CVE-2021-39194

Improper Handling of Missing Values in kaml

Attackers that could provide arbitrary YAML input to an application that uses kaml could cause the application to endlessly loop while parsing the input. This could result in resource starvation and denial of service.

This only affects applications that use polymorphic serialization with the default tagged polymorphism style. Applications using the property polymorphism style are not affected.

YAML input for a polymorphic type that provided a tag but no value for the object would trigger the issue, for example:

!<x>

The following is a sample application that demonstrates this issue:

import com.charleskorn.kaml.Yaml
import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable

@Serializable
private sealed class K {
    @Serializable
    @SerialName("x")
    data class X(
        val property: String? = null,
    ) : K()
}

const val s = """
!<x>
"""

fun main() {
    println("Started.")
    val result = Yaml.default.decodeFromString(K.serializer(), s)
    println("Finished, result is $result")
}

On vulnerable versions of kaml, the decodeFromString() operation hangs and never returns.

CVE-2021-39194 has a CVSS score of 4.3 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.35.3); upgrading removes the vulnerable code path.

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Version 0.35.3 or later contain the fix for this issue.

1. What is CVE-2021-39194? CVE-2021-39194 is a medium-severity security vulnerability in com.charleskorn.kaml:kaml (maven), affecting versions < 0.35.2. It is fixed in 0.35.3.
2. How severe is CVE-2021-39194? CVE-2021-39194 has a CVSS score of 4.3 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of com.charleskorn.kaml:kaml are affected by CVE-2021-39194? com.charleskorn.kaml:kaml (maven) versions < 0.35.2 is affected.
4. Is there a fix for CVE-2021-39194? Yes. CVE-2021-39194 is fixed in 0.35.3. Upgrade to this version or later.
5. Is CVE-2021-39194 exploitable, and should I be worried? Whether CVE-2021-39194 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-39194 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-39194? Upgrade com.charleskorn.kaml:kaml to 0.35.3 or later.

Other vulnerabilities in com.charleskorn.kaml:kaml