Medium

Improper Handling of Missing Values in kaml

Disclosed: 07.09.2021
Updated: 11.10.2026 12:22

Impact

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:

yaml

!<x>

The following is a sample application that demonstrates this issue:

kotlin

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.

Patches

Version 0.35.3 or later contain the fix for this issue.

Affected software

Click a technology to see all vulnerabilities affecting it.

Platform	Package	Vulnerable versions	Fixed in
Java / Kotlin (Maven)	com.charleskorn.kaml:kaml	< 0.35.3	0.35.3

References

CVSS score 4.3/10

CVSS vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
Weakness (CWE): CWE-230, CWE-835
Platform: Java / Kotlin (Maven)
Package: com.charleskorn.kaml:kaml
Disclosed: 07.09.2021
Updated: 11.10.2026 12:22

Sources and licenses

OSV / GitHub Advisory Database - GHSA-fmm9-3gv8-58f4 - CC BY 4.0

Vulnerability history: com.charleskorn.kaml:kaml

All published vulnerabilities of this package (Java / Kotlin (Maven)), newest first. Shows whether this is a one-off slip or a package that needs watching.

Total entries: 2
First → latest: 09.2021 03.2023
Without a fix: 0
Exploited in attacks: 0

Severity breakdown

High 1
Medium 1

Disclosures per year

2021: 1
2022: 0
2023: 1
2024: 0
2025: 0
2026: 0

20.03.2023

kaml has potential denial of service while parsing input with anchors and aliases

High · 7.5 CVE-2023-28118 < 0.53.0 0.53.0
07.09.2021

Improper Handling of Missing Values in kaml this entry

Medium · 4.3 CVE-2021-39194 < 0.35.3 0.35.3

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