# CVE-2026-34873: Mbed TLS client impersonation in TLS 1.3 session resumption

Source: https://junglewise.ai/threats/cve-2026-34873-mbed-tls-client-impersonation-in-tls-1-3-session-resumption  
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-2026-34873 · Severity: critical · CVSS 9.1 · Published 2026-04-01

Mbed TLS is a widely used open-source library that provides secure communication capabilities for devices and applications. A vulnerability has been identified where an attacker can bypass security checks to impersonate a legitimate user during the reconnection process. This could allow unauthorized access to sensitive data or systems that rely on Mbed TLS for identity verification.

Technologies: [Arm Mbed Tls](https://junglewise.ai/threats/technologies/mbed-tls). Vendors: [Arm](https://junglewise.ai/threats/vendors/arm).

## Quick answers

**Is CVE-2026-34873 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-34873 being exploited in the wild.

**How severe is CVE-2026-34873?** Critical severity, with a CVSS score of 9.1, published 1 April 2026.

**Which products and versions are affected?** Arm Mbed Tls 3.5.0 to 3.6.5, 4.0.0.

**Is there a fix for CVE-2026-34873?** A patch was published on 2026-04-07: NIST initial analysis and confirmation of fix versions 3.6.6 and 4.1.0 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-34873 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-34873?** Check whether you run Arm Mbed Tls at an affected version. 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 improper authentication vulnerability (CWE-287) exists in Mbed TLS when a server supporting both TLS 1.2 and 1.3 attempts to resume a TLS 1.3 session. If the server issues a HelloRetryRequest and a man-in-the-middle attacker intercepts it, they can respond with a ClientHello that negotiates TLS 1.2. The server incorrectly proceeds with the resumption using an all-zero master secret, allowing the attacker to complete the handshake without valid credentials. This bypasses client authentication guarantees and can lead to full client impersonation. The issue is fixed in versions 3.6.6 and 4.1.0.

## Affected products

- Arm Mbed TLS 3.5.0 to 3.6.5, 4.0.0
- TrustedFirmware Mbed TLS 3.5.0 to 3.6.5, 4.0.0

## Timeline

- 2026-03-31: advisory: Vendor advisory published by Mbed TLS team
- 2026-04-01: disclosed: CVE-2026-34873 assigned
- 2026-04-07: patched: NIST initial analysis and confirmation of fix versions 3.6.6 and 4.1.0

## 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-client-impersonation-while-resuming-tls13-session/](https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-client-impersonation-while-resuming-tls13-session/)

## Related threats

- [Mbed TLS memory corruption via insecure deserialization of SSL context](https://junglewise.ai/threats/cve-2026-34877-mbed-tls-memory-corruption-via-insecure-deserialization-of-ssl)
- [In Mbed TLS through 4.0.0, there is a compiler-induced timing side channel (in RSA and CBC/ECB decryption) that only…](https://junglewise.ai/threats/cve-2025-66442-in-mbed-tls-through-4-0-0-there-is-a-compiler-induced-timing-side)
- [Arm Mbed TLS Predictable Seed in PRNG on Linux](https://junglewise.ai/threats/cve-2026-34871-arm-mbed-tls-predictable-seed-in-prng-on-linux)
- [Arm Mbed TLS and TF-PSA-Crypto PRNG state cloning vulnerability](https://junglewise.ai/threats/cve-2026-25835-arm-mbed-tls-and-tf-psa-crypto-prng-state-cloning-vulnerability)
- [Mbed TLS out-of-bounds read in mbedtls_lms_import_public_key](https://junglewise.ai/threats/cve-2025-49601-mbed-tls-out-of-bounds-read-in-mbedtls-lms-import-public-key)