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.
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
| Product | Fixed in | Fix | Released |
|---|---|---|---|
| Linux | |||
| mbedtls · Debian 13.5 (trixie) | 3.6.6-0.1~deb13u1 | DEBIAN-CVE-2026-34876 | Apr 2, 2026 |
| mbedtls · Debian 14 | 3.6.6-0.1 | DEBIAN-CVE-2026-34876 | Apr 2, 2026 |
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