CVE-2026-27448 in pyOpenSSL
Summary
by MITRE • 03/18/2026
pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 0.14.0 and prior to version 26.0.0, if a user provided callback to `set_tlsext_servername_callback` raised an unhandled exception, this would result in a connection being accepted. If a user was relying on this callback for any security-sensitive behavior, this could allow bypassing it. Starting in version 26.0.0, unhandled exceptions now result in rejecting the connection.
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Analysis
by VulDB Data Team • 05/04/2026
The vulnerability identified as CVE-2026-27448 affects pyOpenSSL, a Python library that provides a wrapper around the OpenSSL cryptographic library. This security flaw exists in versions ranging from 0.14.0 through 25.0.0, creating a critical security gap in TLS server name indication handling. The issue specifically impacts the `set_tlsext_servername_callback` function which is designed to allow developers to implement custom logic for handling server name indication during TLS handshakes. When a user-provided callback function raises an unhandled exception during TLS negotiation, the vulnerable versions of pyOpenSSL fail to properly terminate the connection, instead allowing the connection to proceed despite the callback failure.
The technical nature of this vulnerability stems from improper exception handling within the TLS extension processing logic. According to CWE-707, this represents a weakness in the design of exception handling mechanisms where an unhandled exception in a security-critical callback function does not result in appropriate connection termination. The flaw operates at the boundary between application-level callback execution and TLS protocol enforcement, creating a scenario where security-sensitive validation logic can be bypassed due to inadequate error recovery procedures. The vulnerability allows an attacker to potentially exploit the callback mechanism for unauthorized access or bypass security controls that depend on proper server name indication handling.
The operational impact of this vulnerability is significant for systems relying on pyOpenSSL for TLS server functionality. Organizations implementing custom server name indication callbacks for certificate selection, access control, or other security-sensitive purposes may find their protections circumvented when exceptions occur during callback execution. This creates a persistent security risk where malicious actors could potentially establish connections that should have been rejected based on callback validation logic. The vulnerability affects any system using pyOpenSSL versions in the affected range, particularly those implementing custom TLS server name indication handling for multi-tenant environments, certificate management, or application-specific security policies.
The mitigation strategy involves upgrading to pyOpenSSL version 26.0.0 or later, where the behavior has been corrected to reject connections when unhandled exceptions occur in the `set_tlsext_servername_callback`. This upgrade ensures that exception handling now properly terminates connections rather than allowing them to proceed. System administrators should also review existing callback implementations to ensure they properly handle potential exceptions and validate input parameters to prevent unexpected callback failures. Additionally, organizations should consider implementing monitoring for TLS connection failures and callback execution errors as part of their security operations procedures. The fix aligns with ATT&CK technique T1566 which involves bypassing security controls through exploitation of implementation flaws, specifically addressing the vulnerability through proper exception handling and connection termination procedures. Organizations should also verify that their TLS configurations properly validate server name indication data and maintain proper audit trails for connection attempts that might trigger callback exceptions.