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,…

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.

Source published

14 Oct 2026 17:36:17 UTC

Source modified

16 Oct 2026 20:12:39 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

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CVSS 4.0 · 9.3 CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N