Vulnerability · Published Apr 2, 2026 · Assigned by mitre
CVE-2026-34876
High 7.5
In plain English
A high-severity out-of-bounds read in the mbedtls package (Debian). There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.4% (higher than 35% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.
Official 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.
Affected packages & fixed versions
| Ecosystem | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian:12 | mbedtls | no fix listed | DEBIAN-CVE-2026-34876 |
| Debian:13 | mbedtls | 3.6.6-0.1~deb13u1 | DEBIAN-CVE-2026-34876 |
| Debian:14 | mbedtls | 3.6.6-0.1 | DEBIAN-CVE-2026-34876 |
| Ubuntu:25.10 | mbedtls | no fix listed | UBUNTU-CVE-2026-34876 |
| Ubuntu:26.04:LTS | mbedtls | no fix listed | UBUNTU-CVE-2026-34876 |
| Ubuntu:26.10 | mbedtls | no fix listed | UBUNTU-CVE-2026-34876 |
Affected products (as reported)
Weakness: CWE-125 · Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N