id: GO-2025-3638
modules:
    - module: github.com/refraction-networking/utls
      versions:
        - introduced: 1.0.0
        - fixed: 1.7.0
      vulnerable_at: 1.6.7
      packages:
        - package: github.com/refraction-networking/utls
          symbols:
            - UConn.clientHandshake
          derived_symbols:
            - Conn.Handshake
            - Conn.HandshakeContext
            - Conn.Read
            - Conn.Write
            - Dial
            - DialWithDialer
            - Dialer.Dial
            - Dialer.DialContext
            - QUICConn.Start
            - Roller.Dial
            - UConn.Handshake
            - UConn.HandshakeContext
            - UConn.Read
            - UConn.Write
            - UQUICConn.HandleData
            - UQUICConn.Start
summary: ServerHellos are accepted without checking TLS 1.3 downgrade canaries in github.com/refraction-networking/utls
description: |-
    Before version 1.7.0, utls did not implement the TLS 1.3 downgrade protection
    mechanism specified in RFC 8446 Section 4.1.3 when using a utls ClientHello
    spec. This allowed an active network adversary to downgrade TLS 1.3 connections
    initiated by a utls client to a lower TLS version (e.g., TLS 1.2) by modifying
    the ClientHello message to exclude the SupportedVersions extension, causing the
    server to respond with a TLS 1.2 ServerHello (along with a downgrade canary in
    the ServerHello random field). Because utls did not check the downgrade canary
    in the ServerHello random field, clients would accept the downgraded connection
    without detecting the attack. This attack could also be used by an active
    network attacker to fingerprint utls connections.
ghsas:
    - GHSA-pmc3-p9hx-jq96
references:
    - advisory: https://github.com/advisories/GHSA-pmc3-p9hx-jq96
    - fix: https://github.com/refraction-networking/utls/commit/f8892761e2a4d29054264651d3a86fda83bc83f9
    - fix: https://github.com/refraction-networking/utls/pull/337
    - report: https://github.com/refraction-networking/utls/issues/181
source:
    id: GHSA-pmc3-p9hx-jq96
    created: 2025-04-23T15:01:24.251871181Z
review_status: REVIEWED