# CVE-2026-53659: http4k: Unbounded gzip decompression in `ServerFilters.GZip` / `RequestFilters.GunZip` allowed memory-exhaustion DoS > An unauthenticated attacker can cause memory exhaustion and service denial by sending small gzip payloads that expand excessively in http4k applications. - **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 14 Sep 2026 - **Updated:** 14 Sep 2026 - **Also known as:** GHSA-g4w2-6h2r-3m3w ## What is CVE-2026-53659? CVE-2026-53659 is a high-severity denial of service vulnerability in http4k, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description http4k is a functional toolkit for Kotlin HTTP applications. Prior to 4.51.0.0, 5.42.0.0, and 6.49.0.0, ServerFilters.GZip, RequestFilters.GunZip, and the underlying Gzip request-body decompression functions impose no limit on decompressed size. An unauthenticated client can send a small gzip-encoded request body that expands to gigabytes, exhausting the JVM heap and denying service to other clients. The fix uses SizeLimitedInputStream to enforce a default 10 MiB limit, causes ServerFilters.GZip and RequestFilters.GunZip to return 413 Request Entity Too Large, and causes other decompression paths to throw SizeLimitExceededException. This issue is fixed in versions 4.51.0.0, 5.42.0.0, and 6.49.0.0. Weakness: CWE-409 (Improper Handling of Highly Compressed Data (Data Amplification)) ## Which versions are affected by CVE-2026-53659? 6 open source packages ship an affected version, including org.http4k:http4k-core. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `org.http4k:http4k-core` | Maven | All versions to 4.48.2.0 | None | | `org.http4k:http4k-core` | Maven | All versions to 4.48.2.0 | None | | `org.http4k:http4k-core` | Maven | From 5.0.0.0 | 5.42.0.0 | | `org.http4k:http4k-core` | Maven | From 5.0.0.0 | 5.42.0.0 | | `org.http4k:http4k-core` | Maven | From 6.0.0.0 | 6.49.0.0 | | `org.http4k:http4k-core` | Maven | From 6.0.0.0 | 6.49.0.0 | ## How do I fix CVE-2026-53659? Upgrade org.http4k:http4k-core to a fixed version for your release line: 5.42.0.0 or 6.49.0.0. ## Is CVE-2026-53659 being exploited? There is no known exploitation of CVE-2026-53659 in the wild and no public exploit code. ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2026-53659 and fix it if it is affected. What it is: CVE-2026-53659 is a high-severity denial of service vulnerability in http4k, published on 14 Sep 2026. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2026-53659 in the wild and no public exploit code. Affected packages and fixed versions: - org.http4k:http4k-core (Maven): affected all versions up to 4.48.2.0; no fix listed - org.http4k:http4k-core (Maven): affected all versions up to 4.48.2.0; no fix listed - org.http4k:http4k-core (Maven): affected from 5.0.0.0; fixed in 5.42.0.0 - org.http4k:http4k-core (Maven): affected from 5.0.0.0; fixed in 5.42.0.0 - org.http4k:http4k-core (Maven): affected from 6.0.0.0; fixed in 6.49.0.0 - org.http4k:http4k-core (Maven): affected from 6.0.0.0; fixed in 6.49.0.0 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2026-53659/ (Markdown: https://insights.sbomify.com/CVE-2026-53659/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2026-53659 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-53659 - OSV: https://osv.dev/vulnerability/CVE-2026-53659 --- From sbomify Insights: https://insights.sbomify.com/CVE-2026-53659/ Summaries are generated and may contain mistakes; the official description above is the record.