CVE-2021-39194Public exploitNetwork attack vector

Denial of service via endless polymorphic parse

Published Sep 7, 2021 · Updated Aug 4, 2024

Denial-of-service vulnerability in Charles Korn kaml before 0.35.3 allows remote authenticated users to interrupt service via crafted YAML. YamlNullInput.decodeElementIndex returned index 0 instead of DECODE_DONE when tagged polymorphic input had no value, so decodeFromString looped indefinitely. The gate is an application that accepts attacker-supplied YAML and uses default tagged polymorphism; property polymorphism is unaffected, and impact is resource starvation and service hang.

CVSS severity4.3
Medium
EPSS probability1.66%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Denial-of-service vulnerability in Charles Korn kaml before 0.35.3 allows remote authenticated users to interrupt service via crafted YAML. YamlNullInput.decodeElementIndex returned index 0 instead of DECODE_DONE when tagged polymorphic input had no value, so decodeFromString looped indefinitely. The gate is an application that accepts attacker-supplied YAML and uses default tagged polymorphism; property polymorphism is unaffected, and impact is resource starvation and service hang.

The record

CVE
CVE-2021-39194
Published
Sep 7, 2021
Updated
Aug 4, 2024
Vendor
Charles Korn
Product
kaml
Classifications
CWE-835, CWE-230, T1499.002
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. Sep 7, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 0.35.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Charles Korn · kamlversion=< 0.35.2

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H
4.0NIST NVDAV:N/AC:L/Au:S/C:N/I:N/A:P
4.3GitHubCVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-835Loop with Unreachable Exit Condition ('Infinite Loop')
CWE-230Improper Handling of Missing Values
T1499.002Service Exhaustion

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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