CVE-2026-34876
CVE-2026-34876 is a high-severity vulnerability in Trustedfirmware Mbed Tls with a CVSS 3.x base score of 7.5. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-125.
Key facts
- Severity: High (CVSS 3.x base score 7.5)
- EPSS exploit prediction: 0% (31st percentile)
- Actively exploited: Not listed in CISA KEV
- EU (EUVD) id: EUVD-2026-18356
- Weakness: CWE-125
- Affected product: Trustedfirmware Mbed Tls
- Published:
- Last modified:
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.
Frequently asked questions
- What is CVE-2026-34876?
- 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.
- How severe is CVE-2026-34876?
- CVE-2026-34876 has a CVSS 3.x base score of 7.5, rated high severity. It is exploitable over network with low attack complexity, requires no privileges and no user interaction. Impact on confidentiality is high, integrity none, and availability none.
- Is CVE-2026-34876 being actively exploited?
- It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (31st percentile), an estimate of the probability of exploitation in the next 30 days.
- What products are affected by CVE-2026-34876?
- CVE-2026-34876 affects Trustedfirmware Mbed Tls. See the affected-products list for the exact vulnerable versions.
- How do I fix CVE-2026-34876?
- Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround. Given its high severity, prioritise patching exposed systems.
- Does CVE-2026-34876 have an EU (EUVD) identifier?
- Yes. CVE-2026-34876 is tracked in the ENISA EU Vulnerability Database (EUVD) as EUVD-2026-18356.
- When was CVE-2026-34876 published?
- CVE-2026-34876 was published on 2026-04-02 and last updated on 2026-06-17.
References
- https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-ccm-finish-boundary-check/
- https://mbed-tls.readthedocs.io/en/latest/tech-updates/security-advisories/
Affected products (1)
- cpe:2.3:a:trustedfirmware:mbed_tls:*:*:*:*:*:*:*:*
More vulnerabilities in Trustedfirmware Mbed Tls
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- CVE-2024-45158 — Critical (CVSS 9.8): An issue was discovered in Mbed TLS 3.6 before 3.6.1. A stack buffer overflow in mbedtls_ecdsa_der_to_raw() and…
- CVE-2023-45199 — Critical (CVSS 9.8): Mbed TLS 3.2.x through 3.4.x before 3.5 has a Buffer Overflow that can lead to remote Code execution.
All CVEs affecting Trustedfirmware Mbed Tls →
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