CVE-2026-103415

CVE-2026-103415 is a critical-severity vulnerability rated 9.3/10 on the CVSS scale. When a TLS 1.3 client processes a HelloRetryRequest, it copies the server's cookie into its second ClientHello without checking that the cookie fits in the outgoing packet. Both the length and the contents of that cookie come from the peer, and the write happens before certificate authentication completes, so a malicious or compromised server can overwrite memory past the caller supplied TLS packet buffer without presenting a certificate. Only the client path is affected.. EPSS estimates a 0.28% chance of exploitation in the next 30 days.