CVE-2026-53914: JetBrains Kotlin RCE Vulnerability

CVE-2026-53914 Overview

CVE-2026-53914 is an insecure deserialization vulnerability [CWE-502] in JetBrains Kotlin versions prior to 2.4.20. The flaw resides in the build cache metadata handling, where untrusted serialized objects are deserialized without validation. Attackers can achieve arbitrary code execution by supplying crafted metadata to a vulnerable Kotlin build environment. JetBrains addressed the issue in Kotlin 2.4.20.

Critical Impact
Unsafe deserialization in build cache metadata enables remote code execution against Kotlin build pipelines, exposing developer workstations and CI/CD infrastructure to full compromise.

Affected Products

JetBrains Kotlin versions prior to 2.4.20
Build environments and CI/CD pipelines consuming Kotlin build cache artifacts
Developer workstations running affected Kotlin compiler and Gradle plugin releases

Discovery Timeline

2026-06-26 - CVE-2026-53914 published to the National Vulnerability Database
2026-06-27 - Last updated in NVD database

Technical Details for CVE-2026-53914

Vulnerability Analysis

The vulnerability stems from the Kotlin build cache subsystem deserializing metadata objects without enforcing type or integrity constraints. Kotlin's build cache stores task outputs and metadata to speed up incremental compilation. When the cache reader consumes attacker-controlled serialized data, it instantiates arbitrary object graphs. This behavior allows gadget chains within the classpath to trigger code execution during deserialization.

Because build tooling routinely runs with the developer's privileges, successful exploitation grants the attacker the same access as the compiling user. In shared CI/CD environments, that access frequently extends to source repositories, signing keys, artifact registries, and cloud credentials.

Root Cause

The root cause is classified under [CWE-502] Deserialization of Untrusted Data. The build cache metadata deserializer accepts serialized objects without a strict allowlist of permissible classes. No integrity signature or authenticated envelope protects cache entries, so tampered or malicious cache payloads reach the deserialization path unchanged.

Attack Vector

Exploitation follows the network attack vector with no authentication or user interaction required. Attackers can plant a malicious build cache entry in a shared remote cache, a compromised dependency artifact, or a mirrored cache backend. When a victim's Kotlin build reads the poisoned metadata, deserialization triggers the gadget chain and executes attacker code inside the build process.

The vulnerability is described in prose because JetBrains has not released a public proof of concept. Refer to the JetBrains Security Issues Fixed advisory for vendor-confirmed technical details.

Detection Methods for CVE-2026-53914

Indicators of Compromise

Unexpected child processes spawned by the Kotlin compiler, Gradle daemon, or Kotlin build task JVMs
Anomalous outbound network connections originating from build agents during compilation phases
Modified or unsigned entries in shared remote build cache backends such as HTTP caches or S3 buckets
New or altered files in developer home directories immediately following a Kotlin build

Detection Strategies

Inventory all Kotlin toolchain versions across developer workstations and CI runners, flagging any release earlier than 2.4.20
Alert on JVM processes loading serialization gadget classes commonly abused in Java deserialization chains
Correlate build cache reads with subsequent process execution or credential access events using EDR telemetry

Monitoring Recommendations

Enable audit logging on remote build cache endpoints and monitor for writes from unexpected identities
Baseline normal Gradle and Kotlin daemon behavior, then alert on deviations such as shell spawns or network egress
Ingest build system logs into a centralized analytics platform to hunt for anomalous cache hit patterns

How to Mitigate CVE-2026-53914

Immediate Actions Required

Upgrade Kotlin to version 2.4.20 or later across all developer workstations, build servers, and CI/CD runners
Invalidate and rebuild any shared remote build caches populated by vulnerable Kotlin versions
Rotate credentials, signing keys, and tokens that were accessible from potentially compromised build environments

Patch Information

JetBrains fixed the unsafe deserialization in Kotlin 2.4.20. Update the Kotlin Gradle plugin, compiler, and any embedded Kotlin runtimes to the patched release. Full remediation details are published in the JetBrains Security Issues Fixed advisory.

Workarounds

Disable remote build cache consumption until all producers and consumers run Kotlin 2.4.20 or later
Restrict write access to shared build caches to trusted CI identities and enforce authenticated transport
Isolate Kotlin builds in ephemeral sandboxed runners with least-privilege credentials and no persistent secrets

bash

# Pin Kotlin to the patched release in Gradle
# gradle.properties
kotlin.version=2.4.20

# Temporarily disable remote build cache in settings.gradle.kts
buildCache {
    remote<HttpBuildCache> {
        isEnabled = false
    }
}