CVE-2026-55199

Analyzed Analyzed - Analysis Complete

libssh2 Pre-Authentication Denial of Service via Malicious SSH Server

Publication date: 2026-06-17

Last updated on: 2026-06-26

Assigner: VulnCheck

Description

libssh2 through 1.11.1, fixed in commit 1762685, contains a pre-authentication denial of service vulnerability in the SSH_MSG_EXT_INFO handler in src/packet.c that allows a malicious SSH server to cause a client CPU exhaustion loop by sending a crafted extension count value. A malicious server can set nr_extensions to 0xFFFFFFFF during key exchange, causing the client to spin in a tight CPU loop for over 60 seconds because return values from _libssh2_get_string() are unchecked and the session timeout does not apply to CPU-bound loops.

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Meta Information

Published

2026-06-17

Last Modified

2026-06-26

Generated

2026-07-08

AI Q&A

2026-06-18

EPSS Evaluated

2026-07-06

NVD

CVE-2026-55199

EUVD

EUVD-2026-37782

Affected Vendors & Products

Vendor	Product	Version / Range
libssh2	libssh2	to 1.11.1 (inc)

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-835	The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.

Attack-Flow Graph

Executive Summary

This vulnerability exists in libssh2 versions through 1.11.1 and involves a pre-authentication denial of service issue in the SSH_MSG_EXT_INFO handler located in src/packet.c.

A malicious SSH server can exploit this by sending a specially crafted extension count value (nr_extensions set to 0xFFFFFFFF) during the key exchange process.

Because the client does not properly check return values from the _libssh2_get_string() function and the session timeout does not apply to CPU-bound loops, the client enters a tight CPU exhaustion loop lasting over 60 seconds.

Impact Analysis

This vulnerability can cause a denial of service condition on the client side by exhausting the CPU resources.

When connecting to a malicious SSH server, the client may become unresponsive or significantly slowed down for over a minute due to the CPU spinning in a tight loop.

This can disrupt normal operations, potentially causing delays or failures in automated processes or user activities relying on SSH connections.

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