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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.

PUBLISHED Reserved 2026-06-16 | Published 2026-06-17 | Updated 2026-06-18 | Assigner VulnCheck




HIGH: 8.2CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

MEDIUM: 5.9CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

Problem types

Loop with Unreachable Exit Condition ('Infinite Loop')

Product status

Default status
unaffected

Any version
affected

17626857d20b3c9a1addfa45979dadcee1cd84a4 (git)
unaffected

Credits

Tristan Madani (@TristanInSec) finder

References

github.com/libssh2/libssh2/pull/1864 (Pull Request) issue-tracking

github.com/...ommit/17626857d20b3c9a1addfa45979dadcee1cd84a4 (Patch Commit) patch

www.vulncheck.com/...cation-dos-via-ssh-msg-ext-info-handler third-party-advisory

cve.org (CVE-2026-55199)

nvd.nist.gov (CVE-2026-55199)

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