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KafkaApplication · Apache

CVE-2026-33557

CRITICAL · 9.1 CVSS v3.1 Published 2026-04-20
Fix available
A fix is available. Upgrade to 4.1.2 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

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 · unedited
A possible security vulnerability has been identified in Apache Kafka. By default, the broker property `sasl.oauthbearer.jwt.validator.class` is set to `org.apache.kafka.common.security.oauthbearer.DefaultJwtValidator`. It accepts any JWT token without validating its signature, issuer, or audience. An attacker can generate a JWT token from any issuer with the `preferred_username` set to any user, and the broker will accept it. We advise the Kafka users using kafka v4.1.0 or v4.1.1 to set the config `sasl.oauthbearer.jwt.validator.class` to `org.apache.kafka.common.security.oauthbearer.BrokerJwtValidator` explicitly to avoid this vulnerability. Since Kafka v4.1.2 and v4.2.0 and later, the issue is fixed and will correctly validate the JWT token.

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 confidence

The default JWT validator in Apache Kafka v4.1.0 and v4.1.1 (DefaultJwtValidator) does not validate JWT signatures, issuers, or audiences. An attacker can generate arbitrary JWT tokens with any preferred_username and the broker will accept them, enabling unauthorized access.

MitigationSet the broker property sasl.oauthbearer.jwt.validator.class to org.apache.kafka.common.security.oauthbearer.BrokerJwtValidator, or upgrade to Kafka v4.1.2+ or v4.2.0+ where the vulnerability is fixed.

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
KafkaApplication
Affected:>= 4.1.0, < 4.1.2

CVSS 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 checks

Work through these to decide whether this CVE applies to you.

  1. Check Kafka broker version
    Run `kafka-broker-api-versions` or check the kafka_2.13-<version>.jar filename in the Kafka lib directory, or query the broker version via JMX if available
    Affected if Version is 4.1.0 or 4.1.1
  2. Verify SASL OAuthBearer is enabled
    Inspect broker configuration file (server.properties) or dynamic broker config for properties containing 'sasl.mechanism=OAUTHBEARER' or 'listener.name.sasl.\*.oauthbearer'
    Affected if OAuthBearer SASL mechanism is configured and enabled on any listener
  3. Confirm default JWT validator is in use
    Check broker configuration for 'sasl.oauthbearer.jwt.validator.class' setting - if absent or set to DefaultJwtValidator, the vulnerable default is active
    Affected if The property is not set, or is explicitly set to org.apache.kafka.common.security.oauthbearer.DefaultJwtValidator
  4. Inspect OAuthBearer token validation configuration
    Review all OAuthBearer-related broker settings including 'sasl.oauthbearer.token.principal.builder', 'oauthbearer.jwks.endpoint.url', 'oauthbearer.issuer.discovery.url', or 'sasl.oauthbearer.sub.claim.name'
    Affected if No signature, issuer, or audience validation is configured - these properties are absent or empty

Environment is affected if running Kafka 4.1.0 or 4.1.1 with SASL OAuthBearer authentication enabled and the default JWT validator (or no validator class) is configured.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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 · scoped
Upgrade available Upgrade to 4.1.2 or later
Fixed in 4.1.2
Interim mitigation

Set the broker property sasl.oauthbearer.jwt.validator.class to org.apache.kafka.common.security.oauthbearer.BrokerJwtValidator, or upgrade to Kafka v4.1.2+ or v4.2.0+ where the vulnerability is fixed.

Recommended fix High confidence

Kafka v4.1.2 or later (or v4.2.0 and later)

  1. 1. Plan upgrade to Kafka v4.1.2 or later (or v4.2.0 and later for the latest stable release).
  2. 2. Review Kafka upgrade documentation for your current version to ensure compatibility.
  3. 3. Back up Kafka configuration files (server.properties, jaas.conf, etc.).
  4. 4. Stop the Kafka broker cluster gracefully.
  5. 5. Upgrade Kafka binaries to version 4.1.2 or later.
  6. 6. Restart Kafka brokers and verify startup.
  7. 7. Verify OAuth Bearer authentication is working correctly with existing JWT tokens.
Caveat Review release notes for any breaking changes between your current version and the target version; minor version upgrades in Kafka typically have limited breaking changes but configuration deprecations may occur.

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Kafka Scoped from the published advisory
  • Consultation3.0 h
  • Implementation2.0 h
  • Testing3.0 h
  • Review / QA1.0 h
9.0 hours of engineering $1,590
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2026-33557 in production — separate from our analysis above.

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What this is

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What belongs here
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  • Version or environment caveats, and links to real fixes
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  • No spam, self-promotion, credentials, or personal data