libssh KEX Memory Leak DoS Vulnerability (CVE-2025-8277)

Vulnerability Summary: libssh Memory Leak and Denial of Service Vulnerability Vulnerability Overview CVE ID: CVE-2025-8277 Vulnerability Type: Memory Leak Severity: CVSSv3 3.1 (AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L) Description: During the key exchange (KEX) process in libssh, if the client sets the flag and repeatedly sends incorrect KEX guesses, a memory leak occurs. Each guess allocates a new temporary key pair without freeing the previous one, eventually leading to memory exhaustion and client crash. Affected Scope Affected Versions: libssh >= 0.6.0, < 0.11.3 Affected Algorithms: - Curve25519 (libcrypto, OpenSSL) - sntrup761x25519 (libcrypto, OpenSSL) - ECDH (libcrypto, mbedtls, OpenSSL) - DH/OH-GEX (OpenSSL, internal implementation) Impact Conditions: - Affects only the client side. - Requires the client to have the flag enabled. - The attacker must be an authenticated client to trigger multiple re-key exchanges. Not Affected: Servers. Remediation Patch Release: A patch for this issue has been released. Upgrade Recommendation: SSH administrators are advised to upgrade to libssh 0.11.3 as soon as possible or apply the patch. Workaround: Do not use KEX guessing (i.e., do not enable ). References RFC 4253 Section 7.1: https://www.rfc-editor.org/rfc/rfc4253#section-7.1 libssh Security Page: https://www.libssh.org/security/ Acknowledgments Original Report: Francesco Rollo Patch Provided by: Francesco Rollo and Jakub Jelen

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libssh KEX Memory Leak DoS Vulnerability (CVE-2025-8277)