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CVE-2026-55199 is a high-severity 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 supply a crafted extension count, such as an extreme nr_extensions value, that drives the client into a loop with an unreachable exit condition. The issue is caused by insufficient validation of the advertised extension count and unchecked return values from _libssh2_get_string(), allowing the client to continue looping even when parsing fails. Because the condition is reachable before authentication, any application using libssh2 as an SSH client can be affected when connecting to an attacker-controlled or attacker-impersonated server.
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25 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
High-severity pre-authentication denial-of-service vulnerability in libssh2 caused by an infinite loop in the SSH_MSG_EXT_INFO handler, triggerable during key exchange with an extreme extension count value.
Another libssh2 vulnerability mentioned as an additional fix to apply, but not described in detail in the content.
A vulnerability affecting libssh2, which is compiled into Cargo, and fixed in Rust 1.96.1.
A libssh2 denial-of-service vulnerability that can trap a connecting client in a CPU loop via a bogus extension count.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.