CVE-2026-25834
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 · uneditedMbed TLS v3.3.0 up to 3.6.5 and 4.0.0 allows Algorithm Downgrade.
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 confidenceMbed TLS versions 3.3.0 through 3.6.5 and 4.0.0 contains an algorithm downgrade vulnerability that allows attackers to force the use of weaker cryptographic algorithms during TLS handshakes, potentially exposing encrypted communications to downgrade attacks.
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>= 3.3.0, < 3.6.6= 4.0.0CVSS 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
- None
- Integrity
- Low
- Availability
- Low
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L
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 versionLocate the Mbed TLS library in your environment and retrieve its version number. Common methods: check library file metadata, examine version headers (e.g., include/mbedtls/version.h), or query the library at runtime via mbedtls_version_get_number() or similar API.Affected if The installed version falls within 3.3.0 to 3.6.5 inclusive, or is exactly 4.0.0.
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Review TLS configuration for cipher suite policyInspect your application's TLS configuration to determine which cipher suites are explicitly allowed or disabled. Look for configuration files, runtime parameters, or code that sets ciphersuite lists via mbedtls_ssl_config_defaults() and related APIs.Affected if Configuration permits legacy or weak cipher suites (e.g., NULL encryption, MD5-based suites, export-grade ciphers) without explicit disabling.
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Check minimum TLS protocol version settingExamine the TLS context configuration to identify the minimum TLS version being negotiated. Verify whether the application explicitly sets a minimum version or relies on default behavior.Affected if The minimum TLS version is not enforced, allowing fallback to TLS 1.1 or lower.
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Verify secure cipher suite orderingReview whether the application prioritizes strong cipher suites or relies on default server cipher preferences that may include weaker options.Affected if The cipher suite preference order is not explicitly configured, potentially allowing downgrade to weaker algorithms during handshake negotiation.
You are affected if your environment runs Mbed TLS version 3.3.0 through 3.6.5 or exactly 4.0.0 AND allows fallback to weak or legacy cipher suites in TLS configuration.
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 a patched version of Mbed TLS once available, and ensure TLS configurations explicitly disable legacy and weak cipher suites while requiring modern TLS versions (1.2 or higher).
Mbed TLS 3.6.6 or later (3.x branch); Mbed TLS 4.0.1 or later (4.x branch)
- 1. Identify the exact Mbed TLS version currently in use by checking project dependencies or library files
- 2. For Mbed TLS 3.x branch (versions 3.3.0 through 3.6.5): upgrade to version 3.6.6 or later
- 3. For Mbed TLS 4.x branch (version 4.0.0): upgrade to version 4.0.1 or later
- 4. Rebuild and recompile any applications or libraries that link against Mbed TLS
- 5. Verify that the upgrade does not introduce regressions in TLS functionality
- 6. Test certificate validation with various certificate chains to confirm the fix is effective
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing4.0 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 $5,120.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2026-25834 — 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-2026-25834 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
- No weaponised exploit code, or anything meant to cause harm
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