Executive Summary

This vulnerability occurs in curl versions 7.58.0 through 8.17.0 when using the libssh backend. During SSH-based transfers with SCP or SFTP that request public key authentication, curl incorrectly attempts to authenticate using a locally running SSH agent even if no agent is explicitly set. This happens due to libssh's API design which falls back to agent authentication. Authentication only succeeds if the local SSH agent has the correct passphrase. This is classified as an improper authentication issue (CWE-287) with low severity. [1, 2]

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Impact Analysis

The vulnerability can lead to unintended authentication attempts using a local SSH agent during SCP or SFTP transfers, potentially allowing authentication without explicit user intent if the local SSH agent holds the correct passphrase. Although the severity is low and no active exploits are known, it could result in unauthorized access if the local SSH agent is compromised or misused. [1, 2]

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Detection Guidance

You can detect this vulnerability by checking the version of curl installed on your system. The vulnerability affects curl versions from 7.58.0 through 8.17.0 when built with the libssh backend. To check the curl version, use the command: `curl --version`. If the version falls within the affected range and uses libssh, your system is vulnerable. Additionally, monitoring SSH-based transfers using SCP or SFTP for unexpected authentication attempts via a local SSH agent may help detect exploitation attempts. However, no specific detection commands for active exploitation are provided. [1, 2]

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Mitigation Strategies

Immediate mitigation steps include upgrading curl to version 8.18.0 or later, where the vulnerability is fixed. Alternatively, you can rebuild curl using the libssh2 backend instead of libssh, or avoid using SCP or SFTP for SSH-based transfers until the update is applied. [1]

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