A server could send a SSHMSGCHANNELREQUEST packet with an exit signal message with a length of max unsigned integer value. The length would then have a value of 1 added to it and used to allocate memory causing a possible memory write out of bounds error or zero byte allocation.

A server could send a value approching unsinged int max number of keyboard prompt requests which could result in an unchecked interger overflow. The value would then be used to allocate memory causing a possible memory write out of bounds error.

A server could send a specially crafted SSH packet with a padding length value greater than the packet length. This would result in a buffer read out of bounds when decompressing the packet or result in a corrupted packet value.

An out of bounds read flaw was discovered in libssh2 before 1.8.1 in the way SFTP packets with empty payloads are parsed. A remote attacker who compromises a SSH server may be able to cause a Denial of Service or read data in the client memory.

The diffiehellmansha256 function in kex.c in libssh2 before 1.7.0 improperly truncates secrets to 128 or 256 bits, which makes it easier for man-in-the-middle attackers to decrypt or intercept SSH sessions via unspecified vectors, aka a "bits/bytes confusion bug."

In libssh2 v1.9.0 and earlier versions, the SSHMSGDISCONNECT logic in packet.c has an integer overflow in a bounds check, enabling an attacker to specify an arbitrary (out-of-bounds) offset for a subsequent memory read. A crafted SSH server may be able to disclose sensitive information or cause a denial of service condition on the client system when a user connects to the server.

In libssh2 before 1.9.0, kexmethoddiffiehellmangroupexchangesha256keyexchange in kex.c has an integer overflow that could lead to an out-of-bounds read in the way packets are read from the server. A remote attacker who compromises a SSH server may be able to disclose sensitive information or cause a denial of service condition on the client system when a user connects to the server. This is related to an libssh2checklength mistake, and is different from the various issues fixed in 1.8.1, such as CVE-2019-3855.

Fabian Bäumer, Marcus Brinkmann, Jörg Schwenk discovered that the SSH protocol was vulnerable to a prefix truncation attack. If a remote attacker was able to intercept SSH communications, extension negotiation messages could be truncated, possibly leading to certain algorithms and features being downgraded. This issue is known as the Terrapin attack. This update adds protocol extensions to mitigate this issue.