CVE-2018-1000520 in mbedTLS
Summary
by MITRE
ARM mbedTLS version 2.7.0 and earlier contains a Ciphersuite Allows Incorrectly Signed Certificates vulnerability in mbedtls_ssl_get_verify_result() that can result in ECDSA-signed certificates are accepted, when only RSA-signed ones should be.. This attack appear to be exploitable via Peers negotiate a TLS-ECDH-RSA-* ciphersuite. Any of the peers can then provide an ECDSA-signed certificate, when only an RSA-signed one should be accepted..
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Analysis
by VulDB Data Team • 03/28/2023
The vulnerability identified as CVE-2018-1000520 affects ARM mbedTLS versions 2.7.0 and earlier, specifically within the mbedtls_ssl_get_verify_result() function. This represents a critical cryptographic flaw that undermines the certificate verification process in TLS implementations. The vulnerability stems from an incorrect validation mechanism that fails to properly enforce signature algorithm constraints during certificate verification. When peers negotiate TLS-ECDH-RSA-* ciphersuites, the system incorrectly accepts ECDSA-signed certificates while properly rejecting RSA-signed ones, creating a significant security gap in the certificate validation process.
The technical flaw manifests in the improper handling of certificate signature algorithms within the SSL/TLS handshake process. The mbedTLS library fails to maintain strict enforcement of the expected signature algorithm based on the negotiated ciphersuite, allowing for certificate forgery attacks. This vulnerability specifically impacts the TLS-ECDH-RSA-* ciphersuite family where the cryptographic key exchange mechanism expects RSA-signed certificates but accepts ECDSA signatures due to inadequate verification logic. The flaw essentially creates a bypass mechanism that allows attackers to substitute ECDSA-signed certificates for the expected RSA-signed certificates, potentially enabling man-in-the-middle attacks and certificate impersonation.
Operationally, this vulnerability presents a severe risk to any system utilizing affected mbedTLS versions in TLS implementations. The attack vector requires only that peers negotiate a TLS-ECDH-RSA-* ciphersuite, making it relatively accessible to potential attackers. Once exploited, the vulnerability can enable attackers to establish fraudulent TLS connections, impersonate legitimate services, and potentially intercept or modify encrypted communications. The impact extends beyond simple certificate validation failures to encompass broader trust model compromises, as the system incorrectly validates certificates that should not be accepted based on the negotiated cryptographic parameters. This vulnerability aligns with CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) and represents a failure in proper cryptographic protocol enforcement.
The security implications of this vulnerability extend to the fundamental trust mechanisms underlying TLS communications. Attackers can exploit this flaw to perform certificate substitution attacks, where they present ECDSA-signed certificates in contexts where RSA signatures are required and validated. This misalignment between expected and actual cryptographic signatures creates opportunities for authentication bypasses and cryptographic downgrade attacks. The vulnerability also maps to ATT&CK technique T1552.001 (Unsecured Credentials) and T1046 (Network Service Scanning) as attackers can leverage this weakness to establish unauthorized connections and potentially gain access to sensitive communications channels. Organizations using affected mbedTLS versions should prioritize immediate patching and deployment of updated library versions that properly enforce signature algorithm validation based on negotiated ciphersuites.
Mitigation strategies should include immediate upgrading to mbedTLS versions that address this vulnerability, typically those beyond 2.7.0. System administrators should conduct thorough inventory assessments to identify all affected systems and applications utilizing the vulnerable library. Additional defensive measures include implementing strict certificate validation policies, monitoring for anomalous certificate signature patterns, and deploying network intrusion detection systems to identify potential exploitation attempts. The vulnerability highlights the importance of maintaining up-to-date cryptographic libraries and proper security testing of cryptographic implementations to prevent such critical validation failures that can compromise entire communication infrastructures.