CVE-2021-25763
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 1.4.2, weak cipher suites were enabled by default.
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 confidenceIn JetBrains Ktor versions prior to 1.4.2, weak cipher suites were enabled by default in the SSL/TLS configuration, potentially allowing attackers to force the use of weaker cryptographic algorithms for encrypted communications.
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< 1.4.2CVSS 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
- Low
- Integrity
- None
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/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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Check the installed Ktor versionInspect your build configuration file (build.gradle.kts, build.gradle, or pom.xml) for the ktor dependency and note the version number. For example: `implementation("io.ktor:ktor-server-core:$ktorVersion")`Affected if The Ktor version listed is lower than 1.4.2 (e.g., 1.4.0, 1.4.1, or any 1.x version below 1.4.2)
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Verify if SSL/TLS is configured in the applicationSearch the application source code for SSL/TLS configuration: look for calls to `sslConnector`, `https`, or loading of SSL configuration files (e.g., `apply { from("secure.conf") }`). Also check the application.conf for `ktor.ssl` block or similar SSL settings.Affected if SSL/TLS is configured and the Ktor version is below 1.4.2 - the default weak cipher suites will be in effect unless explicitly overridden
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Inspect the cipher suite configurationIf SSL/TLS is configured, examine the SSL configuration block for explicit `cipherSuites` or `trustStore` settings. Look for any custom cipher suite configuration in the application code or configuration file that loads the SSL settings.Affected if No custom strong cipher suites are configured, meaning the application relies on Ktor's default cipher suite settings (which are weak in versions before 1.4.2)
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Check for default configuration fallbackIf you cannot find explicit cipher suite configuration, verify that the SSL configuration is using Ktor's defaults. This typically means the SSL config does not include a `cipherSuites` list, relying on the JVM or Ktor default behavior.Affected if The SSL configuration relies on defaults without specifying strong cipher suites, and the Ktor version is below 1.4.2
You are affected if your project uses Ktor version below 1.4.2 AND has SSL/TLS configured without explicitly overriding the default cipher suites with strong algorithms.
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.
From vendor data1.4.2
Upgrade Ktor to version 1.4.2 or later where strong cipher suites are the default, or explicitly configure strong cipher suites in the application SSL configuration.
- Consultation2.0 h
- Implementation2.0 h
- Testing2.0 h
- Review / QA1.0 h
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Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-25763 — 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-2021-25763 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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A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.
- 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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