CVE-2025-8277: 
Linux Debian vulnerability analysis and mitigation

A memory exhaustion vulnerability (CVE-2025-8277) was discovered in libssh's key exchange (KEX) handling mechanism. The vulnerability affects libssh versions 0.6.0 and later, identified in September 2025. The flaw exists in the library's handling of key exchange processes when clients repeatedly send incorrect KEX guesses (Red Hat CVE, Red Hat Bugzilla).

The vulnerability occurs when an authenticated client repeatedly performs rekeying with incorrect firstkexpacketfollows guesses. During these operations, libssh allocates new ephemeral key pairs without properly freeing old ones stored in session->nextcrypto. This flaw affects multiple KEX algorithms including Curve25519, ECDH, sntrup761x25519, and DH-GEX across various crypto backends (libgcrypt, OpenSSL, mbedTLS). The vulnerability has been assigned a CVSS v3.1 base score of 3.1 (Low) with the vector string CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:U/C:N/I:N/A:L (NVD).

The vulnerability can lead to gradual memory exhaustion on the client side, potentially resulting in system crashes, particularly when using libgcrypt. This impacts application stability and availability. The issue specifically affects the client side and does not impact server operations (Red Hat CVE).

The vulnerability has been reported and is currently being tracked. System administrators and developers using libssh should monitor for updates and patches from their respective vendors. The issue has been assigned CWE-401 (Missing Release of Memory after Effective Lifetime) (NVD).