CVE-2021-43203
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 · uneditedIn JetBrains Ktor before 1.6.4, nonce verification during the OAuth2 authentication process is implemented improperly.
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 JetBrains Ktor before 1.6.4, the nonce verification mechanism during OAuth2 authentication is improperly implemented. The nonce is intended to prevent replay attacks by ensuring each authentication request is unique, but the flawed implementation allows this check to be bypassed, potentially enabling attackers to replay or intercept valid authentication flows.
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< 1.6.4CVSS 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
- High
- Availability
- None
CVSS:3.1/AV:N/AC:L/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 Ktor installation and versionReview your project's build configuration (build.gradle, pom.xml, or similar) and locate the Ktor dependency declaration. Check the declared version number.Affected if The Ktor version listed is below 1.6.4 (for example, 1.6.3, 1.6.2, 1.5.x, etc.)
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Verify OAuth2 feature is in useSearch your application source code for OAuth2-related imports and installations, such as 'io.ktor.features.OAuth' or calls to 'install(OAuth)'. Also check for OAuth2 client configuration in your routing or application setup.Affected if OAuth2 authentication feature is installed and configured in your application code.
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Confirm nonce verification is activeReview your OAuth2 configuration to determine if the default nonce parameter handling is being used. Examine the authentication flow configuration where the OAuth provider is defined.Affected if OAuth2 authentication is configured without custom nonce verification logic overriding the default implementation.
You are affected if your application uses Ktor with a version below 1.6.4 and has OAuth2 authentication enabled using the default nonce verification mechanism.
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 · scoped1.6.4
Upgrade Ktor to version 1.6.4 or later to obtain the fixed OAuth2 nonce verification implementation. After upgrading, thoroughly test all OAuth2 authentication flows to ensure the nonce mechanism functions correctly.
1.6.4
- Update the Ktor dependency in your build configuration (Gradle or Maven) to version 1.6.4 or later
- Run your build tool's dependency update command (e.g., ./gradlew dependencies or mvn dependency:tree) to verify the new version is resolved
- Rebuild your application and run tests to ensure the upgrade does not break existing functionality
Generated from the published advisory — verify against the referenced sources before acting.
- Consultation2.0 h
- Implementation4.0 h
- Testing8.0 h
- Review / QA4.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $4,864.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-43203 — 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-2021-43203 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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