Heap Out-of-Bounds Read in libssh GSSAPI Key Exchange

Executive Summary

This vulnerability is in libssh during server-side GSSAPI key exchange. A client can send a short Curve25519 public key without proper length validation. The server copies this key using memcpy without checking its length, causing an out-of-bounds heap read. This allows a remote unauthenticated attacker to disclose small amounts of server memory.

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Detection Guidance

Detecting this vulnerability requires monitoring SSH server logs for malformed GSSAPI key exchange attempts. Check for SSH_MSG_KEXGSS_INIT messages with unusually short Curve25519 public keys. Use tcpdump or Wireshark to capture SSH traffic and inspect key exchange packets for anomalies.

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Impact Analysis

An attacker could exploit this to read small portions of server memory. This might expose sensitive data like session keys, user credentials, or other confidential information processed by the server. The impact is limited to information disclosure and requires no authentication.

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Compliance Impact

This vulnerability does not directly impact compliance with GDPR, HIPAA, or similar standards as it involves a low-severity memory disclosure issue in libssh during SSH key exchange. Standards like GDPR and HIPAA focus on data protection and privacy, but this flaw only exposes small amounts of server memory without direct access to sensitive data.

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Mitigation Strategies

Update libssh to the latest patched version immediately. If patching is not possible, disable GSSAPI-based authentication in the SSH server configuration by setting GSSAPIAuthentication no in sshd_config. Restrict SSH access via firewall rules to trusted IPs as a temporary measure.

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