libre is a generic library for real-time communications with asynchronous input and output support. Prior to 4.8.1, the websockdecode() function in src/websock/websock.c contains an integer overflow when validating a masked WebSocket frame that uses the 64-bit extended length encoding. The expression 4 + hdr->len can wrap when hdr->len is close to UINT64MAX, causing the mbufgetleft() bounds check to pass. The subsequent XOR unmasking loop then writes beyond the heap buffer. Applications using websockaccept() or websockacceptproto() to implement a WebSocket server are affected, and exploitation can cause attacker-controlled heap corruption or denial of service after the HTTP WebSocket upgrade handshake. This issue is fixed in version 4.8.1.