CVE-2026-34876
Mbed TLS 3.x before 3.6.6 vulnerability
Mbed · 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.
MSRC advisory May 19, 2026
Alert details
- Source feed
- Microsoft MSRC
- CVE ID
- CVE-2026-34876
- Affected products
- 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. · An 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. · An
What happened
Information published.
What it means for your business
An 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. (CVE-2026-34876) was added to Microsoft’s security update guidance. Information published. PremierePC tracks MSRC advisories for Upstate SC businesses — patch or open a ticket if you need help verifying exposure.
EPSS probability
0.39%
Sources
Related briefs
Microsoft Excel Remote Code Execution vulnerability
Microsoft Excel Remote Code Execution vulnerability (CVE-2026-68801) was added to Microsoft’s security update guidance. Updated an acknowledgement. This is an informational change only. If you need help checking exposure, call (864) 335-9223.
Microsoft Office Remote Code Execution vulnerability
Microsoft Office Remote Code Execution vulnerability (CVE-2026-64903) was added to Microsoft’s security update guidance. Updated an acknowledgement. This is an informational change only. If you need help checking exposure, call (864) 335-9223.
Microsoft Office Information Disclosure vulnerability
Microsoft Office Information Disclosure vulnerability (CVE-2026-64899) was added to Microsoft’s security update guidance. Updated an acknowledgement. This is an informational change only. If you need help checking exposure, call (864) 335-9223.
Windows Universal Plug and Play (UPnP) Device Host Elevation of Privilege vulnerability
Windows Universal Plug and Play (UPnP) Device Host Elevation of Privilege vulnerability (CVE-2026-58547) was added to Microsoft’s security update guidance. Updated an acknowledgement. This is an informational change only. If you need help checking exposure, call (864) 335-9223.
Need help patching?
PremierePC monitors KEV alerts for managed clients and helps teams prioritize remediation before attackers do.