Once the TLS handshake completes, QUICConn.HandleData buffers data and passes it to handlePostHandshakeMessage every time the buffer contains a complete message. The size check is wrong, however, so it can pass along a partial message, triggering a panic when handlePostHandshakeMessage tries to read the remainder of the message.

In addition, HandleData doesn't limit the amount of data it can buffer. It should reject messages larger than maxHandshake.

Thanks to @marten-seemann for reporting this issue.

Normally, we'd consider this a PRIVATE track vulnerability, but this is a very new API and the only known user (quic-go) has already released a workaround in a patch release, so we're calling it PUBLIC track.

The panic due to partial messages is CVE-2023-39321.
The lack of a limit on buffered post-handshake data is CVE-2023-39322.