External risk intelligence

JetBrains Kotlin Unsafe Deserialization Code Execution

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-53914

This vulnerability exists in the build cache metadata processing of a programming language compiler. Build caches are developer-centric, internal components used during the software development lifecycle, and are not intended to be exposed to or reachable from the public internet.

Deserialization

Jetbrains Kotlin

before 2.4.20

Halo Surface Signal: 1 out of 5 — much less likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in JetBrains Kotlin, impacting its build cache metadata processing. This issue could allow for code execution, presenting a significant risk if exploited. The primary concern is to confirm if your development environments utilize affected versions of Kotlin and are potentially exposed.

Code execution risk via build cache metadata.
Protects developer tooling and build processes.
Confirm Kotlin usage and exposure in your environments.

Attack Path

How an attacker could exploit the issue

An attacker could achieve code execution by exploiting a flaw in how Kotlin's build cache metadata is handled. This vulnerability is accessible over the network and does not require any prior authentication or user interaction. Successful exploitation could allow an attacker to run arbitrary code on the affected system.

Network access required.
Unsafe deserialization of build cache metadata.
Leads to arbitrary code execution.

Live Threat

Current exploitation, exposure, and threat context

When supported by the advisory, unsafe deserialization in the build cache metadata of JetBrains Kotlin could allow for code execution. This could impact the integrity and availability of development environments and potentially lead to the execution of arbitrary code on the build system.

Build cache metadata.
Unsafe deserialization of build cache data.
Arbitrary code execution on the build system.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability in Kotlin's build cache metadata could allow for code execution if exploited via unsafe deserialization. Teams responsible for the development toolchain, including application owners and platform engineers, should investigate its presence. The first step is to identify all instances of the affected Kotlin versions, determine their exposure, and confirm criticality to prioritize remediation.

Application and platform teams own resolution.
Verify build cache exposure and Kotlin versions.
Plan remediation based on business impact.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is JetBrains Kotlin?

Kotlin is a statically typed programming language widely used for building cross-platform applications, including Android mobile apps, server-side services, and multiplatform libraries. It includes a compiler and various development tools that manage how code is translated and built. This vulnerability specifically impacts the build cache component, which is designed to speed up compilation by storing and retrieving metadata from previous builds.

How does CVE-2026-53914 work?

This issue is classified as CWE-502, which is Deserialization of Untrusted Data. In plain terms, the software takes data from the build cache metadata and reconstructs it into an object without sufficient validation. Because the process is unsafe, an attacker can craft malicious metadata that, when processed, tricks the system into executing unauthorized commands or code on the machine performing the build.

Do I need to worry about local builds?

The trigger requires the build cache metadata to be accessible over the network. If your Kotlin build cache is strictly local to a developer's machine and not shared or reachable by other systems, the risk of external exploitation is significantly lower. The vulnerability does not trigger during standard coding activities if the attacker cannot reach or influence the specific metadata files being processed by the build tool.

How relevant is this CVE-2026-53914 to me?

According to Halo Surface Signal, this vulnerability is very unlikely to be exploited externally. Because the issue resides in developer-centric build cache components rather than public-facing web applications, these systems should naturally remain internal. You should care if your build infrastructure is shared, networked, or managed in a way that makes it potentially reachable from outside your secure development environment.

What should I do to address this vulnerability?

Start by identifying all environments in your organization running Kotlin versions earlier than 2.4.20. Focus on build servers, continuous integration pipelines, and any shared developer workstations that utilize network-accessible build caches. Once identified, prioritize updating the Kotlin toolchain to version 2.4.20 or later to patch the deserialization flaw. Consult with your platform engineering team to verify that build cache configurations follow internal security standards.

References

www.jetbrains.com/privacy-security/issues-fixed/

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.