CVE-2026-53659
HIGHCVE-2026-53659 is a high-severity (CVSS 7.5) vulnerability in org.http4k:http4k-core. O3 Security confirms whether CVE-2026-53659 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
http4k: Unbounded gzip decompression in `ServerFilters.GZip` / `RequestFilters.GunZip` allowed memory-exhaustion DoS
Real-World Exposure
org.http4k:http4k-core☕org.http4k:http4k-core☕org.http4k:http4k-coreReal-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/GunZipfilters with custom versions that wrap the decompressedInputStreamin 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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | org.http4k:http4k-core | ≥ 6.0.0.0&&< 6.49.0.0 | 6.49.0.0 |
| ☕Maven | org.http4k:http4k-core | ≥ 5.0.0.0&&< 5.42.0.0 | 5.42.0.0 |
| ☕Maven | org.http4k:http4k-core | all versions | No fix |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.http4k:http4k-core. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
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.
How O3 protects you
O3 pinpoints whether CVE-2026-53659 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-53659. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-53659 in your dependencies?
O3 detects CVE-2026-53659 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.