CVE-2017-5243 in Nexpose Hardware Appliance
Summary
by MITRE
The default SSH configuration in Rapid7 Nexpose hardware appliances shipped before June 2017 does not specify desired algorithms for key exchange and other important functions. As a result, it falls back to allowing ALL algorithms supported by the relevant version of OpenSSH and makes the installations vulnerable to a range of MITM, downgrade, and decryption attacks.
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Analysis
by VulDB Data Team • 10/14/2019
The vulnerability identified as CVE-2017-5243 affects Rapid7 Nexpose hardware appliances that were shipped prior to June 2017, representing a critical security flaw in the default SSH configuration implementation. This issue stems from the absence of explicit algorithm specifications for key exchange and other essential cryptographic functions within the SSH daemon configuration. The vulnerability operates at the foundational level of network security infrastructure, where improper cryptographic configuration can undermine the entire security posture of affected systems. The default configuration essentially permits the use of all available algorithms supported by the OpenSSH version installed on these appliances, creating an expansive attack surface that adversaries can exploit to compromise system integrity and confidentiality.
The technical flaw manifests through the lack of explicit cryptographic algorithm restrictions in the SSH server configuration files, specifically within the sshd_config file. This absence allows the SSH implementation to utilize any supported algorithm for key exchange, encryption, and message authentication, including potentially weak or deprecated algorithms that may be vulnerable to various cryptographic attacks. The vulnerability directly maps to CWE-327, which addresses the use of weak or broken cryptographic algorithms, and CWE-310, which covers cryptographic issues related to key management and algorithm selection. When systems fall back to accepting all available algorithms, they become susceptible to downgrade attacks where adversaries can force connections to use weaker cryptographic methods, and man-in-the-middle attacks that exploit the lack of proper algorithm validation and selection.
The operational impact of this vulnerability extends beyond simple cryptographic weakness to encompass significant security risks for organizations utilizing affected Nexpose appliances. Attackers can leverage this flaw to perform cryptographic downgrade attacks, forcing connections to use insecure key exchange algorithms such as diffie-hellman-group1-sha1, which is vulnerable to computational attacks. The vulnerability also enables decryption attacks where adversaries can potentially exploit the lack of proper algorithm restrictions to intercept and manipulate encrypted communications between the appliance and its administrators. Additionally, the configuration allows for potential MITM attacks that can compromise the integrity of SSH sessions, enabling unauthorized access to system management interfaces and potentially leading to full system compromise. This vulnerability affects the core security mechanisms of the appliance, undermining trust in the communication channels and potentially exposing sensitive network information.
Mitigation strategies for CVE-2017-5243 require immediate configuration updates to restrict the cryptographic algorithms accepted by the SSH daemon. Organizations must explicitly define the acceptable key exchange algorithms, encryption ciphers, and MAC algorithms in the sshd_config file, typically by specifying strong algorithms such as diffie-hellman-group14-sha256, [email protected], and hmac-sha2-256. The configuration should disable weak algorithms including those using SHA-1 hashing, 1024-bit RSA keys, and deprecated Diffie-Hellman groups. Security administrators should implement the principle of least privilege by configuring only the necessary cryptographic algorithms required for legitimate operations, thereby reducing the attack surface. The remediation process aligns with ATT&CK technique T1566, which covers credential access through network sniffing and man-in-the-middle attacks, and T1071, which addresses application layer protocol usage for communication. Organizations should also implement regular security audits to verify that SSH configurations remain compliant with current security standards and that no weak algorithms are inadvertently enabled through configuration updates or system patches. The vulnerability demonstrates the critical importance of proper cryptographic configuration management and the potential consequences of default security settings that do not adequately consider modern threat landscapes and cryptographic best practices.