SNI disclosure via ECH-bypassing speculative connection
Published May 27, 2025 · Updated Apr 13, 2026
Information disclosure in Mozilla Firefox and Thunderbird before 139 allows man-in-the-middle attackers to read SNI despite encrypted DNS. An Alt-Svc validation path creates a speculative HTTP/3 connection without first looking up the HTTPS record, so the connection receives no ECH configuration and sends plaintext SNI. Encrypted DNS and an Alt-Svc-triggered speculative connection are required; the resulting leak reveals the requested hostname to a network observer.
Summary
What happened
Information disclosure in Mozilla Firefox and Thunderbird before 139 allows man-in-the-middle attackers to read SNI despite encrypted DNS. An Alt-Svc validation path creates a speculative HTTP/3 connection without first looking up the HTTPS record, so the connection receives no ECH configuration and sends plaintext SNI. Encrypted DNS and an Alt-Svc-triggered speculative connection are required; the resulting leak reveals the requested hostname to a network observer.
The record
- CVE
- CVE-2025-5270
- Published
- May 27, 2025
- Updated
- Apr 13, 2026
- Vendor
- Unknown vendor
- Product
- Unknown product
- Classifications
- CWE-319, T1040
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- May 27, 2025CVE publishedPublication date reported by the CVE source.
- Apr 13, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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What is affected?
Affected products and versions are unavailable in this record.
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Weakness, pattern, technique
Public exploit references
- Plaintext SNI reproduction procedureproof of concept · demonstrated
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Technologies
Your stack
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Your stack
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