[mysqlnd] Fix OK packet message length buffer over-read - #23497
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The OK packet message-length varint is read after the last bounds check, so a length-encoded integer at the end of a packet can advance p past header.size and even past the end of the 4096-byte command buffer. The old MIN(net_len, buf_len - (p - begin)) clamp then underflows and passes an unclamped attacker-controlled length to mnd_pestrndup(), reading heap memory beyond both the packet and its allocation. Reject a message length that extends past the payload, matching php_mysqlnd_auth_response_read() from GHSA-h35g-vwh6-m678; an audit found no further readers using the vulnerable buf_len clamp.
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php_mysqlnd_ok_read() reads the length-encoded message length after its last bounds check, so a hostile varint at the end of an OK packet filling the 4096-byte buffer pushes p past header.size and the buffer end, underflows the MIN(net_len, buf_len - (p - begin)) clamp, and mnd_pestrndup() copies attacker-controlled bytes beyond the allocation. Smaller packets left stale command-buffer content in the message. A message length that extends past the payload is now rejected, matching php_mysqlnd_auth_response_read() from GHSA-h35g-vwh6-m678.