# CVE-2026-25834: Mbed TLS algorithm downgrade in TLS 1.2 client

Source: https://junglewise.ai/threats/cve-2026-25834-mbed-tls-algorithm-downgrade-in-tls-1-2-client  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-01. Updated: 2026-06-05  
License: CC BY 4.0 with a link back

CVE-2026-25834 · Severity: medium · CVSS 6.5 · Published 2026-04-01

Mbed TLS is a widely used cryptographic library that helps secure communications for embedded devices and applications. A vulnerability in certain versions allows a malicious or misconfigured server to force a client to use weaker security settings than intended during a TLS 1.2 connection. This could lead to a bypass of corporate security policies, potentially allowing the use of outdated or insecure encryption methods that the client was configured to reject.

## Quick answers

**Is CVE-2026-25834 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-25834 being exploited in the wild.

**How severe is CVE-2026-25834?** Medium severity, with a CVSS score of 6.5, published 1 April 2026.

**Which products and versions are affected?** TrustedFirmware Mbed TLS 3.3.0 to 3.6.5, 4.0.0.

**Is there a fix for CVE-2026-25834?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-25834 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-25834?** Check whether anything you run is named in the advisory. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An algorithm downgrade vulnerability exists in Mbed TLS 1.2 client implementations. When a server ignores the signature algorithms extension provided in the Client Hello, the client fails to validate the server's choice against its local security policy configured via mbedtls_ssl_conf_sig_algs(). Instead, the client accepts any signature algorithm supported at compile-time, even if it was explicitly excluded from the runtime configuration. This allows a network-positioned attacker (acting as a server) to bypass intended cryptographic restrictions. The issue is resolved in Mbed TLS 3.6.6 and 4.1.0.

## Affected products

- TrustedFirmware Mbed TLS 3.3.0 to 3.6.5, 4.0.0

## Timeline

- 2026-03-31: advisory: Vendor advisory published by Mbed TLS team
- 2026-04-01: disclosed: CVE published to NVD

## References

- [https://mbed-tls.readthedocs.io/en/latest/security-advisories/](https://mbed-tls.readthedocs.io/en/latest/security-advisories/)
- [https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-sigalg-injection/](https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-sigalg-injection/)