CVE-2016-9015 in Python urllib3 Library
Summary
by MITRE
Versions 1.17 and 1.18 of the Python urllib3 library suffer from a vulnerability that can cause them, in certain configurations, to not correctly validate TLS certificates. This places users of the library with those configurations at risk of man-in-the-middle and information leakage attacks. This vulnerability affects users using versions 1.17 and 1.18 of the urllib3 library, who are using the optional PyOpenSSL support for TLS instead of the regular standard library TLS backend, and who are using OpenSSL 1.1.0 via PyOpenSSL. This is an extremely uncommon configuration, so the security impact of this vulnerability is low.
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Analysis
by VulDB Data Team • 07/19/2020
The CVE-2016-9015 vulnerability resides within the Python urllib3 library versions 1.17 and 1.18, specifically affecting a critical security mechanism that governs TLS certificate validation. This flaw manifests when users employ the optional PyOpenSSL support for TLS operations rather than relying on the standard library's built-in TLS backend. The vulnerability becomes particularly pronounced when the OpenSSL 1.1.0 version is utilized through PyOpenSSL, creating a specific combination that bypasses proper certificate validation procedures. The underlying technical issue stems from how the library handles certificate verification when PyOpenSSL is enabled, leading to a scenario where malicious actors could potentially intercept communications without detection, undermining the fundamental security assurances that TLS protocols are designed to provide.
The operational impact of this vulnerability extends beyond simple certificate validation failures to encompass broader security implications that could compromise sensitive data transfers. When the affected configurations are present, attackers can exploit this weakness to perform man-in-the-middle attacks, potentially gaining access to confidential information transmitted between clients and servers. The vulnerability creates a path for information leakage attacks where intercepted data could be read, modified, or redirected without the knowledge of either party in the communication. This represents a significant risk to applications that rely on urllib3 for network communications, particularly those handling sensitive data such as authentication credentials, personal information, or proprietary business data. The security implications are particularly concerning given that TLS certificate validation is a cornerstone of secure internet communications, and any failure in this mechanism fundamentally undermines the trust model that secure network protocols depend upon.
The vulnerability's low security impact classification stems from the extremely uncommon nature of the specific configuration required to exploit it. This configuration involves simultaneously using urllib3 versions 1.17 or 1.18, enabling PyOpenSSL support, and utilizing OpenSSL 1.1.0 through PyOpenSSL, creating a narrow attack surface that many developers would not encounter in typical deployments. According to CWE standards, this vulnerability would be categorized under CWE-295 which addresses improper certificate validation, specifically focusing on weak or missing certificate validation mechanisms. The ATT&CK framework would classify this under T1566 which covers credential access through man-in-the-middle attacks, and potentially T1041 for data transmission methods that could be exploited. Organizations should consider this vulnerability in their risk assessment frameworks, particularly those with legacy systems that might still be using these specific library versions, as the combination of factors required for exploitation makes it less likely to be targeted by automated attacks but still represents a potential threat vector that security teams should monitor and address proactively. The recommended mitigation involves upgrading to newer versions of urllib3 where this specific validation issue has been resolved, ensuring that the library is not configured to use the problematic PyOpenSSL integration, and implementing additional monitoring for any suspicious network activity that might indicate exploitation attempts.