CVE-2026-34873
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 3.5.0 through 4.0.0. Client impersonation can occur while resuming a TLS 1.3 session.
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 · moderate confidenceA vulnerability in Mbed TLS versions 3.5.0 through 4.0.0 allows client impersonation during TLS 1.3 session resumption. The flaw enables an attacker to hijack or forge session resumes, bypassing authentication mechanisms.
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>= 3.5.0, < 3.6.6>= 4.0.0, < 4.1.0CVSS 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
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/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 installed Mbed TLS versionLocate the Mbed TLS library in your environment and retrieve its version number. This is typically found via library file metadata, version headers, or runtime version API calls (such as mbedtls_version_get_number() or similar).Affected if The installed version is >= 3.5.0 and < 3.6.6, OR >= 4.0.0 and < 4.1.0
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Confirm TLS 1.3 support is presentCheck whether your Mbed TLS build includes TLS 1.3 protocol support. This is usually configurable at compile time (MBEDTLS_SSL_PROTO_TLS1_3) or can be verified through runtime capability queries.Affected if TLS 1.3 is compiled in or enabled in the build
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Verify TLS 1.3 session resumption is enabledInspect the SSL/TLS configuration to determine whether session resumption (either stateless or stateful) is enabled for TLS 1.3 connections. Check for relevant configuration flags such as MBEDTLS_SSL_SESSION_TICKETS or equivalent session ticket handling settings.Affected if TLS 1.3 session resumption is enabled in the configuration
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Determine if the deployment uses client certificate authentication with resumptionReview the application or service configuration to see if it uses Mbed TLS as a server that authenticates clients via certificates AND employs TLS 1.3 session resumption.Affected if The environment acts as a TLS server using both client certificate authentication and TLS 1.3 session resumption
You are affected if your Mbed TLS version falls within 3.5.0 to 3.6.5 or 4.0.0 to 4.0.x AND TLS 1.3 session resumption with client certificate authentication is enabled in your environment.
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 · scoped3.6.64.1.0
Update Mbed TLS to a version beyond 4.0.0 when a patched release becomes available. If immediate patching is not possible, consider disabling TLS 1.3 session resumption as a temporary workaround and audit affected TLS implementations.
Mbed TLS 3.6.6 (for 3.x branch) or Mbed TLS 4.1.0 (for 4.x branch)
- 1. Identify the current Mbed TLS version in use by checking project dependencies or library version macros (e.g., MBEDTLS_VERSION_NUMBER)
- 2. For Mbed TLS 3.x versions (3.5.0 to 3.6.5): Upgrade to Mbed TLS 3.6.6 or later
- 3. For Mbed TLS 4.x versions (4.0.0 to 4.0.x): Upgrade to Mbed TLS 4.1.0 or later
- 4. Update the dependency in your build system (e.g., package.json, conan, cmake, or submodule) to point to the new version
- 5. Rebuild the project to link against the updated Mbed TLS library
- 6. Run TLS functionality tests and verify session resumption works correctly
- 7. Deploy the updated application
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing6.0 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 $5,600.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-34873 — 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-2026-34873 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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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
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