. For SeaMonkey: check Help > About. For ESR variants, the version will show 'ESR' designation.

Affected if Version matches any affected range: Firefox <=34.0.5, Firefox =31.0/31.1.0/31.1.1/31.3.0, Firefox ESR =31.2, SeaMonkey <=2.31, Thunderbird <=31.3.0
Check if HTTP proxy with authentication is configured

In the browser, go to Options > Advanced > Network > Settings, or check system proxy settings. Look for configured proxy servers that require authentication (username/password).

Affected if A proxy requiring authentication is configured and active - this is required for the vulnerability to be exploitable
Verify proxy authentication can trigger 407 responses

Test browsing through the configured proxy to any HTTPS site. A 407 Proxy Authentication Required response would contain Set-Cookie headers that could be mishandled.

Affected if The proxy returns 407 responses with Set-Cookie headers and those cookies override or set session cookies for the destination domain

A user is affected if they run an affected product version AND use an HTTP proxy that requires authentication, allowing the proxy to inject cookies via 407 responses.

Generated from the published advisory. Verify against your own configuration.

Remediation

From vendor data

Upgrade available Upgrade to a release after 34.0.5

Interim mitigation

Update affected Mozilla products to version 35.0 or later for Firefox, 31.4 or later for Firefox ESR and Thunderbird, and 2.32 or later for SeaMonkey. Avoid using untrusted or compromised HTTP proxy servers.

Fix this in Firefox Scoped from the published advisory

Consultation2.0 h
Implementation2.0 h
Testing2.0 h

6.0 hours of engineering $1,060

Get the upgrade done

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

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References

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2014-8639 in production — separate from our analysis above.

Notes0 Guidelines

Know something about CVE-2014-8639?

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

Be the first to add a field note for this CVE — a mitigation you’ve verified, a version caveat, or a link to a working fix. Sign in above to contribute.

What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here

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
No spam, self-promotion, credentials, or personal data

At a glance

A fix existsUpgrade to after 34.0.5 or later

Severity 6.8 Medium

Exploit likelihood EPSS, next 30 days 1.9%

Affects: Firefox

Remediation

A high-priority fix

Fix this in Firefox 6 hrs · Low

Typical engagement from $1,060

Get a fix See the full breakdown ↓

A high-priority vulnerability. Send your environment and a specialist will scope CVE-2014-8639 and return a fixed quote and timeline.

You’ll deal with our team directly and deploy the reviewed fix to staging first — never straight to production.

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.

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.

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.

CVSS 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.

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.

Check your environment

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.

Scan for this in your stack

References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Practitioner notes

No notes yet

Official description

Technical summary

Affected products & versions

CVSS breakdown

Am I affected?

Remediation

References