Heap Buffer Overflow in libssh2 Client Library

Executive Summary

CVE-2026-58050 is a high-severity integer overflow vulnerability in libssh2 versions up to and including 1.11.1. The vulnerability occurs in the publickey subsystem when libssh2 reads a 32-bit attribute count from a malicious SSH server's response. This attribute count is used to calculate memory allocation without proper bounds checking. On 32-bit platforms, this calculation can overflow, resulting in an undersized buffer.

Because of this overflow, an attacker-controlled SSH server can exploit the vulnerability by causing the attribute-parsing loop to write beyond the allocated memory buffer, leading to a heap buffer overflow in the libssh2 client.

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Detection Guidance

Detection of CVE-2026-58050 involves identifying if your system uses a vulnerable version of libssh2 (up to 1.11.1) and monitoring for suspicious or malformed SSH publickey-subsystem responses that could trigger the integer overflow.

Since the vulnerability is triggered by a malicious SSH server sending crafted publickey-subsystem responses, network detection could focus on capturing and analyzing SSH traffic for abnormal attribute counts or malformed packets in the publickey subsystem.

There are no explicit detection commands provided in the available resources. However, you can check the libssh2 version on your system using commands like:

• ldconfig -p | grep libssh2
• dpkg -l | grep libssh2 (on Debian/Ubuntu)
• rpm -qa | grep libssh2 (on RedHat/CentOS)

For network detection, using packet capture tools like tcpdump or Wireshark to filter SSH traffic and inspect publickey-subsystem responses for unusually large attribute counts could help identify exploitation attempts.

Example tcpdump command to capture SSH traffic:

• tcpdump -i <interface> port 22 -w ssh_traffic.pcap

Then analyze ssh_traffic.pcap in Wireshark for anomalies in the publickey subsystem messages.

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Impact Analysis

This vulnerability can lead to a heap buffer overflow in the libssh2 client when connecting to a malicious SSH server. Such a buffer overflow can cause crashes, memory corruption, or potentially allow an attacker to execute arbitrary code on the client system.

Since the vulnerability is exploitable remotely by a malicious SSH server, it poses a significant security risk, especially on 32-bit platforms where the integer overflow occurs.

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Compliance Impact

The provided context and resources do not contain information regarding the impact of CVE-2026-58050 on compliance with common standards and regulations such as GDPR or HIPAA.

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Mitigation Strategies

The vulnerability affects libssh2 versions up to and including 1.11.1. To mitigate this vulnerability, you should upgrade libssh2 to a version later than 1.11.1 where this integer overflow issue is fixed.

Additionally, avoid connecting to untrusted or malicious SSH servers, especially on 32-bit platforms where the overflow occurs.

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