How severe is CVE-2026-53659?

CVE-2026-53659 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-53659?

CVE-2026-53659 affects the following packages: org.http4k:http4k-core (Maven), org.http4k:http4k-core (Maven), org.http4k:http4k-core (Maven). Ecosystems affected: Maven.

How do I fix CVE-2026-53659?

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 CVE-2026-53659 is resolved across your whole dependency graph.

How do I detect CVE-2026-53659 in my Maven dependencies?

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.

How do I mitigate CVE-2026-53659 if there is no patch (or I can't update yet)?

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.

Is CVE-2026-53659 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-53659 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-53659?

CVE-2026-53659 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 48th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-53659?

CVE-2026-53659 is classified as CWE-409 (CWE-409). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-53659 published, and has it been updated?

CVE-2026-53659 was published on September 14, 2026 and was last updated on October 2, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

HIGH severity

CVE-2026-53659 — http4k-core

HIGHFix: http4k/http4k@2618fe0

CVE-2026-53659 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 asGHSA-g4w2-6h2r-3m3w

Published

Sep 14, 2026

Updated

Oct 2, 2026

Affected

3 pkgs

Patched

2 / 3

Exploits

None indexed

Exploitation data as of Oct 1, 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 CVE-2026-53659.

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

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

How urgent is this, really

CVE-2026-53659 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

Line	Fixed in	Edition
v6.x (Community)	6.49.0.0	Community
v5.x (LTS)	5.42.0.0	Enterprise — contact [email protected]
v4.x (LTS)	4.51.0.0	Enterprise — 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

Vulnerability introduced: 2618fe08f9
Fix release: v6.49.0.0
Background: CWE-409 — Improper Handling of Highly Compressed Data

Affected Packages

3 total 2 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.http4k:http4k-core`	≥ 6.0.0.0&&< 6.49.0.0	6.49.0.0`org.http4k:http4k-core:6.49.0.0`
☕Maven	`org.http4k:http4k-core`	≥ 5.0.0.0&&< 5.42.0.0	5.42.0.0`org.http4k:http4k-core:5.42.0.0`
☕Maven	`org.http4k:http4k-core`	all versions	No fix

Detection & mitigation playbook

Open-source dependency

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.
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 CVE-2026-53659 is resolved across your whole dependency graph.
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