Go crypto/tls CVE-2026-42505

Encrypted Client Hello (ECH) is a TLS 1.3 extension that encrypts the sensitive inner ClientHello - including the Server Name Indication (SNI) - inside an unencrypted outer ClientHello envelope, preventing passive observers from identifying the intended server. TLS session resumption uses Pre-Shared Keys (PSKs), each identified by a PSK identity value derived from a prior completed session. The root cause (CWE-201: Exposure of Sensitive Information Through Sent Data) is that Go's crypto/tls implementation fails to suppress PSK identity values when constructing the outer ClientHello during an ECH handshake. Because PSK identities are session-specific and can be correlated to prior connections or to specific servers, their presence in cleartext directly undermines ECH's anonymity guarantees. The affected product is identified by CPE cpe:2.3:a:go_standard_library:crypto/tls:*:*:*:*:*:*:*:* and spans all three active Go release branches.