limit Diffie-Hellman Group Exchange prime sizes

ggrandes · Jun 28, 2020 · d0ac8d7 · d0ac8d7
1 parent f091d64
commit d0ac8d7
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diff --git a/src/main/java/org/javastack/sftpserver/ModuliFilter.java b/src/main/java/org/javastack/sftpserver/ModuliFilter.java
@@ -0,0 +1,82 @@
+package org.javastack.sftpserver;
+
+import java.io.BufferedReader;
+import java.io.BufferedWriter;
+import java.io.File;
+import java.io.FileOutputStream;
+import java.io.IOException;
+import java.io.InputStreamReader;
+import java.io.OutputStreamWriter;
+import java.net.URL;
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.List;
+
+/**
+ * @see org.apache.sshd.server.kex.Moduli
+ */
+public class ModuliFilter {
+	public static final int MODULI_TYPE_SAFE = 2;
+	public static final int MODULI_TESTS_COMPOSITE = 0x01;
+
+	public static ArrayList<String> filterModuli(final URL url, final int minSize, final int maxSize)
+			throws IOException {
+		final ArrayList<String> parsed = new ArrayList<>();
+		try (BufferedReader r = new BufferedReader(new InputStreamReader(url.openStream(), StandardCharsets.UTF_8))) {
+			for (String line = r.readLine(); line != null; line = r.readLine()) {
+				line = line.trim();
+				if (line.isEmpty()) {
+					continue;
+				}
+
+				if (line.startsWith("#")) {
+					continue;
+				}
+
+				String[] parts = line.split("\\s+");
+				// Ensure valid line
+				if (parts.length != 7) {
+					continue;
+				}
+
+				// Discard moduli types which are not safe
+				int type = Integer.parseInt(parts[1]);
+				if (type != MODULI_TYPE_SAFE) {
+					continue;
+				}
+
+				// Discard untested moduli
+				int tests = Integer.parseInt(parts[2]);
+				if (((tests & MODULI_TESTS_COMPOSITE) != 0) || ((tests & ~MODULI_TESTS_COMPOSITE) == 0)) {
+					continue;
+				}
+
+				// Discard untried
+				int tries = Integer.parseInt(parts[3]);
+				if (tries == 0) {
+					continue;
+				}
+
+				// Discard unwanted sizes
+				int size = Integer.parseInt(parts[4]);
+				if ((size < minSize) || (size > maxSize)) {
+					continue;
+				}
+
+				parsed.add(line);
+			}
+
+			return parsed;
+		}
+	}
+
+	public static void writeModuli(final File file, final List<String> lines) throws IOException {
+		try (BufferedWriter r = new BufferedWriter(
+				new OutputStreamWriter(new FileOutputStream(file), StandardCharsets.UTF_8))) {
+			for (final String str : lines) {
+				r.write(str);
+				r.newLine();
+			}
+		}
+	}
+}
diff --git a/src/main/java/org/javastack/sftpserver/Server.java b/src/main/java/org/javastack/sftpserver/Server.java
@@ -21,6 +21,7 @@
 import java.io.InputStream;
 import java.io.InputStreamReader;
 import java.io.OutputStream;
+import java.net.URL;
 import java.nio.file.FileSystem;
 import java.nio.file.LinkOption;
 import java.nio.file.Path;
@@ -64,6 +65,7 @@
 import org.apache.sshd.server.channel.ChannelSession;
 import org.apache.sshd.server.channel.ChannelSessionFactory;
 import org.apache.sshd.server.command.Command;
+import org.apache.sshd.server.kex.Moduli;
 import org.apache.sshd.server.keyprovider.AbstractGeneratorHostKeyProvider;
 import org.apache.sshd.server.keyprovider.SimpleGeneratorHostKeyProvider;
 import org.apache.sshd.server.scp.ScpCommandFactory;
@@ -244,6 +246,56 @@ private void hackVersion() {
 		PropertyResolverUtils.updateProperty(sshd, ServerFactoryManager.SERVER_IDENTIFICATION, "SSHD");
 	}
 
+	/**
+	 * Filter the moduli file contains prime numbers and generators used by
+	 * Diffie-Hellman Group Exchange.
+	 * 
+	 * @see org.apache.sshd.common.kex.BuiltinDHFactories#dhgex256
+	 *      diffie-hellman-group-exchange-sha256
+	 * @see org.apache.sshd.common.kex.BuiltinDHFactories#dhgex
+	 *      diffie-hellman-group-exchange-sha1
+	 * @see org.apache.sshd.server.kex.DHGEXServer
+	 * @see org.apache.sshd.server.kex.Moduli
+	 * @see http://manpages.ubuntu.com/manpages/focal/man5/moduli.5.html
+	 */
+	private void hackModuliDHGEX() {
+		URL srcModuli = null;
+		final File sysLinuxModuli = new File("/etc/ssh/moduli");
+		if (sysLinuxModuli.canRead()) {
+			try {
+				srcModuli = sysLinuxModuli.toURI().toURL();
+				LOG.info("Linux moduli file: " + sysLinuxModuli);
+			} catch (IOException e) {
+			}
+		} else {
+			final String moduliPath = Moduli.INTERNAL_MODULI_RESPATH;
+			srcModuli = Moduli.class.getResource(moduliPath);
+			if (srcModuli == null) {
+				LOG.warn("Missing internal moduli file: " + moduliPath);
+			}
+		}
+		if (srcModuli != null) {
+			final File newModuli = new File(System.getProperty("java.io.tmpdir", "/tmp/"), "moduli.sftpd");
+			if (!newModuli.exists() // create
+					|| (newModuli.length() <= 0) // empty
+					|| (System.currentTimeMillis() - newModuli.lastModified() > TimeUnit.DAYS.toMillis(1))) { // 1day
+				try {
+					LOG.info("Filtering moduli file:" + srcModuli.toExternalForm());
+					final List<String> data = ModuliFilter.filterModuli(srcModuli, //
+							db.getMinSizeDHGEX(), db.getMaxSizeDHGEX());
+					ModuliFilter.writeModuli(newModuli, data);
+				} catch (IOException e) {
+					LOG.error("Error filtering moduli: " + e, e);
+				}
+			}
+			if ((newModuli != null) && newModuli.canRead() && (newModuli.length() > 0)) {
+				LOG.warn("Using moduli file: " + newModuli);
+				PropertyResolverUtils.updateProperty(sshd, ServerFactoryManager.MODULI_URL,
+						newModuli.toURI().toString());
+			}
+		}
+	}
+
 	public void start() {
 		LOG.info("Starting");
 		logger = new ServiceLogger();
@@ -252,6 +304,7 @@ public void start() {
 		sshd = SshServer.setUpDefaultServer();
 		LOG.info("SSHD " + sshd.getVersion());
 		hackVersion();
+		hackModuliDHGEX();
 		setupFactories();
 		setupKeyPair();
 		setupScp();
@@ -335,6 +388,8 @@ public boolean authenticate(final String username, final PublicKey key, final Se
 	static class Config {
 		// @see https://stribika.github.io/2015/01/04/secure-secure-shell.html
 		// @see http://manpages.ubuntu.com/manpages/focal/man5/sshd_config.5.html
+		public static final int DEFAULT_DHGEX_MIN = 2000;
+		public static final int DEFAULT_DHGEX_MAX = 8200;
 		/**
 		 * man 5 sshd_config : KexAlgorithms
 		 * 
@@ -383,6 +438,8 @@ static class Config {
 		public static final String PROP_KEX_ALGORITHMS = "kexalgorithms";
 		public static final String PROP_CIPHERS = "ciphers";
 		public static final String PROP_MACS = "macs";
+		public static final String PROP_DHGEX_MIN = "dhgex-min";
+		public static final String PROP_DHGEX_MAX = "dhgex-max";
 		// HtPasswd config
 		public static final String PROP_HTPASSWD = BASE + "." + "htpasswd";
 		public static final String PROP_HT_HOME = "homedirectory";
@@ -477,6 +534,22 @@ public String getMacs() {
 			return value;
 		}
 
+		public int getMinSizeDHGEX() {
+			final String value = getValue(PROP_DHGEX_MIN);
+			if (value == null) {
+				return DEFAULT_DHGEX_MIN;
+			}
+			return Integer.parseInt(value);
+		}
+
+		public int getMaxSizeDHGEX() {
+			final String value = getValue(PROP_DHGEX_MAX);
+			if (value == null) {
+				return DEFAULT_DHGEX_MAX;
+			}
+			return Integer.parseInt(value);
+		}
+
 		// User config
 		private final String getValue(final String user, final String key) {
 			if ((user == null) || (key == null))

diff --git a/src/main/resources/sftpd.policy b/src/main/resources/sftpd.policy
@@ -52,6 +52,7 @@ grant {
   permission java.io.FilePermission "${java.io.tmpdir}${/}-", "read, write, delete";
   //
   permission java.io.FilePermission "/etc/resolv.conf", "read";
+  permission java.io.FilePermission "/etc/ssh/moduli", "read";
   permission java.io.FilePermission "${sftp.home}/-", "read";
   permission java.io.FilePermission "${sftp.home}/keys/hostkey.ser", "read, write";
   permission java.io.FilePermission "${sftp.home}/keys/hostkey.pem", "read, write";