Rsync versions before 3.4.3 contain an off-by-one out-of-bounds stack write vulnerability in the establish proxy connection() function in socket.c that allows network attackers to corrupt stack memory by sending a malformed HTTP proxy response. Attackers can exploit this by positioning themselves between the client and proxy or controlling the proxy server to send a response line of 1023 or more bytes without a newline terminator, causing a null byte to be written to an out-of-bounds stack address when the RSYNC PROXY environment variable is set.