CVE-2026-27459: pyOpenSSL Buffer Overflow Vulnerability

CVE-2026-27459 Overview

CVE-2026-27459 is a buffer overflow vulnerability in pyOpenSSL, a Python wrapper around the OpenSSL library. The vulnerability exists in versions 22.0.0 through 25.x.x and occurs when a user-provided callback to set_cookie_generate_callback returns a cookie value greater than 256 bytes. When this happens, pyOpenSSL overflows an OpenSSL-provided buffer, potentially leading to memory corruption and code execution.

Critical Impact

This buffer overflow vulnerability could allow attackers to corrupt memory or potentially achieve code execution by exploiting applications that use custom DTLS cookie generation callbacks with improperly validated cookie lengths.

Affected Products

• pyOpenSSL versions >= 22.0.0 and < 26.0.0
• Applications using set_cookie_generate_callback with custom callbacks
• DTLS-enabled Python applications using vulnerable pyOpenSSL versions

Discovery Timeline

• 2026-03-18 - CVE-2026-27459 published to NVD
• 2026-03-18 - Last updated in NVD database

Technical Details for CVE-2026-27459

Vulnerability Analysis

This vulnerability is classified as CWE-120 (Buffer Copy without Checking Size of Input). The flaw exists in the DTLS cookie generation callback handling mechanism within pyOpenSSL. When developers implement custom cookie generation callbacks using set_cookie_generate_callback, the library failed to validate that the returned cookie value fits within the OpenSSL-defined buffer size limit (DTLS1_COOKIE_LENGTH, which is 255 bytes).

The vulnerability is network-accessible and requires high attack complexity with a specific active precondition. Exploitation would require an attacker to influence the behavior of a vulnerable application's cookie generation callback to return an oversized cookie value, which would then overflow the internal OpenSSL buffer.

Root Cause

The root cause is insufficient input validation in the callback wrapper code within src/OpenSSL/SSL.py. When a user-defined callback returned a cookie value, the code directly copied the cookie bytes into the OpenSSL-provided output buffer without first verifying that the cookie length did not exceed DTLS1_COOKIE_LENGTH. This allowed arbitrarily long cookie values to overflow the fixed-size buffer.

Attack Vector

The attack vector is network-based, targeting applications that implement DTLS (Datagram Transport Layer Security) with custom cookie generation callbacks. An attacker would need to trigger conditions where the application's callback returns an oversized cookie. The exploitation path requires:

1. Target application uses pyOpenSSL versions 22.0.0 to 25.x.x
2. Application implements set_cookie_generate_callback with a custom callback
3. Attacker can influence the callback to return a cookie longer than 255 bytes
4. The overflow corrupts adjacent memory, potentially enabling code execution

The following patch demonstrates the fix applied in version 26.0.0:

     def __init__(self, callback: _CookieGenerateCallback) -> None:
         _CallbackExceptionHelper.__init__(self)

+        max_cookie_len = getattr(_lib, "DTLS1_COOKIE_LENGTH", 255)
+
         @wraps(callback)
         def wrapper(ssl, out, outlen):  # type: ignore[no-untyped-def]
             try:
                 conn = Connection._reverse_mapping[ssl]
                 cookie = callback(conn)
+                if len(cookie) > max_cookie_len:
+                    raise ValueError(
+                        f"Cookie too long (got {len(cookie)} bytes, "
+                        f"max {max_cookie_len})"
+                    )
                 out[0 : len(cookie)] = cookie
                 outlen[0] = len(cookie)
                 return 1

Source: GitHub Commit Details

Detection Methods for CVE-2026-27459

Indicators of Compromise

• Application crashes or unexpected terminations in DTLS-enabled Python services
• Memory corruption errors in processes using pyOpenSSL for DTLS operations
• Unusual behavior in applications implementing custom set_cookie_generate_callback handlers
• Stack traces indicating buffer overflows in OpenSSL or pyOpenSSL library code

Detection Strategies

• Audit Python dependencies using pip list or pip freeze to identify pyOpenSSL versions between 22.0.0 and 25.x.x
• Review application code for usage of set_cookie_generate_callback with custom callbacks
• Implement static code analysis to identify custom callback implementations that may return variable-length cookies
• Use memory sanitizers (ASan/MSan) in development environments to detect buffer overflow conditions

Monitoring Recommendations

• Monitor application logs for ValueError exceptions related to cookie length validation after upgrading
• Implement runtime monitoring for memory corruption indicators in DTLS-enabled services
• Set up dependency scanning in CI/CD pipelines to flag vulnerable pyOpenSSL versions
• Track process crashes and core dumps that may indicate exploitation attempts

How to Mitigate CVE-2026-27459

Immediate Actions Required

• Upgrade pyOpenSSL to version 26.0.0 or later immediately
• Audit all custom set_cookie_generate_callback implementations to ensure cookies do not exceed 255 bytes
• Review DTLS-enabled applications for exposure to untrusted network input
• Consider temporarily disabling DTLS functionality if immediate patching is not possible

Patch Information

The vulnerability has been fixed in pyOpenSSL version 26.0.0. The patch adds proper length validation to the cookie generation callback wrapper, raising a ValueError if a callback returns a cookie longer than DTLS1_COOKIE_LENGTH bytes. Organizations should upgrade to version 26.0.0 or later to remediate this vulnerability.

For detailed patch information, refer to the GitHub Security Advisory GHSA-5pwr-322w-8jr4 and the GitHub Commit Details.

Workarounds

• Implement cookie length validation within custom callback functions before returning the cookie value
• Ensure all custom callbacks truncate or reject cookies exceeding 255 bytes
• Add application-level input validation for any data that influences cookie generation
• Consider using fixed-length cookie generation strategies as a temporary measure

# Upgrade pyOpenSSL to patched version
pip install --upgrade pyopenssl>=26.0.0

# Verify installed version
pip show pyopenssl | grep Version