Pre-authentication 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 the SSH key-exchange phase. libssh2 does not properly sanity-check the advertised extension count and also fails to adequately handle error returns from _libssh2_get_string(). A malicious SSH server can send a crafted SSH_MSG_EXT_INFO message with nr_extensions set to 0xFFFFFFFF, causing the client to iterate in a tight CPU-bound loop. Because the loop is CPU-bound, the normal session timeout does not interrupt it. The issue is fixed by commit 1762685.
Are you exposed to this one?
Mallory correlates every CVE against your assets, your vendors, and active adversary campaigns. Know which vulnerabilities matter for you, not just which ones are loud.
Impact, mitigation & remediation
What it means. What to do now. Patch path, mitigations, and the assume-compromise checklist.
Impact
What an attacker gets, and what they’ve been doing with it.
Mitigation
If you can’t patch tonight, do this now.
Remediation
Patch, then assume compromise.
Exploits
No public exploits tracked yet. Mallory keeps watching.
No public exploit code observed for this vulnerability.
Affected products & vendors
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
Recent activity
6 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
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.
The version that knows your environment.
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.