CVE-2023-23931: Cryptography.io Buffer Mutation Vulnerability

CVE-2023-23931 Overview

CVE-2023-23931 is a vulnerability in the Python cryptography package that allows immutable objects to be mutated through the Cipher.update_into method. This library is widely used to expose cryptographic primitives and recipes to Python developers. The vulnerability occurs because Cipher.update_into incorrectly accepts Python objects that implement the buffer protocol but provide only immutable buffers, such as bytes objects. This allows fundamental rules of Python to be violated, potentially resulting in corrupted cryptographic output.

Critical Impact

Applications using the cryptography library may produce corrupted output when immutable buffer objects are passed to Cipher.update_into, leading to potential data integrity issues and unpredictable application behavior.

Affected Products

• cryptography.io cryptography (versions 1.8 and later until patched)
• Python applications utilizing the Cipher.update_into method
• Systems with vulnerable cryptography package installations

Discovery Timeline

• February 7, 2023 - CVE CVE-2023-23931 published to NVD
• November 3, 2025 - Last updated in NVD database

Technical Details for CVE-2023-23931

Vulnerability Analysis

This vulnerability represents an Improper Check for Unusual or Exceptional Conditions (CWE-754). The Cipher.update_into method in the cryptography library was designed to write encrypted or decrypted data directly into a provided buffer for performance optimization. However, the method failed to properly validate whether the provided buffer was mutable before writing to it.

In Python, the buffer protocol allows objects to expose their internal memory for direct access. Some objects like bytes implement this protocol but only provide read-only (immutable) access. The Cipher.update_into method did not distinguish between mutable and immutable buffers, allowing writes to occur on objects that should be immutable.

This behavior violates Python's fundamental guarantee that immutable objects remain unchanged after creation. When cryptographic operations write to an immutable buffer, the resulting data corruption can lead to unpredictable application behavior, failed cryptographic operations, or security-sensitive data being incorrectly processed.

Root Cause

The root cause is the missing validation check in Cipher.update_into that should verify the buffer's mutability before attempting write operations. The method accepted any object implementing the buffer protocol without verifying that the buffer supports write operations. This oversight has been present in the codebase since the update_into method was originally introduced in cryptography version 1.8.

Attack Vector

The vulnerability can be exploited over the network in scenarios where an attacker can influence the input parameters passed to cryptographic operations. An attacker could potentially craft inputs that cause immutable buffer mutation, resulting in:

• Corrupted cryptographic output that may bypass integrity checks
• Unpredictable application behavior due to violated Python invariants
• Potential memory corruption scenarios in certain Python implementations

The fix ensures that an exception is now properly raised when an immutable buffer is passed to Cipher.update_into, preventing the corruption from occurring. For technical implementation details, refer to the GitHub Security Advisory.

Detection Methods for CVE-2023-23931

Indicators of Compromise

• Unexpected behavior in applications using cryptographic operations with the cryptography library
• Corrupted or unexpected output from encryption/decryption operations
• Application crashes or errors related to buffer operations in cryptographic contexts
• Log entries indicating memory or buffer-related anomalies during crypto operations

Detection Strategies

• Audit application code for usage of Cipher.update_into method with immutable objects like bytes
• Implement dependency scanning to identify vulnerable versions of the cryptography package
• Monitor application logs for unexpected cryptographic output or buffer-related errors
• Use Software Composition Analysis (SCA) tools to track vulnerable library versions

Monitoring Recommendations

• Enable verbose logging for cryptographic operations to detect anomalous behavior
• Implement integrity checks on cryptographic output to identify corruption
• Monitor package management systems for updates to the cryptography library
• Set up alerts for dependency vulnerability notifications from security advisories

How to Mitigate CVE-2023-23931

Immediate Actions Required

• Upgrade the cryptography package to the latest patched version immediately
• Review application code for instances of Cipher.update_into usage with immutable buffers
• Replace any bytes objects passed to Cipher.update_into with mutable bytearray objects
• Run comprehensive tests on cryptographic functionality after upgrading

Patch Information

The vulnerability has been addressed by the cryptography maintainers. The fix adds proper validation to raise an exception when an immutable buffer is passed to Cipher.update_into. Users should upgrade to the patched version as indicated in the GitHub Security Advisory. The specific commit containing the fix is available in the GitHub Commit Details.

Additional advisories have been published by Debian LTS and NetApp for affected distributions and products.

Workarounds

• Use bytearray objects instead of bytes when calling Cipher.update_into to ensure mutable buffers are used
• Implement wrapper functions that validate buffer mutability before passing to cryptographic methods
• Consider using the standard Cipher.update method which returns data rather than writing to a buffer
• Add application-level checks to verify buffer types before cryptographic operations

# Upgrade cryptography package to patched version
pip install --upgrade cryptography

# Verify installed version
pip show cryptography | grep Version

# Check for vulnerable versions in requirements
pip-audit --requirement requirements.txt