libssh2 CVE-2026-58051

Exploitation requires that the victim is a libssh2 client (version ≤ 1.11.1) that initiates an SSH connection to a server the attacker controls or has compromised, and that the connection uses the publickey subsystem so the vulnerable list-growth code in publickey.c is exercised. The attacker must send a malformed publickey response crafted to cause a parse failure that reaches the cleanup path (libssh2_publickey_list_free) before the new list entry is initialized. Limiting factors: the attacker cannot reach a passive client - the victim must actively connect outbound to the malicious server; the publickey subsystem code path must be used; and AC:H reflects that steering the uninitialized attrs pointer to an attacker-useful value (beyond a crash) is difficult, so the most dependable impact is denial of service rather than guaranteed code execution.

This is a genuine, prioritize-worthy client-side risk because exploitation is network-reachable with no authentication (PR:N, UI:N) and public PoC code exists, but several signals temper it. The provided CVSS 4.0 vector (AV:N/AC:H/AT:N/PR:N/UI:N/VC:N/VI:L/VA:H) scores 8.3 (High) and emphasizes availability impact (VA:H) with limited integrity impact (VI:L) and explicitly no confidentiality impact (VC:N) - notable because the input Tags label this 'Information Disclosure,' which directly conflicts with VC:N; treat the information-disclosure characterization as unconfirmed and likely a tagging artifact rather than a demonstrated leak. AC:H reflects that the attacker must steer an uninitialized pointer to a useful value through a precisely malformed response, so reliable code execution is non-trivial while a denial-of-service crash is straightforward. The threat model is also inverted from typical server CVEs: the victim is the connecting client, so risk concentrates in software that initiates outbound SSH to untrusted or attacker-controlled servers. No EPSS score was supplied, and the issue is not in CISA KEV, so widespread automated exploitation is not currently indicated.

An attacker stands up or compromises an SSH server and lures or coerces a libssh2-based client (for example an automation job, sync tool, or appliance) into connecting and negotiating the publickey subsystem. The server returns a deliberately malformed publickey response that forces a parse failure into the cleanup path, causing the client to free an uninitialized attrs pointer; publicly available PoC code demonstrates triggering this calc/cleanup flaw. With AC:H, the most reliable outcome is a client crash (DoS), while shaping the uninitialized pointer toward exploitable memory corruption is harder but conceivable.