libssh2 CVE-2026-58050

Exploitation requires the victim libssh2 client (through 1.11.1) to initiate a connection to an attacker-controlled or compromised SSH server and to enter the publickey subsystem code path, and the client must be built for a 32-bit architecture so that num_attrs * sizeof(libssh2_publickey_attribute) overflows 32-bit size_t. The attacker needs no credentials on the client (PR:N) and no separate user click beyond the client establishing the SSH session, but the attack is bounded by AC:H factors: it does not work against 64-bit builds, it depends on the application actually exercising the publickey subsystem, and it requires steering the victim to the malicious server. These limiting factors - 32-bit-only, untrusted-server-required - substantially narrow real-world exposure below what the network attack vector alone implies.

The provided CVSS 4.0 vector (AV:N/AC:H/AT:N/PR:N/UI:N, VC:L/VI:L/VA:H, base 8.3) captures the key tension: the attack is network-reachable and needs no privileges, but AC:H reflects real constraints - the victim client must connect to an attacker-controlled SSH server and the target must be a 32-bit build for the multiplication to overflow, which excludes the now-dominant 64-bit deployments. Availability impact is High (memory corruption / crash) while confidentiality and integrity are only Low, consistent with a heap overflow whose reliable code-execution outcome is uncertain. Publicly available exploit code (VulnCheck-reported, with a GitHub PoC) raises practical risk, but there is no CISA KEV listing and no EPSS score supplied, so there is no evidence of widespread active exploitation - treat this as a credible, prove-able bug rather than a mass-exploited one. Net priority is moderate-to-high specifically for organizations running 32-bit clients (embedded/IoT, legacy systems) that initiate SSH connections to untrusted endpoints.

An attacker stands up a malicious SSH server (or compromises one) and waits for or lures a 32-bit libssh2 client to connect, for example an automated backup or sync job that uses libcurl SCP. During the publickey exchange the server returns a crafted response with a huge 32-bit attribute count that overflows the allocation, and the subsequent attribute-parsing loop writes past the undersized heap buffer, crashing the client or potentially corrupting heap memory for code execution. A public PoC for this calculation already exists (github.com/bikini/exploitarium libssh2-publickey-list-calc-poc).