CVE-2026-53914: Kotlin Build Cache Code Execution Vulnerability – Patch to 2.4.20

Technical summary

CVE-2026-53914 is a remote code execution flaw stemming from unsafe deserialization in Kotlin's build cache metadata layer (CWE-502). The vulnerability exists in Kotlin versions prior to 2.4.20. The attack surface is local; an attacker must have high privilege on the system running the Kotlin compiler or build process. The CVSS 3.1 score of 6.7 (MEDIUM) reflects the local attack vector and requirement for high privilege, but accounts for the confidentiality impact and potential cascade effects across containerized or multi-tenant build environments (scope change).

Business impact

For development teams using Kotlin, this vulnerability primarily affects build pipeline security. If an attacker gains high-level access to a build machine—whether through supply chain compromise, insider threat, or lateral movement—they can inject malicious code into compiled artifacts. This breaks the integrity of the build-to-deployment chain and could poison downstream applications. Organizations relying on Kotlin in CI/CD pipelines should assess whether their build infrastructure meets the privilege assumptions this vulnerability requires, and whether build caches are stored or shared in ways that could amplify the risk.

Affected systems

JetBrains Kotlin before version 2.4.20 is affected. This includes development environments, CI/CD pipelines, and any system running the Kotlin compiler with build caching enabled. The scope of impact depends on how build caches are stored—local-only caches pose lower risk than shared or remote caches accessible to multiple users or services.

Exploitability

Practical exploitation requires high privilege on the target system, which limits the attack surface. The vulnerability is not trivial to exploit remotely and is not listed on the CISA KEV catalog, indicating it is not currently known to be weaponized in the wild. However, in environments where multiple developers or services share build infrastructure, or where build caches are stored on shared network volumes, the privilege barrier could be lower than expected.

Remediation

Upgrade JetBrains Kotlin to version 2.4.20 or later. This is the primary mitigation. For teams unable to upgrade immediately, isolate build caches to single-user or single-service systems and restrict file system access to the Kotlin compiler and its cache directories.

Patch guidance

Verify the exact patched version against the JetBrains Kotlin release notes and advisory. Kotlin 2.4.20 and later contain the fix. Test the upgrade in a non-production environment first, as Kotlin compiler updates may affect build reproducibility or compatibility with plugins. Check for any interim build-cache artifacts created by vulnerable versions and consider invalidating them before upgrading.

Detection guidance

Monitor for file system access to Kotlin build cache directories (typically .gradle/cache or project-specific cache paths) from unexpected processes or users. Log compiler invocations and note any abnormal behavior in artifact generation. Endpoint Detection and Response (EDR) tools should flag unusual child process spawning from Kotlin/Java runtime environments. Review CI/CD pipeline logs for unexpected code or artifact changes correlated with build execution.

Why prioritize this

Although the CVSS score is MEDIUM, prioritize this for Kotlin-heavy organizations because: (1) build-chain integrity is critical to software supply chain security, (2) many teams do not regularly patch development tools, and (3) compromise of a build artifact can affect all downstream consumers. Prioritize systems with shared or remote build caches higher than isolated developer machines.

Risk score, explained

The CVSS 3.1 score of 6.7 reflects a local attack vector (AV:L) with high privilege requirement (PR:H), which correctly constrains the immediate exploitability. However, the scope change (S:C) and confidentiality impact (C:H) acknowledge that code execution in a build environment can escape its immediate context and compromise the entire application ecosystem. Organizations with mature supply-chain security practices should consider this a higher practical risk than the base score suggests.

Frequently asked questions