CVE-2018-7749 in AsyncSSH
Summary
by MITRE
The SSH server implementation of AsyncSSH before 1.12.1 does not properly check whether authentication is completed before processing other requests. A customized SSH client can simply skip the authentication step.
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Analysis
by VulDB Data Team • 02/21/2023
The vulnerability identified as CVE-2018-7749 affects the AsyncSSH library version 1.12.1 and earlier, representing a critical flaw in the SSH server implementation that undermines fundamental authentication mechanisms. This weakness stems from inadequate validation of authentication status before processing subsequent SSH protocol requests, creating a pathway for malicious actors to bypass the standard authentication workflow entirely. The vulnerability operates at the core of SSH protocol security by allowing unauthorized access through a simple manipulation of the client-server communication sequence.
The technical flaw manifests as a failure in the server-side implementation to verify that authentication has been successfully completed before accepting and processing additional SSH protocol commands. This occurs because the SSH server does not maintain proper state tracking to ensure that all required authentication steps have been fulfilled before proceeding with request handling. Attackers can exploit this by crafting a custom SSH client that skips the standard authentication sequence, allowing immediate access to server resources without proper credentials. The vulnerability essentially creates a race condition or state management flaw where the server processes requests without proper authorization verification, violating fundamental security principles of access control.
From an operational impact perspective, this vulnerability enables unauthorized access to SSH servers running vulnerable versions of AsyncSSH, potentially allowing attackers to execute arbitrary commands, access sensitive data, or establish persistent access to affected systems. The implications extend beyond simple unauthorized access as the vulnerability can be leveraged to perform privilege escalation attacks or maintain covert access to compromised systems. Security professionals must consider that this flaw can be exploited by adversaries with minimal technical expertise, making it particularly dangerous in environments where SSH servers are extensively used for administrative access to critical infrastructure.
The vulnerability aligns with CWE-284, which addresses improper access control in software implementations, and can be mapped to ATT&CK technique T1021.004 for remote services exploitation. Organizations using AsyncSSH servers should prioritize immediate patching to version 1.12.1 or later, as this update resolves the authentication state checking mechanism that was previously bypassable. Additional mitigations include implementing network-level access controls, monitoring SSH connection attempts for anomalous behavior, and ensuring that only authorized clients can establish connections to SSH servers. Security teams should also consider implementing intrusion detection systems that can identify attempts to exploit this specific vulnerability pattern, as the attack methodology is relatively straightforward and detectable through behavioral analysis of SSH traffic.