CVE-2026-25832: Mbed TLS 1.3 Client Configuration Vulnerability

CVE-2026-25832 Overview

CVE-2026-25832 affects Mbed TLS versions 3.6.x before 3.6.7 and 4.1.x before 4.1.2. The Transport Layer Security (TLS) 1.3 client accepts a HelloRetryRequest (HRR) message that selects a key exchange group the client never advertised in its ClientHello. This constitutes a protocol state machine flaw categorized under [CWE-669] (Incorrect Resource Transfer Between Spheres). An attacker positioned on the network path can influence the negotiated key share by injecting a manipulated HRR referencing an unadvertised group.

Critical Impact
A network attacker can coerce a TLS 1.3 client into completing a handshake using a group parameter it never offered, undermining negotiation integrity.

Affected Products

Mbed TLS 3.6.x prior to 3.6.7
Mbed TLS 4.1.x prior to 4.1.2
Applications and embedded devices linking the affected Mbed TLS client library for TLS 1.3

Discovery Timeline

2026-09-14 - CVE-2026-25832 published to the National Vulnerability Database (NVD)
2026-09-14 - Last updated in NVD database

Technical Details for CVE-2026-25832

Vulnerability Analysis

The flaw resides in the Mbed TLS 1.3 client-side handshake logic. During a standard TLS 1.3 handshake, the client sends a ClientHello advertising supported groups and one or more key shares. If the server cannot use the offered key shares, it responds with a HelloRetryRequest selecting a group from those the client advertised. The client must then resend a ClientHello with a key share for that specific group.

The affected Mbed TLS versions fail to validate that the group named in the HRR was present in the client's original supported_groups extension. The client proceeds to generate and send a key share for the attacker-chosen group. This weakens the guarantees of the negotiation and can enable downgrade-style manipulation of the key exchange.

Root Cause

The root cause is missing input validation on the server-selected group value carried in HelloRetryRequest. The client accepts the field without cross-referencing it against the locally advertised supported group list. This violates RFC 8446 Section 4.1.4, which requires the client to abort the handshake with an illegal_parameter alert when the selected group was not offered.

Attack Vector

Exploitation requires an active network attacker capable of intercepting or impersonating the TLS server during handshake. The attacker crafts an HelloRetryRequest naming a group of their choice, typically one the client library supports but did not advertise for this session. The client then completes the handshake using the attacker-influenced parameters. The attack complexity is high because the attacker must occupy an on-path position and complete the handshake within TLS timing constraints. Refer to the Mbed TLS Advisory 2026-07 on HRR for the maintainers' technical description.

Detection Methods for CVE-2026-25832

Indicators of Compromise

TLS 1.3 handshakes containing a HelloRetryRequest whose selected_group value does not appear in the preceding ClientHellosupported_groups extension.
Sessions where the client's second ClientHello carries a key share for a group absent from its first ClientHello.
Unexpected downgrades in negotiated key exchange group compared with the client's published defaults.

Detection Strategies

Deploy TLS-aware network sensors that parse handshake extensions and correlate supported_groups with the HRR-selected group.
Perform passive TLS fingerprinting to flag clients running vulnerable Mbed TLS versions (3.6.x before 3.6.7, 4.1.x before 4.1.2).
Inventory embedded and IoT deployments to identify firmware bundling affected Mbed TLS builds.

Monitoring Recommendations

Log full TLS handshake metadata at gateways and inspect for anomalous HRR-driven group changes.
Alert on repeated handshake failures or unusual retry patterns from Mbed TLS clients, which may indicate exploitation attempts.
Track library versions across build pipelines using Software Bill of Materials (SBOM) data to detect vulnerable versions.

How to Mitigate CVE-2026-25832

Immediate Actions Required

Upgrade Mbed TLS to version 3.6.7 or later on the 3.6.x branch, or to 4.1.2 or later on the 4.1.x branch.
Rebuild and redeploy all applications, firmware images, and containers that statically link the affected Mbed TLS library.
Audit third-party dependencies and vendor firmware for embedded Mbed TLS components.

Patch Information

The Mbed TLS maintainers released fixed versions 3.6.7 and 4.1.2 that enforce validation of the HRR-selected group against the client's advertised supported_groups. Consult the Mbed TLS Security Advisories index and the Mbed TLS Advisory 2026-07 on HRR for release notes and commit references.

Workarounds

Where patching is not immediately feasible, restrict client configurations to a single supported group so any HRR referencing a different group is trivially anomalous.
Terminate outbound TLS at a hardened, patched proxy that performs its own TLS 1.3 handshake with the destination.
Constrain affected devices to trusted networks until patched builds are deployed.

bash

# Verify installed Mbed TLS version
strings /path/to/binary | grep -i 'mbed TLS'

# Example: build against a patched release from source
git clone https://github.com/Mbed-TLS/mbedtls.git
cd mbedtls
git checkout v3.6.7   # or v4.1.2
cmake -B build -DENABLE_TESTING=Off
cmake --build build