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

High severity GitHub Reviewed Published Jun 16, 2026 in http4k/http4k • Updated Aug 17, 2026

Package

maven org.http4k:http4k-core (Maven)

Affected versions

>= 5.0.0.0, < 5.42.0.0
<= 4.48.2.0
>= 6.0.0.0, < 6.49.0.0

Patched versions

5.42.0.0
6.49.0.0

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

Line Fixed in Edition
v6.x (Community) 6.49.0.0 Community
v5.x (LTS) 5.42.0.0 Enterprise — contact enterprise@http4k.org
v4.x (LTS) 4.51.0.0 Enterprise — contact enterprise@http4k.org

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

References

@daviddenton daviddenton published to http4k/http4k Jun 16, 2026
Published to the GitHub Advisory Database Aug 17, 2026
Reviewed Aug 17, 2026
Last updated Aug 17, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Network
Attack complexity
Low
Privileges required
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(50th percentile)

Weaknesses

Improper Handling of Highly Compressed Data (Data Amplification)

The product does not handle or incorrectly handles a compressed input with a very high compression ratio that produces a large output. Learn more on MITRE.

CVE ID

CVE-2026-53659

GHSA ID

GHSA-g4w2-6h2r-3m3w

Source code

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