CVE-2026-19553 in Python
Summary
by MITRE • 09/30/2026
ssl.SSLContext.wrap_bio() didn't require the server_hostname argument to not be None if ssl.SSLContext.check_hostname was set. Due to a missing parameter check in SSLObject, if the server_hostname argument isn't supplied then hostname verification would be silently skipped.
This defect could lead to programs where certificate hostname verification *appeared* to be succeeding with SSLContext.check_hostname = True and no ValueError being raised due to misconfiguration.
If the program passes a server_hostname value that isn't an empty string or None to any of these APIs then certificate hostname verification proceeds as expected and the program is not affected by this vulnerability.
Mitigating this vulnerability doesn't require updating Python or applying the patch. To mitigate, pass a valid non-None and non-empty server_hostname value to SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls() and certificate hostname verification will proceed as expected. Upgrading to the latest version of Python or applying the patch only changes the behavior from silently skipping hostname verification to raising a ValueError, similar to SSLContext.wrap_socket(), when server_hostname isn't supplied.
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified in this CVE description relates to a critical flaw within the Python standard library's Secure Sockets Layer implementation, specifically affecting the ssl.SSLContext.wrap_bio() method and related asynchronous APIs such as asyncio.create_connection() and asyncio.loop.start_tls(). The core technical issue stems from an insufficient parameter validation mechanism within the SSLObject class. When developers configure an SSL context with hostname verification enabled by setting ssl.SSLContext.check_hostname to True, they expect that any incoming certificate will be rigorously validated against a specified server identity. However, due to this defect, if the server_hostname argument is omitted or explicitly passed as None, the system fails to raise an exception indicating misconfiguration. Instead, it silently bypasses the hostname verification process entirely. This behavior creates a dangerous false sense of security where applications believe they are performing mutual authentication and integrity checks when, in reality, they are accepting any certificate presented by the peer without validating its identity against the expected host name.
From a technical perspective, this flaw represents a significant deviation from the principle of least privilege and secure defaults. In cryptographic protocols like TLS, hostname verification is essential to prevent man-in-the-middle attacks where an attacker might present a validly signed but malicious certificate for a different domain. By allowing None or missing values to pass through without triggering an error, the implementation violates expected security postures defined in industry standards such as CWE-295 Improper Certificate Validation and CWE-347 Improper Verification of Cryptographic Signature. The absence of explicit validation means that the application logic proceeds under the assumption that a secure channel has been established with the correct entity, while an attacker could potentially intercept traffic by presenting a certificate for any domain, provided it is signed by a trusted root authority. This silent failure mode is particularly insidious because standard error handling routines will not catch this condition, allowing vulnerable code to continue operating in an insecure state without developer awareness.
The operational impact of this vulnerability is severe for applications relying on automated SSL/TLS connections where the server hostname might be dynamically determined or inadvertently left unset during initialization phases. If a program passes a valid non-empty string for server_hostname, the verification proceeds correctly and the application remains secure against identity spoofing attacks associated with this specific flaw. However, in scenarios involving configuration management systems, dynamic service discovery tools, or legacy codebases where hostname parameters might be conditionally assigned based on environment variables that could resolve to null values, the risk is substantial. Attackers exploiting this weakness can perform man-in-the-middle attacks by intercepting communications and presenting certificates for unrelated but trusted domains. Since no ValueError is raised, logging mechanisms typically used for security auditing will not record a failed handshake attempt due to hostname mismatch, making detection of such intrusions significantly more difficult for security operations teams relying on standard TLS failure logs.
Mitigation strategies focus primarily on code-level adjustments rather than immediate software updates, although upgrading Python or applying the official patch is recommended as it changes the behavior from silent skipping to raising a ValueError when server_hostname is not supplied. This aligns with best practices outlined in ATT&CK technique T1078 Valid Accounts and related network-based attack vectors where identity verification is bypassed. To immediately mitigate this vulnerability, developers must ensure that all calls to SSLContext.wrap_bio(), asyncio.create_connection(), or asyncio.loop.start_tls() include a valid, non-None, and non-empty server_hostname argument. This explicit provision forces the underlying cryptographic library to perform the necessary comparison between the certificate's subject alternative names or common name and the expected host identifier. By enforcing this parameter requirement at the application layer, organizations can restore the intended security controls until they are able to upgrade their Python environments to versions that enforce these checks natively through exception handling mechanisms similar to those in SSLContext.wrap_socket().