Impact Analysis

The vulnerability can cause client-side application instability and availability issues by exhausting system memory through repeated incorrect key exchange attempts. This can lead to crashes or denial-of-service conditions on the client side, especially when using cryptographic backends like libgcrypt. [1]

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Executive Summary

This vulnerability is a memory exhaustion issue in the libssh library's key exchange (KEX) process. When an authenticated client repeatedly sends incorrect guesses during rekeying attempts, libssh fails to free previously allocated memory for ephemeral key pairs. This causes a memory leak that gradually consumes system memory, potentially leading to client-side crashes and denial-of-service conditions. [1]

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Detection Guidance

This vulnerability involves a memory leak triggered by an authenticated client repeatedly sending incorrect key exchange guesses to libssh. Detection would involve monitoring client-side memory usage for abnormal increases during SSH sessions using libssh versions 0.6.0 and later. Network detection could focus on identifying repeated rekeying attempts with incorrect first_kex_packet_follows parameters from authenticated clients. However, no specific detection commands are provided in the available resources. [1]

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Mitigation Strategies

Immediate mitigation steps include updating libssh to a version later than 0.6.0 where this memory leak issue is fixed. Additionally, restricting or monitoring authenticated client rekey attempts to prevent repeated incorrect KEX guesses can reduce exploitation risk. Since the vulnerability affects client-side memory, ensuring clients use patched versions and limiting exposure to untrusted servers or clients is advisable. [1]

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