CVE-2026-27459 in pyOpenSSL
Summary
by MITRE • 03/18/2026
pyOpenSSL is a Python wrapper around the OpenSSL library. Starting in version 22.0.0 and prior to version 26.0.0, if a user provided callback to `set_cookie_generate_callback` returned a cookie value greater than 256 bytes, pyOpenSSL would overflow an OpenSSL provided buffer. Starting in version 26.0.0, cookie values that are too long are now rejected.
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Analysis
by VulDB Data Team • 05/04/2026
The vulnerability identified as CVE-2026-27459 affects the pyOpenSSL library, a critical Python component that serves as a wrapper for the OpenSSL cryptographic library. This library is widely used across enterprise environments and security applications for implementing SSL/TLS protocols and cryptographic operations. The flaw manifests in versions between 22.0.0 and 25.0.0 where the library fails to properly validate cookie values returned by user-defined callbacks, creating a potential buffer overflow condition that could be exploited by malicious actors.
The technical implementation of this vulnerability occurs within the `set_cookie_generate_callback` function which allows developers to define custom cookie generation logic. When a user-provided callback function returns a cookie value exceeding 256 bytes, the pyOpenSSL library does not properly check the length before passing it to OpenSSL's internal buffer management system. This creates a classic buffer overflow scenario where data written to a fixed-size buffer exceeds its allocated capacity, potentially corrupting adjacent memory locations and allowing for arbitrary code execution or denial of service conditions.
From an operational perspective, this vulnerability presents significant risks to systems relying on pyOpenSSL for secure communications, particularly web applications, API gateways, and network security appliances. The buffer overflow could be triggered through crafted cookie values in HTTP requests, potentially leading to application crashes, memory corruption, or in severe cases, remote code execution depending on the system architecture and memory layout. The impact extends beyond individual applications to entire infrastructure components that depend on OpenSSL for secure communications, making this a critical vulnerability for security teams to address immediately.
The vulnerability aligns with CWE-121, which describes stack-based buffer overflow conditions, and represents a classic example of improper input validation in cryptographic libraries. From an attack framework perspective, this vulnerability could be categorized under the MITRE ATT&CK technique T1203, which covers exploitation of software vulnerabilities, specifically targeting the credential access and execution phases of an attack lifecycle. The fix implemented in version 26.0.0 demonstrates proper defensive programming by introducing explicit length validation that rejects cookie values exceeding the safe threshold, thereby preventing the buffer overflow condition from occurring. Organizations should prioritize upgrading to pyOpenSSL version 26.0.0 or later to remediate this vulnerability and ensure continued secure operation of their cryptographic implementations.