Part of the SFTP handshake involves "extensions", which are key/value pairs, comprised of strings. In SSH, strings are encoded for network transport as a 32-bit length, followed by the bytes. The mod_sftp module currently places no bounds/length limitations when reading these SFTP extension key/value data from the network. A malicious attacker might attempt to encode large values, and allocate more memory than is necessary, causing excessive resource usage or the FTP daemon to crash.

This update limits the amount of memory allocated to handle these extensions.