CVE-2018-7750 in SSH Server
Summary
by MITRE
transport.py in the SSH server implementation of Paramiko before 1.17.6, 1.18.x before 1.18.5, 2.0.x before 2.0.8, 2.1.x before 2.1.5, 2.2.x before 2.2.3, 2.3.x before 2.3.2, and 2.4.x before 2.4.1 does not properly check whether authentication is completed before processing other requests, as demonstrated by channel-open. A customized SSH client can simply skip the authentication step.
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Analysis
by VulDB Data Team • 12/20/2024
The vulnerability identified as CVE-2018-7750 represents a critical authentication bypass flaw within the Paramiko SSH server implementation that affects multiple version ranges spanning from 1.17.6 through 2.4.1. This issue resides in the transport.py module which governs the core SSH protocol handling mechanisms. The flaw manifests when the SSH server fails to validate authentication completion status before processing subsequent requests, creating a window where unauthorized clients can manipulate the authentication flow. Specifically, during channel-open operations, the server does not properly verify that the client has successfully authenticated before accepting additional commands, allowing malicious actors to exploit this weakness through crafted SSH client implementations.
The technical nature of this vulnerability stems from improper state validation within the SSH protocol stack where the server maintains an inconsistent authentication state. According to CWE-284, this represents an improper access control condition where the system allows operations to proceed without proper authorization checks. The flaw enables what attackers can essentially perform a fast authentication bypass by crafting a custom SSH client that skips the authentication phase entirely, effectively allowing unauthorized access to SSH server resources. This vulnerability directly impacts the principle of least privilege and violates fundamental security assumptions that authentication must be completed before any meaningful operations can occur.
Operationally, this vulnerability creates significant risk for systems running affected Paramiko versions as it allows attackers to establish SSH connections without proper credentials, potentially leading to complete system compromise. The impact extends beyond simple unauthorized access to include potential privilege escalation, data exfiltration, and lateral movement within networks where SSH servers are deployed. Attackers can leverage this vulnerability to establish persistent access points, execute arbitrary commands, and maintain control over compromised systems. The vulnerability's exploitation requires minimal sophistication since it targets the fundamental authentication flow rather than requiring complex cryptographic attacks or system-level privileges.
Mitigation strategies for CVE-2018-7750 primarily involve immediate patching of all affected Paramiko versions to their respective secure releases, with particular attention to the version ranges mentioned in the vulnerability description. Organizations should implement comprehensive network monitoring to detect unusual SSH connection patterns and authentication bypass attempts. According to ATT&CK framework, this vulnerability maps to T1021.004 which covers SSH and remote desktop protocol usage, making it a critical target for defensive measures. Additional controls include implementing strict SSH server configurations, enabling detailed logging of authentication events, and deploying intrusion detection systems that can identify anomalous behavior patterns consistent with authentication bypass attempts. Network segmentation and access control lists should be reviewed to minimize potential blast radius if exploitation occurs, while regular security assessments should verify that all SSH implementations are updated to secure versions.