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An mvfst QUIC server retained CRYPTO frame data received at an encryption level (Initial or Handshake) that the TLS handshake had already moved past. Because the transport treated this data as delivered, it did not count against the CRYPTO stream buffer limit. A remote, unauthenticated client could complete a handshake, withhold its acknowledgment of the server's HANDSHAKE_DONE frame so that Handshake keys stay in use, and keep sending Handshake-level CRYPTO data at increasing offsets, causing unbounded memory growth on the server and leading to denial of service. This issue affects mvfst versions prior to v2026.11.09.00.
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