CVE-2026-34876
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedAn 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.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceAn out-of-bounds read vulnerability exists in mbedtls_ccm_finish() in Mbed TLS 3.x before 3.6.6. The function fails to validate the tag_len parameter against the fixed 16-byte internal authentication buffer, allowing attackers to read adjacent CCM context data when the multipart CCM API is invoked with an oversized tag_len value.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data>= 3.1.0, < 3.6.6CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- High
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify Mbed TLS library versionInspect the version header file (mbedtls/version.h or similar) or run: grep -r 'MBEDTLS_VERSION' in the library headers, or check the shared library with: ldd <your_binary> | grep mbedtlsAffected if The version is 3.1.0 or higher but lower than 3.6.6
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Locate CCM multipart API usageSearch source code for calls to mbedtls_ccm_start, mbedtls_ccm_update, or mbedtls_ccm_finish functions (or their AES-CCM variants)Affected if The codebase uses the multipart CCM encryption API (as opposed to single-shot mbedtls_ccm_encrypt_and_tag or mbedtls_ccm_decrypt_and_tag)
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Inspect CCM context initialization for tag_lenSearch for mbedtls_ccm_init and mbedtls_ccm_setkey calls, then examine the tag_len parameter value passed to the context or the authentication tag length configurationAffected if tag_len is set to a value greater than 16 bytes (for example, 24 or 32) without explicit validation in user code before calling CCM functions
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Check for vulnerable code path in mbedtls_ccm_finishIf using Mbed TLS library directly, examine the source file ccm.c for the mbedtls_ccm_finish function; verify if tag_len validation against the 16-byte internal buffer existsAffected if The library source lacks validation of tag_len against 16 bytes before buffer access in mbedtls_ccm_finish (this requires source code review of the library)
You are affected if you use Mbed TLS version 3.1.0 through 3.6.5, your code invokes the multipart CCM API, and tag_len values exceeding 16 bytes are passed to CCM functions without pre-validation in your application code.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
dbcve · scoped3.6.6
Upgrade to Mbed TLS 3.6.6 or later. If upgrade is not immediately feasible, ensure applications using the multipart CCM API validate tag_len does not exceed 16 bytes before calling mbedtls_ccm_finish().
3.6.6
- Download Mbed TLS version 3.6.6 from the official source (e.g., GitHub releases or package manager)
- Replace the existing mbedtls library files (libmbedtls.so, libmbedcrypto.so, libmbedx509.so or corresponding static libraries) with version 3.6.6
- Rebuild any applications that are linked against the mbedtls library to ensure they use the updated version
- Verify the version by checking mbedtls version information at runtime
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation2.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $2,512.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-34876 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2026-34876 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Version or environment caveats, and links to real fixes
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