Denial of Service (DoS) Affecting com.charleskorn.kaml:kaml package, versions [,0.35.3)

com.charleskorn.kaml:kaml is a YAML support for kotlinx.serialization.

Affected versions of this package are vulnerable to Denial of Service (DoS). An attacker 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 exhaustion 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.

PoC

# 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")
}

Denial of Service (DoS) describes a family of attacks, all aimed at making a system inaccessible to its intended and legitimate users.

Unlike other vulnerabilities, DoS attacks usually do not aim at breaching security. Rather, they are focused on making websites and services unavailable to genuine users resulting in downtime.

One popular Denial of Service vulnerability is DDoS (a Distributed Denial of Service), an attack that attempts to clog network pipes to the system by generating a large volume of traffic from many machines.

When it comes to open source libraries, DoS vulnerabilities allow attackers to trigger such a crash or crippling of the service by using a flaw either in the application code or from the use of open source libraries.

Two common types of DoS vulnerabilities:

• High CPU/Memory Consumption- An attacker sending crafted requests that could cause the system to take a disproportionate amount of time to process. For example, commons-fileupload:commons-fileupload.

• Crash - An attacker sending crafted requests that could cause the system to crash. For Example, npm ws package

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