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.

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

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 This could lead to a bypass of corporate security policies, potentially allowing the use of outdated or insecure encryption methods that the client was configured to reject.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

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