Vulnerability in Http4k

CVE-2026-53659

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.

Published · last modified .

CVSS v3 metric

CVSS v3 base score 7.5 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

EPSS exploit prediction

EPSS: 0.006 (50.1th percentile), scored .

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 50.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS trend (30 days)EPSS over the last 30 days for CVE-2026-53659: held from 0.006 to 0.006.

EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000

Affected products

  • Http4k — versions < 4.51.0.0, >= 5.0.0.0, < 5.42.0.0, >= 6.0.0.0, < 6.49.0.0

Weakness classification (CWE)

References

Frequently asked questions

What is CVE-2026-53659?
CVE-2026-53659 is a high-severity vulnerability in Http4k, classified under Improper Handling of Highly Compressed Data (Data Amplification). CVSS score: 7.5/10. Published 2026-09-14.
How severe is CVE-2026-53659?
High severity. CVSS v3 base score is 7.5 out of 10.