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CVE-2025-0239

Alt-Svc ALPN validation failure when redirected

MEDIUM
4
CVSS Score
Published: January 7, 2025 at 04:07 PM UTC
Updated: October 5, 2026 at 03:10 PM UTC
2 products affected across 1 vendorFirefoxThunderbird

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Severity
MEDIUM4
Exploit prediction
EPSS 0.2%
SSVC
No known exploit

Description

When using Alt-Svc, ALPN did not properly validate certificates when the original server is redirecting to an insecure site. This vulnerability affects Firefox < 134, Firefox ESR < 128.6, Thunderbird < 134, and Thunderbird < 128.6.

Affected

Our own answer: ranges merged, affected separated from platform-only, with confidence and review flags.

Firefox· Mozilla
95%
v < 128.6.0v < 134.0
Vendor states this is fixed in 128.6 (unaffected)
Affected range stops below 128.6.0, so it looks safe — our inference, not a vendor statement
Vendor states this is fixed in 134 (unaffected)
Affected range stops below 134.0, so it looks safe — our inference, not a vendor statement
Thunderbird· Mozilla
95%
v < 128.6.0129.0 ≤ v < 134.0
Vendor states this is fixed in 128.6 (unaffected)
Affected range stops below 128.6.0, so it looks safe — our inference, not a vendor statement
Vendor states this is fixed in 134 (unaffected)
Affected range stops below 134.0, so it looks safe — our inference, not a vendor statement

References (6)

Issue TrackingPermissions Required
Source: [email protected]
https://bugzilla.mozilla.org/show_bug.cgi?id=1929156
Vendor Advisory
Source: [email protected]
https://www.mozilla.org/security/advisories/mfsa2025-01/
Vendor Advisory
Source: [email protected]
https://www.mozilla.org/security/advisories/mfsa2025-02/
Vendor Advisory
Source: [email protected]
https://www.mozilla.org/security/advisories/mfsa2025-04/
Vendor Advisory
Source: [email protected]
https://www.mozilla.org/security/advisories/mfsa2025-05/
Source: af854a3a-2127-422b-91ae-364da2661108
https://lists.debian.org/debian-lts-announce/2025/01/msg00004.html

SSVC — Action Prioritization

SSVCNo known exploitNot automatablePartial impact

SSVC signals action urgency (what to patch first), separate from CVSS which measures severity.

CVSS

4MEDIUMCISA-ADP
Attack Vector
LOCAL
Complexity
LOW
Privileges
NONE
User Interaction
NONE
Scope
UNCHANGED
Confidentiality
NONE
Integrity
LOW
Availability
NONE
Vector?

Quick Info

StatusMODIFIED
CWE
CWE-295

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