GHSA-fmm9-3gv8-58f4
MEDIUMImproper Handling of Missing Values in kaml
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Blast Radius
com.charleskorn.kaml:kamlReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
Description
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:
!<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.
Patches
Version 0.35.3 or later contain the fix for this issue.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | com.charleskorn.kaml:kaml | all versions | 0.35.3 |
Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for com.charleskorn.kaml:kaml. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update com.charleskorn.kaml:kaml to 0.35.3 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-fmm9-3gv8-58f4 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether GHSA-fmm9-3gv8-58f4 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-fmm9-3gv8-58f4. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-fmm9-3gv8-58f4 in your dependencies?
O3 detects GHSA-fmm9-3gv8-58f4 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.