CVE-2026-34871: Mbed TLS PRNG Seed Vulnerability

CVE-2026-34871 Overview

A cryptographic vulnerability has been discovered in Mbed TLS and TF-PSA-Crypto involving a predictable seed in the Pseudo-Random Number Generator (PRNG). This weakness (CWE-338: Use of Cryptographically Weak Pseudo-Random Number Generator) can allow attackers with local access to potentially predict cryptographic keys or other security-sensitive random values generated by the affected libraries.

Critical Impact

Predictable PRNG seeding can compromise the confidentiality and integrity of cryptographic operations, potentially allowing attackers to predict generated keys, nonces, or other security-critical random values used in TLS connections and cryptographic protocols.

Affected Products

• Mbed TLS versions before 3.6.6
• Mbed TLS 4.x versions before 4.1.0
• TF-PSA-Crypto versions before 1.1.0

Discovery Timeline

• 2026-04-01 - CVE-2026-34871 published to NVD
• 2026-04-01 - Last updated in NVD database

Technical Details for CVE-2026-34871

Vulnerability Analysis

This vulnerability falls under the category of Insecure Random Number Generation. The affected versions of Mbed TLS and TF-PSA-Crypto contain a flaw in how the PRNG is seeded, resulting in predictable entropy sources being used during initialization. Cryptographic random number generators require high-quality entropy to ensure that generated values cannot be predicted or reproduced by attackers.

When the PRNG is initialized with a predictable seed, the entire sequence of random numbers it produces becomes deterministic. An attacker who can determine or predict the seed value can reconstruct the random number sequence, potentially compromising private keys, session keys, initialization vectors, and other cryptographic materials.

The vulnerability requires local access to exploit, as the attacker would need to observe system state or timing information to predict the seed value. However, successful exploitation could lead to high confidentiality and integrity impact, enabling decryption of encrypted communications or forging of cryptographic signatures.

Root Cause

The root cause of this vulnerability is the use of insufficiently random seed material when initializing the PRNG. The affected code path uses a seed source that lacks adequate entropy, making the initial state of the random number generator predictable under certain conditions. This violates fundamental cryptographic principles that require true randomness for security-sensitive operations.

Attack Vector

The attack vector is local, requiring the attacker to have some level of access to the target system. An attacker could exploit this vulnerability by:

1. Observing or predicting the state of the system at the time the PRNG was seeded
2. Reconstructing the seed value based on available information
3. Regenerating the same sequence of "random" numbers produced by the PRNG
4. Using the predicted values to compromise cryptographic operations such as key generation or session establishment

The vulnerability mechanism involves the entropy collection process within the PRNG initialization routine. When insufficient entropy is available or when predictable sources are used, the resulting seed becomes vulnerable to prediction attacks. For detailed technical information about the specific entropy source issue, refer to the Mbed TLS Security Advisory 2026-03.

Detection Methods for CVE-2026-34871

Indicators of Compromise

• Unusual patterns in cryptographic operations or key generation activities
• Evidence of local process monitoring or memory inspection attempts
• Anomalous access to entropy sources such as /dev/random or /dev/urandom
• Signs of timing analysis attacks against cryptographic operations

Detection Strategies

• Monitor for suspicious local processes attempting to access or monitor entropy sources
• Implement integrity checking for cryptographic libraries and their dependencies
• Review application logs for anomalous cryptographic operation failures or retries
• Use software composition analysis (SCA) tools to identify vulnerable Mbed TLS and TF-PSA-Crypto versions in your environment

Monitoring Recommendations

• Enable comprehensive logging for cryptographic operations in applications using Mbed TLS
• Monitor file access patterns to entropy sources on systems running affected software
• Implement runtime application self-protection (RASP) to detect abnormal library behavior
• Establish baseline behavior for cryptographic operations and alert on deviations

How to Mitigate CVE-2026-34871

Immediate Actions Required

• Inventory all applications and systems using Mbed TLS or TF-PSA-Crypto libraries
• Prioritize patching systems that handle sensitive cryptographic operations
• Consider regenerating cryptographic keys that may have been generated using affected versions
• Review and rotate TLS certificates and session keys on affected systems

Patch Information

The vulnerability has been addressed in the following versions:

• Mbed TLS: Upgrade to version 3.6.6 or later (for 3.x branch)
• Mbed TLS: Upgrade to version 4.1.0 or later (for 4.x branch)
• TF-PSA-Crypto: Upgrade to version 1.1.0 or later

For official patch details and security advisories, consult the Mbed TLS Security Advisories page.

Workarounds

• If immediate patching is not possible, consider using external entropy sources with higher quality randomness
• Implement additional entropy mixing at the application level before cryptographic operations
• Restrict local access to systems running affected library versions
• Monitor cryptographic operations more closely until patches can be applied

# Verify Mbed TLS version in your environment
# Check installed package version (Debian/Ubuntu)
dpkg -l | grep mbedtls

# Check installed package version (RHEL/CentOS)
rpm -qa | grep mbedtls

# Verify library version programmatically
strings /usr/lib/libmbedtls.so | grep -i version