Pre-authentication CPU exhaustion DoS in libssh2 SSH_MSG_EXT_INFO handler
CVE-2026-55199 is a pre-authentication denial-of-service vulnerability in libssh2 affecting versions through 1.11.1. The flaw is in the SSH_MSG_EXT_INFO handler in src/packet.c during SSH key exchange. A malicious SSH server can send a crafted extension count value, specifically setting nr_extensions to 0xFFFFFFFF, which causes the client to enter an excessive processing loop. The issue is caused by insufficient sanity checking of the advertised extension count and unchecked return values from _libssh2_get_string(). As a result, a libssh2-based client can spin in a tight CPU-bound loop for more than 60 seconds before authentication completes.
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Recent activity
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A high-severity pre-authentication denial-of-service vulnerability in libssh2 caused by failure to validate the advertised extension count during SSH key exchange, leading to prolonged CPU consumption and client hangs.
A high-severity pre-authentication denial-of-service vulnerability in libssh2 caused by failure to sanity-check the advertised SSH extension count during key exchange, leading to prolonged CPU consumption and client hang.
A pre-authentication denial-of-service vulnerability in libssh2's SSH_MSG_EXT_INFO handler that can let a malicious SSH server trigger client CPU exhaustion via a crafted extension count value.
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