Introducing a Security Support Policy for the Kotlin Standard Library

Anton Yalyshev

Upgrade rhythms vary significantly among Kotlin’s user base. Some teams update whenever a new release lands without a second thought. On the other hand, a team inside a regulated organization moves on a multi-quarter cycle and treats every dependency as something that has to be reviewed, approved, and then frozen in production for a while.

Among all of these audiences, Kotlin’s adoption on the JVM keeps growing. Around half of Kotlin developers today write server-side applications, including in segments like payment infrastructure and banking, where some teams have been running Kotlin in production for years. A large portion of teams in segments like these work in environments where every production dependency goes through a formal security review.

In environments like these, platform teams run into a deceptively simple question: “Which Kotlin versions are supported?” Until today, we didn’t have a clean answer. This post introduces one.

Growing adoption means stronger compatibility and security guarantees

As more code depends on Kotlin, the language becomes more useful – and more constrained. People building on it expect that what they wrote yesterday will keep working tomorrow, that changes will be predictable, and that the team behind Kotlin will treat compatibility as a deliberate choice rather than an afterthought.

A few aspects of Kotlin already work this way. Source-level language stability means code written today keeps compiling on new releases. A documented deprecation cycle replaces silent breaking changes, so anything we remove is announced, and developers are given time to adapt. The Language Committee is the formal body that approves significant changes to the language. And the standard library carries its own backward-compatibility contract for public APIs across versions.

These commitments grew naturally as Kotlin became a load-bearing part of large codebases. Each one was added when it became clear we owed our users that guarantee.

But one key thing has been missing from that list, namely an answer to the question, “How long is a Kotlin release supported for security fixes?” For most teams, the gap is invisible. But there are organizations whose dependency reviews depend on the answer – and for them, this gap is everything.

Why the lack of a support policy was a problem

Kotlin’s release model is built around a steady cadence of stable releases. The latest stable release, whether it’s a language or tooling release, is the recommended baseline. Bug fixes and language development flow forward into the next release rather than backward through patches. For most teams, this works out perfectly – upgrading is straightforward, and there is little need to think about “support” as a separate concept.

For organizations that need a documented support signal, the consequences are concrete:

• Compliance teams cannot list Kotlin as a supported dependency in their standard process, because there is no formal end-of-support date to record.
• Each new Kotlin version in production triggers an individual security review instead of inheriting a documented support status.
• Procurement and vendor assessment frameworks ask for supplier documentation that doesn’t exist in this form.
• In the event of platform freezes, the absence of a policy means “upgrade immediately or lose support”. This approach doesn’t fit the way organizations like this actually manage dependencies.

Kotlin’s user base continues to grow, and that growth includes environments where the absence of a documented answer carries real cost. Addressing that absence is the next step.

Introducing a security support policy for Kotlin

• Each Kotlin release line (e.g., 2.4.x) is supported for security fixes for 18 months from the release date of its .0 version.
• Security fixes are backported to all release lines within an active support window and released as the latest patch in each line.
• Scope: the JVM kotlin-stdlib runtime artifact.

Why the JVM standard library specifically? The concern this policy primarily addresses is code running in production on the JVM – the runtime component every JVM Kotlin application links against and ships to its servers. That’s the layer compliance and security review processes focus on when assessing dependency freshness, and that’s where documented support actually unblocks decisions. Compile-time tooling – the compiler, Gradle and Maven plugins – sit in build infrastructure, not in the production runtime, and are governed differently. This policy targets exactly where the support is needed.

How patches are released. When a security issue is found and fixed, the fix is first added to a release based on the latest Kotlin release line and is then backported to every other release line still in support. Patches go out as the next patch release in each affected line – for example, if 2.4.20 is the current stable release in the 2.4 release line, the next patch release is 2.4.21. Each release line keeps its own version numbering, so a team that has qualified 2.4 for production can stay on 2.4 to receive the fix, without crossing into a new release line.

Releases ship together. Each security patch is a full Kotlin release – it goes through the standard release pipeline and ships the complete set of artifacts. You update one Kotlin version in your build and get the patched standard library. Patches for all affected supported lines are published simultaneously.

CVE and advisory. Security issues are assigned CVE identifiers where applicable and published on the JetBrains Fixed Security Issues page through the established JetBrains Security advisory process.

The policy applies to Kotlin lines released from launch onward (2.4 and later). Earlier lines remain on the previous model and are not retroactively covered.

The current list of supported release lines, their end-of-support dates, and the latest patch version in each line is maintained on a dedicated support page on kotlinlang.org. That page is the canonical reference for which versions are currently supported.

How a release line evolves

The policy is easier to follow if you watch one release line evolve from the first release (.0) until the end of support. Below is an example of what this looks like for 2.4 (with an approximate timeline; the exact dates do not matter for this example).

1. 2.4.0 ships. The 2.4 line enters its 18-month security support window.
2. A security report comes in shortly after release. The security fix is added to the release line and is shipped as 2.4.10. (Note: the x.x.10 slot follows the established Kotlin convention for the first bug fix on x.x.0.)
3. Months later, 2.4.20 ships as the next regular release in the 2.4 line. It includes all prior security fixes.
4. A new security report comes in after 2.4.20. The security fix goes out as 2.4.21. The latest patch in the 2.4 line is now 2.4.21 – the version supported teams should be on.
5. 2.5.0 ships, opening its own 18-month window. The 2.4 line is still in support; both lines now receive security backports.
6. 2.6.0 ships, opening the 2.6 line. By this point, 2.5 has had its own regular cycle and is on 2.5.20; 2.4 is still in support at 2.4.21. A new security issue is reported and confirmed. The fix is added to the latest stable release  as 2.6.10, and is simultaneously backported to the still-supported 2.5 and 2.4 lines as 2.5.21 and 2.4.22 – all three releases on the same day.
7. The 2.4 line reaches the end of support 18 months after 2.4.0. Security fixes stop for that line.

Two practical takeaways: first, you can stay on the release line you’ve qualified for production and still receive security fixes – you do not need to skip releases. Second, when a fix is published, it becomes available on all supported lines at the same time. There is no “the latest line is patched, your older line will get it eventually” gap.

What is not changing

• Kotlin’s release process is unchanged. Bug fixes, language and library features, and performance improvements continue to ship in new releases the way they always have. Older still-supported lines receive only security backports.
• Each new Kotlin release is still the recommended baseline for new projects. The security support window exists for organizations that need to stay on a specific minor line for compliance reasons – we still do not recommend delaying upgrades.

FAQ and where to go next

Q: What counts as a security fix under this policy? A: Issues with confirmed security impact – vulnerabilities of the kind tracked by CVE, where the documented and correct use of an API leads to security impact. Application-level misuse and issues caused by passing unvalidated user input into stdlib APIs are not covered.

Q: Will I have to upgrade to get a security fix? A: Only within your release line. The fix is shipped as the next patch in that line – for example, 2.4.20 → 2.4.21. You do not need to jump to a newer release to receive it.

Q: Where do I see which lines are currently supported? A: The dedicated support page on kotlinlang.org. You can find the link below.

Q: My team uses libraries that depend on a different standard library version. Does that matter? A: Yes. Only one version of kotlin-stdlib ends up on the classpath after dependency resolution, and which specific version that is depends on your build tool. Gradle’s default resolution picks the highest version requested among all your direct and transitive dependencies – so if a transitive dependency pulls in a newer standard library, that newer version is the one running, not the patched one you set in your build. Maven uses “nearest wins” by default. In either case, pin the resolved standard library explicitly (via a BOM, version constraints, or strict-version rules) to make sure the patched version you want is the one that runs.

Where to go next:

• Support page (current supported lines, end-of-support dates, latest patches): kotl.in/stdlib-security
• Security policy: kotlinlang.org/docs/security.html