# CVE-2026-34876: mbedtls: An issue was discovered in Mbed TLS 3.x before 3.6.6

CVE-2026-34876 is fixed in mbedtls on 1 platform, first by DEBIAN-CVE-2026-34876, released April 2, 2026. It is not reported as exploited. Patch Radar priority: High.

- Web page: https://patchblox.com/security/cve/CVE-2026-34876
- Priority: High
- CVSS: 7.5 (high)
- EPSS: 0.0041
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Apr 2, 2026

## Description

An issue was discovered in Mbed TLS 3.x before 3.6.6. An out-of-bounds read vulnerability in mbedtls_ccm_finish() in library/ccm.c allows attackers to obtain adjacent CCM context data via invocation of the multipart CCM API with an oversized tag_len parameter. This is caused by missing validation of the tag_len parameter against the size of the internal 16-byte authentication buffer. The issue affects the public multipart CCM API in Mbed TLS 3.x, where mbedtls_ccm_finish() can be invoked directly by applications. In Mbed TLS 4.x versions prior to the fix, the same missing validation exists in the internal implementation; however, the function is not exposed as part of the public API. Exploitation requires application-level invocation of the multipart CCM API.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | mbedtls Debian 13.5 (trixie) | 3.6.6-0.1~deb13u1 | [DEBIAN-CVE-2026-34876](https://patchblox.com/security/data/fix/linux/debian-cve-2026-34876.md) | Apr 2, 2026 |
| Linux | mbedtls Debian 14 | 3.6.6-0.1 | [DEBIAN-CVE-2026-34876](https://patchblox.com/security/data/fix/linux/debian-cve-2026-34876.md) | Apr 2, 2026 |

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Data as of 2026-11-08T00:53:09Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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