CVE-2019-6110 in OpenSSH
Summary
by MITRE
In OpenSSH 7.9, due to accepting and displaying arbitrary stderr output from the server, a malicious server (or Man-in-The-Middle attacker) can manipulate the client output, for example to use ANSI control codes to hide additional files being transferred.
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Analysis
by VulDB Data Team • 12/18/2025
The vulnerability identified as CVE-2019-6110 represents a significant security flaw in OpenSSH version 7.9 that exploits the client's handling of server-side stderr output. This issue stems from the client's design to accept and display arbitrary error messages from the server during the connection process, creating an attack surface that malicious actors can leverage to manipulate user perception of file transfers. The vulnerability is particularly concerning because it operates at the user interface level, making it difficult to detect and potentially leading to successful deception attacks.
The technical flaw manifests when an attacker-controlled server or man-in-the-middle position can inject ANSI control codes into the stderr stream that the OpenSSH client processes and displays. These control codes allow the attacker to manipulate the visual output of the client, potentially hiding or obscuring files that are actually being transferred or processed. The vulnerability is classified under CWE-200, which deals with information exposure, and specifically relates to CWE-119, which involves insufficient protection of data within a system. The attack vector exploits the trust relationship between SSH client and server, where the client's default behavior of displaying all stderr output without proper sanitization creates opportunities for malicious interference.
The operational impact of this vulnerability extends beyond simple information hiding, as it can facilitate more sophisticated attacks such as credential theft, data exfiltration, or file manipulation during transfer operations. When users observe what appears to be normal file transfer activity, they may be unknowingly witnessing only a portion of the actual operations occurring, with the malicious server concealing additional files or activities through the ANSI control code manipulation. This vulnerability aligns with ATT&CK technique T1071.004, which covers application layer protocol: ssh, and specifically targets the client-side deception capabilities that can be leveraged to bypass security monitoring systems.
Mitigation strategies for CVE-2019-6110 focus primarily on updating to patched versions of OpenSSH where the handling of stderr output has been improved to prevent arbitrary control code injection. System administrators should also implement network-level monitoring to detect unusual stderr output patterns and consider disabling verbose error display in production environments where security is paramount. Additional protective measures include implementing proper certificate validation, using SSH key-based authentication, and maintaining up-to-date security patches across all SSH client implementations to prevent exploitation of this and similar vulnerabilities in the broader SSH ecosystem.