CVE-2020-36477
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 before 2.24.0. The verification of X.509 certificates when matching the expected common name (the cn argument of mbedtls_x509_crt_verify) with the actual certificate name is mishandled: when the subjecAltName extension is present, the expected name is compared to any name in that extension regardless of its type. This means that an attacker could impersonate a 4-byte or 16-byte domain by getting a certificate for the corresponding IPv4 or IPv6 address (this would require the attacker to control that IP address, though).
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 confidenceIn Mbed TLS before 2.24.0, the certificate verification function mbedtls_x509_crt_verify incorrectly handles common name matching when the subjectAltName extension is present. Instead of only comparing the expected CN to DNS names in the SAN extension, it compares to ANY name type in the SAN, including IP addresses. This allows an attacker with control over an IP address to get a certificate for that IP and impersonate a service using that IP address as a hostname.
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< 2.24.0CVSS 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
- High
- Privileges
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality
- None
- Integrity
- High
- Availability
- None
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/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 Mbed TLS library versionLocate the Mbed TLS library binary or header file and retrieve its version information, typically found in version.h or through runtime version API (mbedtls_version_check)Affected if The installed version is earlier than 2.24.0
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Verify certificate verification usageSearch codebase or binary for usage of the function mbedtls_x509_crt_verify which performs certificate chain validationAffected if The function mbedtls_x509_crt_verify is being used for certificate validation
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Inspect certificate chain validation flowTrace the code path where certificates are verified and check whether subjectAltName (SAN) extension processing occurs during verificationAffected if Certificates containing a SAN extension are being validated using mbedtls_x509_crt_verify
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Review certificate inputs for IP addressesExamine the certificates being validated in the environment to determine if any contain IP addresses in the SAN extension fieldAffected if Certificates with IP addresses in the SAN extension are being validated against hostnames using this function
You are affected if Mbed TLS version is below 2.24.0 AND you use mbedtls_x509_crt_verify to validate certificates that may contain IP addresses in the subjectAltName extension.
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 · scoped2.24.0
Upgrade Mbed TLS to version 2.24.0 or later. Additionally, ensure certificate chain validation is properly configured and consider implementing certificate pinning for high-security connections.
Mbed TLS 2.24.0
- Identify the current version of Mbed TLS being used in your project or system
- Upgrade Mbed TLS to version 2.24.0 or later
- After upgrading, verify that certificate verification now properly validates the subjectAltName type before comparing names (e.g., only comparing DNS names to expected hostnames, not IP addresses)
- Test your application's certificate validation to ensure legitimate certificates are still accepted
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation4.0 h
- Implementation8.0 h
- Testing12.0 h
- Review / QA4.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2020-36477 — 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-2020-36477 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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