Python ssl Module Certificate Verification Bypass Vulnerability (CVE-2019-9636) Analysis and Patch

Vulnerability Summary: Python Module Certificate Verification Bypass (CVE-2019-9636) 1. Vulnerability Overview Vulnerability Type: Certificate Verification Bypass. Core Principle: In Python's module, when the server sends a certificate chain, if the intermediate Certificate Authority (CA) certificate contains the extension with but lacks the extension (or the extension does not include ), Python's verification logic incorrectly accepts the certificate. Consequence: An attacker can forge an intermediate CA certificate to issue legitimate-looking server certificates. This causes clients (such as , , and other libraries depending on Python's SSL library) to erroneously trust the forged certificate, enabling Man-in-the-Middle (MITM) attacks. 2. Affected Scope Affected Component: Python module (underlying dependency: OpenSSL). Affected Versions: Python 3.7.x (versions prior to 3.7.4) Python 3.6.x (versions prior to 3.6.9) Python 2.7.x (versions prior to 2.7.16) Trigger Condition: The certificate chain sent by the server contains an intermediate CA certificate that lacks the extension. 3. Remediation Official Patch: Upgrade Python to secure versions (3.7.4, 3.6.9, 2.7.16, or later). Code-Level Fix (Based on the Commit in the Screenshot): Modified the function in . Key Logic Change: Explicitly set when creating the SSL Context. Enhanced Verification Logic: Added checks for the extension in the or related verification processes. If a certificate is marked as a CA ( ), it must contain the permission; otherwise, verification fails. 4. POC / Exploitation Code Extraction The screenshot displays the code comparison before and after the fix. Below are the key logic snippets before the fix (vulnerable) (from ): Underlying C Code Fix Logic ( ): The screenshot shows that strict certificate chain verification logic was added in to ensure that if is present, must be validated. Brief Description of Exploitation Scenario (POC Approach): 1. The attacker generates a self-signed root CA. 2. The attacker uses the root CA to issue an intermediate CA certificate, which contains but does not contain . 3. The attacker uses the intermediate CA to issue a server certificate for a target domain (e.g., ). 4. The [Root CA, Intermediate CA] is sent as the certificate chain to the client. 5. Vulnerable Python Version: Verification passes (because the old logic assumes that as long as is present, it is valid, ignoring the missing ). 6. Fixed Python Version: Verification fails (because the new logic mandates that a CA must have ).

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Python ssl Module Certificate Verification Bypass Vulnerability (CVE-2019-9636) Analysis and Patch

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