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HIGH severity

GHSA-g4w2-6h2r-3m3w — http4k-core

HIGH

GHSA-g4w2-6h2r-3m3w is a high-severity (CVSS 7.5) CWE-409 vulnerability in org.http4k:http4k-core. A fix is available for org.http4k:http4k-core — see the affected versions and patch details below.

http4k: Unbounded gzip decompression in `ServerFilters.GZip` / `RequestFilters.GunZip` allowed memory-exhaustion DoS

Also known asCVE-2026-53659
Published
Updated
Affected
3 pkgs
Patched
2 / 3
Exploits
None indexed
Exploitation data as of Oct 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

  • CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
  • CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for GHSA-g4w2-6h2r-3m3w.

EPSS Exploitation Probability

via FIRST.org ↗
0.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs48th percentile — riskier than 48% of all scored CVEsHighest risk
0.13%0.46%0.80%1.13%0.6%0.6%Oct 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-g4w2-6h2r-3m3w by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 382,205 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

3 pkgs affected
☕org.http4k:http4k-core☕org.http4k:http4k-core☕org.http4k:http4k-core

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

Impact

ServerFilters.GZip and RequestFilters.GunZip (and the underlying Gzip functions used to decompress request bodies) did not impose any cap on the decompressed size. A small malicious gzip-encoded request body (on the order of kilobytes) could decompress to gigabytes, exhausting the JVM heap and denying service to other clients.

Who is affected: any http4k server that accepts gzip-encoded requests via ServerFilters.GZip or RequestFilters.GunZip. Exploitable by any unauthenticated client. The vulnerability was introduced on 2017-08-01 (commit 2618fe08f9) and was present for ~9 years.

Patches

LineFixed inEdition
v6.x (Community)6.49.0.0Community
v5.x (LTS)5.42.0.0Enterprise — contact [email protected]
v4.x (LTS)4.51.0.0Enterprise — contact [email protected]

The fix caps decompression at 10MB by default; oversized requests through ServerFilters.GZip / RequestFilters.GunZip now return 413 Request Entity Too Large, and decompressing elsewhere throws SizeLimitExceededException. The keyed hmacSHA256 helper and other safe paths are unaffected.

Workarounds

For deployments that cannot upgrade immediately:

  • Replace the GZip / GunZip filters with custom versions that wrap the decompressed InputStream in a size-limited reader, or
  • Strip gzip-encoded request support at the edge (CDN, reverse proxy, or load balancer).

References

Affected Packages

3 total 2 fixed
EcosystemPackageVulnerable rangeFix
☕Mavenorg.http4k:http4k-core≥ 6.0.0.0&&< 6.49.0.06.49.0.0org.http4k:http4k-core:6.49.0.0
☕Mavenorg.http4k:http4k-core≥ 5.0.0.0&&< 5.42.0.05.42.0.0org.http4k:http4k-core:5.42.0.0
☕Mavenorg.http4k:http4k-coreall versionsNo fix

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4k:http4k-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update org.http4k:http4k-core to 6.49.0.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-g4w2-6h2r-3m3w is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact `ServerFilters.GZip` and `RequestFilters.GunZip` (and the underlying `Gzip` functions used to decompress request bodies) did not impose any cap on the decompressed size. A small malicious gzip-encoded request body (on the order of kilobytes) could decompress to gigabytes, exhausting the JVM heap and denying service to other clients. **Who is affected:** any http4k server that accepts gzip-encoded requests via `ServerFilters.GZip` or `RequestFilters.GunZip`. Exploitable by any unauthenticated client. The vulnerability was introduced on 2017-08-01 (commit `2618fe08f9`) and was prese
O3 Security · Impact-Aware SCA

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GHSA-g4w2-6h2r-3m3w: http4k-core | O3 Security