CVE-2025-59438: Arm Mbed TLS Timing Discrepancy Flaw

CVE-2025-59438 Overview

CVE-2025-59438 is an observable timing discrepancy vulnerability in Arm Mbed TLS through version 3.6.4. The flaw is classified under [CWE-208] (Observable Timing Discrepancy) and enables a remote attacker to distinguish between valid and invalid cryptographic padding by measuring the time taken to process a message. This class of side-channel weakness underpins classic padding oracle attacks against block cipher modes such as CBC.

The vulnerability is network-exploitable without authentication or user interaction. Successful exploitation may leak plaintext bits from encrypted traffic, undermining the confidentiality guarantees expected from TLS. No public exploit is currently available, and the issue is not listed in the CISA Known Exploited Vulnerabilities catalog.

Critical Impact
Remote attackers can leverage measurable timing differences during padding validation to recover portions of encrypted plaintext, weakening TLS confidentiality for applications embedding vulnerable Mbed TLS builds.

Affected Products

Arm Mbed TLS versions up to and including 3.6.4
Applications and firmware statically linking vulnerable Mbed TLS builds
Embedded and IoT devices bundling Mbed TLS for TLS or cryptographic operations

Discovery Timeline

2025-10-21 - CVE-2025-59438 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-59438

Vulnerability Analysis

Mbed TLS is a widely deployed C library providing TLS and cryptographic primitives for embedded systems. The vulnerability stems from the padding validation path exhibiting measurable timing differences between well-formed and malformed padding. An attacker positioned to submit chosen ciphertexts and observe response latency can infer whether padding is valid.

This behavior enables a padding oracle attack. By repeatedly querying the oracle and modifying ciphertext bytes, an attacker can progressively recover plaintext without knowing the key. The attack targets confidentiality only, with no direct impact on integrity or availability.

The exposure is limited to protocols and modes that rely on the vulnerable padding routine, most notably CBC-mode cipher suites. Applications that have already migrated to AEAD constructions such as AES-GCM or ChaCha20-Poly1305 are not affected by this specific timing path.

Root Cause

The root cause is non-constant-time handling of invalid padding errors within Mbed TLS through 3.6.4. Cryptographic code must execute in a data-independent manner to prevent secret-dependent branches or memory accesses from creating measurable side channels. In this case, error handling and validation logic diverges based on padding correctness, producing observable timing variance across the network.

Attack Vector

Exploitation requires the attacker to interact with a TLS endpoint or cryptographic service that uses Mbed TLS for padded decryption. The attacker submits crafted ciphertexts, measures response timing across many samples to average out network jitter, and uses the results to distinguish valid from invalid padding. Refer to the Mbed TLS Advisory: Invalid Padding Error for authoritative technical detail.

Detection Methods for CVE-2025-59438

Indicators of Compromise

Abnormally high volumes of TLS handshakes or record decryption failures from a single source over short intervals
Repeated bad_record_mac or padding-related TLS alerts originating from the same client identity
Traffic patterns showing systematic ciphertext bit-flipping across sessions targeting the same endpoint

Detection Strategies

Inventory all binaries, firmware images, and containers that statically link Mbed TLS and identify versions at or below 3.6.4
Monitor TLS servers for elevated rates of decryption failures and alert on statistical anomalies in error-response timing
Use software composition analysis (SCA) tooling to flag Mbed TLS dependencies in build pipelines

Monitoring Recommendations

Capture TLS alert telemetry and correlate padding-related alerts with source IP reputation and session cardinality
Instrument application logs to record decryption failure counts per client and per key material
Feed TLS handshake and error telemetry into a centralized data lake for cross-source correlation and historical baselining

How to Mitigate CVE-2025-59438

Immediate Actions Required

Upgrade Mbed TLS to a fixed release published after 3.6.4 per the vendor advisory
Prefer AEAD cipher suites such as AES-GCM or ChaCha20-Poly1305 over CBC-mode suites where negotiation allows
Audit third-party firmware and appliances for embedded Mbed TLS builds and coordinate updates with vendors

Patch Information

Arm has published guidance in the Mbed TLS Advisory: Invalid Padding Error. Consult the full Mbed TLS Security Advisory index for fixed version details and rebuild instructions. Applications must be recompiled against the patched library and redeployed; simply updating source without rebuilding dependent binaries is insufficient.

Workarounds

Disable CBC-mode cipher suites in TLS server configuration and negotiate only AEAD suites
Restrict network exposure of services that perform padded decryption to trusted networks where feasible
Apply rate limiting and anomaly-based blocking on repeated decryption failures from the same source

bash

# Example: restrict OpenSSL/compatible server config to AEAD suites only
ssl_protocols TLSv1.2 TLSv1.3;
ssl_ciphers 'TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384';
ssl_prefer_server_ciphers on;