Merge pull request '75.1 TLS для SMTP: STARTTLS и неявный TLS (465)' (#9) from feat/75-1-smtp-starttls into master

Reviewed-on: #9

README.md

@@ -13,6 +13,7 @@
 ```
 
 - **Приём (SMTP).** Команды разбираются по строкам (`LineBasedFrameDecoder`): работает и пачка команд в одном пакете, и строка, разорванная на два пакета. Письмо сохраняется или ставится в очередь **до** ответа `250`; если сохранить не удалось — `451`, и отправитель повторит. Работа с базой и DNS идёт не в сетевом потоке.
+- **TLS.** `STARTTLS` на порту SMTP (RFC 3207) и, если задан `SMTPS_PORT`, неявный TLS (RFC 8314, обычно 465); TLS 1.2/1.3. После `STARTTLS` сессия начинается заново (новый `EHLO`), а команды, присланные открытым текстом вместе со `STARTTLS`, не выполняются — соединение закрывается (CVE-2011-0411). `AUTH` предлагается только внутри TLS (или без него при `SMTP_AUTH_WITHOUT_TLS=true`). Сертификат — свой (`MAIL_TLS_CERT` + `MAIL_TLS_KEY`, например Let's Encrypt) или самоподписанный: создаётся один раз в `MAIL_TLS_DIR` и потом переиспользуется.
 - **Не открытый релей.**
   - Письмо на свой домен принимается от кого угодно.
   - Письмо на чужой домен — только после `AUTH`, иначе `550 5.7.1 Relaying denied`.
@@ -59,6 +60,10 @@ API: `GET /api/config` (без входа), `/api/me`, `/api/me/password`, `/api
 | `MAIL_REGISTRATION` | открыть регистрацию | `false` |
 | `MAIL_CATCH_ALL` | ящик-ловушка: письма на адреса без своего ящика здесь (и на чужие домены) — сюда, наружу ничего; ящик создаётся при старте | — |
 | `SMTP_PORT` | порт SMTP | `3025` |
+| `MAIL_TLS` | TLS для SMTP: `STARTTLS` и порт `SMTPS_PORT` | `true` |
+| `MAIL_TLS_CERT`, `MAIL_TLS_KEY` | свой сертификат: PEM-цепочка и ключ PKCS#8; без них — самоподписанный | — |
+| `MAIL_TLS_DIR` | где лежит самоподписанный сертификат | `./tls` (в образе `/app/data/tls`) |
+| `SMTPS_PORT` | порт неявного TLS (465); `0` — выключен | `0` |
 | `SMTP_AUTH_WITHOUT_TLS` | разрешить `AUTH` без шифрования — **только** в закрытой сети (docker, ВМ) | `false` |
 | `OUTBOUND_MODE` | `off` / `mx` / `smarthost` | `off` |
 | `OUTBOUND_HOST`, `OUTBOUND_PORT`, `OUTBOUND_USERNAME`, `OUTBOUND_PASSWORD`, `OUTBOUND_TLS` | smarthost (`starttls` / `ssl` / `none`) | —, `587`, —, —, `starttls` |
@@ -97,7 +102,8 @@ cd frontend && npm install && npx ng serve                         # http://loca
 
 ## Не сделано
 
-- [ ] TLS: STARTTLS для SMTP и IMAP. Пока `AUTH` — только в закрытой сети.
+- [ ] TLS для IMAP (вместе с переписыванием IMAP). SMTP — сделано: `STARTTLS` и порт 465.
+- [ ] Сертификат Let's Encrypt для настоящей почты (75.6): сейчас — свой файл или самоподписанный.
 - [ ] IMAP выключен: упрощённый, блокирует сетевой поток (`.block()`). Переписать и проверить с Thunderbird.
 - [ ] Пересылка не берёт вложения исходного письма — их надо прикрепить заново.
 - [ ] Встроенные картинки HTML-писем (`cid:`) показываются как вложения, а не внутри письма.

backend/Dockerfile

@@ -24,11 +24,11 @@ RUN chmod +x gradlew && ./gradlew :backend:bootJar --no-daemon -x test
 FROM eclipse-temurin:21-jre
 # Ubuntu image, curl included; own user: "mail" already exists (uid 8)
 RUN groupadd -r -g 10001 app && useradd -r -u 10001 -g app -d /app -s /usr/sbin/nologin app \
-    && mkdir -p /app/data/attachments /app/data/dkim-keys && chown -R app:app /app/data
+    && mkdir -p /app/data/attachments /app/data/dkim-keys /app/data/tls && chown -R app:app /app/data
 WORKDIR /app
 COPY --from=build /app/backend/build/libs/*.jar app.jar
 USER app
-ENV MAIL_ATTACHMENTS_DIR=/app/data/attachments DKIM_KEYS_DIR=/app/data/dkim-keys MAIL_PASSWORDS_FILE=/app/data/initial-passwords.txt
+ENV MAIL_ATTACHMENTS_DIR=/app/data/attachments DKIM_KEYS_DIR=/app/data/dkim-keys MAIL_PASSWORDS_FILE=/app/data/initial-passwords.txt MAIL_TLS_DIR=/app/data/tls
 EXPOSE 8080 3025
 HEALTHCHECK --interval=30s --timeout=5s --start-period=60s --retries=3 \
     CMD curl -fsS http://localhost:8080/actuator/health || exit 1

backend/src/main/java/com/mailserver/config/MailServerConfig.java

@@ -3,12 +3,16 @@ package com.mailserver.config;
 import com.mailserver.delivery.SmtpDeliveryClient;
 import com.mailserver.smtp.MailRouter;
 import com.mailserver.smtp.SmtpServer;
+import com.mailserver.smtp.SmtpTls;
+import io.netty.handler.ssl.SslContext;
 import org.springframework.beans.factory.annotation.Value;
+import org.springframework.boot.autoconfigure.condition.ConditionalOnExpression;
 import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.scheduling.annotation.EnableScheduling;
 
+import java.nio.file.Path;
 import java.time.Duration;
 import java.util.Locale;
 
@@ -17,16 +21,46 @@ import java.util.Locale;
 @EnableScheduling
 public class MailServerConfig {
 
+    /**
+     * Certificate for SMTP TLS: mailserver.tls.cert-file + key-file (PEM), or a self-signed one in mailserver.tls.dir.
+     * mailserver.tls.enabled=false — no TLS at all.
+     */
+    @Bean
+    public SmtpTlsContext smtpTlsContext(@Value("${mailserver.tls.enabled:true}") boolean enabled,
+                                         @Value("${mailserver.tls.cert-file:}") String certFile,
+                                         @Value("${mailserver.tls.key-file:}") String keyFile,
+                                         @Value("${mailserver.tls.dir:./tls}") String dir,
+                                         @Value("${mailserver.smtp.hostname:localhost}") String hostname) throws Exception {
+        return new SmtpTlsContext(enabled ? SmtpTls.serverContext(certFile, keyFile, Path.of(dir), hostname) : null);
+    }
+
+    public record SmtpTlsContext(SslContext context) {
+    }
+
     @Bean(initMethod = "start", destroyMethod = "stop")
     @ConditionalOnProperty(name = "mailserver.smtp.enabled", havingValue = "true", matchIfMissing = true)
-    public SmtpServer smtpServer(MailRouter router,
+    public SmtpServer smtpServer(MailRouter router, SmtpTlsContext tls,
                                  @Value("${mailserver.smtp.host:0.0.0.0}") String host,
                                  @Value("${mailserver.smtp.port:3025}") int port,
                                  @Value("${mailserver.smtp.hostname:localhost}") String hostname,
                                  @Value("${mailserver.smtp.max-message-bytes:26214400}") long maxBytes,
                                  @Value("${mailserver.smtp.auth-without-tls:false}") boolean authWithoutTls,
                                  @Value("${mailserver.smtp.idle-timeout:5m}") Duration idleTimeout) {
-        return new SmtpServer(new SmtpServer.Settings(host, port, hostname, maxBytes, authWithoutTls, idleTimeout), router);
+        return new SmtpServer(new SmtpServer.Settings(host, port, hostname, maxBytes, authWithoutTls, idleTimeout,
+                tls.context(), false), router);
+    }
+
+    /** Implicit TLS (RFC 8314, usually port 465) — only when mailserver.smtps.port is set and TLS is on. */
+    @Bean(initMethod = "start", destroyMethod = "stop")
+    @ConditionalOnExpression("${mailserver.smtps.port:0} > 0 and ${mailserver.tls.enabled:true}")
+    public SmtpServer smtpsServer(MailRouter router, SmtpTlsContext tls,
+                                  @Value("${mailserver.smtp.host:0.0.0.0}") String host,
+                                  @Value("${mailserver.smtps.port}") int port,
+                                  @Value("${mailserver.smtp.hostname:localhost}") String hostname,
+                                  @Value("${mailserver.smtp.max-message-bytes:26214400}") long maxBytes,
+                                  @Value("${mailserver.smtp.idle-timeout:5m}") Duration idleTimeout) {
+        return new SmtpServer(new SmtpServer.Settings(host, port, hostname, maxBytes, false, idleTimeout,
+                tls.context(), true), router);
     }
 
     /**

backend/src/main/java/com/mailserver/smtp/SmtpServer.java

@@ -1,13 +1,22 @@
 package com.mailserver.smtp;
 
+import io.netty.buffer.ByteBuf;
+import io.netty.buffer.ByteBufUtil;
+import io.netty.channel.ChannelHandlerContext;
+import io.netty.channel.ChannelPipeline;
 import io.netty.handler.codec.LineBasedFrameDecoder;
+import io.netty.handler.codec.MessageToMessageDecoder;
+import io.netty.handler.ssl.SslContext;
+import io.netty.handler.ssl.SslHandler;
 import io.netty.handler.timeout.ReadTimeoutHandler;
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 import reactor.core.publisher.Flux;
 import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Schedulers;
+import reactor.netty.Connection;
 import reactor.netty.DisposableServer;
+import reactor.netty.NettyOutbound;
 import reactor.netty.tcp.TcpServer;
 
 import java.io.ByteArrayOutputStream;
@@ -32,7 +41,10 @@ import java.util.concurrent.TimeUnit;
  *   <li>the message is stored or queued before the client gets 250 — on failure 451 and the client retries;</li>
  *   <li>database and DNS work runs on a worker thread, not on the event loop.</li>
  * </ul>
- * No STARTTLS yet: AUTH is offered only when {@code authWithoutTls} is on (a closed network: inside a VM or a docker network).
+ * TLS (RFC 3207 STARTTLS, or implicit TLS on a separate port, RFC 8314): with a {@code tls} context STARTTLS is
+ * offered; AUTH only over TLS, or without it when {@code authWithoutTls} is on (a closed network: VM, docker network).
+ * After STARTTLS the session starts over (new EHLO), and lines sent in plain text together with STARTTLS are not
+ * executed — the connection is closed (command injection, CVE-2011-0411).
  */
 public class SmtpServer {
 
@@ -42,9 +54,32 @@ public class SmtpServer {
     static final int MAX_LINE = 64 * 1024;
     static final int MAX_RECIPIENTS = 100;
     private static final int MAX_BAD_COMMANDS = 20;
+    // Compared by identity: TLS starts right after exactly this reply is written
+    private static final String STARTTLS_READY = new String("220 2.0.0 Ready to start TLS\r\n");
 
+    /**
+     * @param tls         certificate for STARTTLS / implicit TLS; null — no TLS
+     * @param implicitTls TLS from the first byte (port 465), no STARTTLS
+     */
     public record Settings(String host, int port, String hostname, long maxMessageBytes, boolean authWithoutTls,
+                           Duration idleTimeout, SslContext tls, boolean implicitTls) {
+
+        public Settings(String host, int port, String hostname, long maxMessageBytes, boolean authWithoutTls,
                         Duration idleTimeout) {
+            this(host, port, hostname, maxMessageBytes, authWithoutTls, idleTimeout, null, false);
+        }
+    }
+
+    /** A line of input and whether it came over TLS. */
+    record Line(byte[] bytes, boolean tls) {
+    }
+
+    /** Marks each line: decoded behind an SslHandler (encrypted) or not. */
+    private static final class LineTagger extends MessageToMessageDecoder<ByteBuf> {
+        @Override
+        protected void decode(ChannelHandlerContext ctx, ByteBuf frame, List<Object> out) {
+            out.add(new Line(ByteBufUtil.getBytes(frame), ctx.pipeline().get(SslHandler.class) != null));
+        }
     }
 
     private final Settings settings;
@@ -57,32 +92,70 @@ public class SmtpServer {
     }
 
     public void start() {
-        server = TcpServer.create()
+        if (settings.implicitTls() && settings.tls() == null) {
+            throw new IllegalStateException("Implicit TLS needs a certificate");
+        }
+        TcpServer tcp = TcpServer.create()
                 .host(settings.host())
                 .port(settings.port())
                 .doOnConnection(connection -> connection
                         .addHandlerFirst("smtp-idle", new ReadTimeoutHandler(settings.idleTimeout().toSeconds(), TimeUnit.SECONDS))
-                        .addHandlerLast("smtp-lines", new LineBasedFrameDecoder(MAX_LINE, true, false)))
+                        .addHandlerLast("smtp-lines", new LineBasedFrameDecoder(MAX_LINE, true, false))
+                        .addHandlerLast("smtp-tag", new LineTagger()));
+        if (settings.implicitTls()) {
+            tcp = tcp.secure(spec -> spec.sslContext(settings.tls()));
+        }
+        server = tcp
                 .handle((inbound, outbound) -> {
+                    Connection[] connection = {null};
                     String[] remoteIp = {"unknown"};
                     inbound.withConnection(c -> {
+                        connection[0] = c;
                         if (c.address() instanceof InetSocketAddress address && address.getAddress() != null) {
                             remoteIp[0] = address.getAddress().getHostAddress();
                         }
                     });
-                    Session session = new Session(remoteIp[0]);
+                    Session session = new Session(remoteIp[0], settings.implicitTls());
-                    Flux<String> replies = inbound.receive().asByteArray()
+                    Flux<String> replies = inbound.receiveObject().cast(Line.class)
                             .concatMap(session::onLine)
                             .onErrorResume(e -> {
                                 log.debug("SMTP {}: connection closed: {}", session.remoteIp, e.toString());
                                 return Mono.just("421 4.4.2 " + settings.hostname() + " closing connection\r\n");
                             })
                             .takeUntil(reply -> session.closing);
-                    return outbound.sendString(Flux.concat(
+                    // One reply at a time, each written before the next line is handled: after "220 Ready to start TLS"
-                            Mono.just("220 " + settings.hostname() + " ESMTP ready\r\n"), replies), StandardCharsets.US_ASCII);
+                    // the TLS handler goes in only when the reply has left in plain text
+                    return Flux.concat(Mono.just("220 " + settings.hostname() + " ESMTP ready\r\n"), replies)
+                            .concatMap(reply -> send(outbound, reply)
+                                    .then(Mono.defer(() -> reply == STARTTLS_READY ? startTls(connection[0], session) : Mono.<Void>empty())))
+                            .then();
                 })
                 .bindNow(Duration.ofSeconds(10));
-        log.info("SMTP server listening on {}:{} ({})", settings.host(), port(), settings.hostname());
+        log.info("SMTP server listening on {}:{} ({}{})", settings.host(), port(), settings.hostname(),
+                settings.implicitTls() ? ", implicit TLS" : settings.tls() != null ? ", STARTTLS" : ", no TLS");
+    }
+
+    private static Mono<Void> send(NettyOutbound outbound, String reply) {
+        return outbound.sendString(Mono.just(reply), StandardCharsets.US_ASCII).then();
+    }
+
+    /** TLS handler in front, fresh line decoder (no plain-text leftovers mixed into encrypted input). */
+    private Mono<Void> startTls(Connection connection, Session session) {
+        ChannelPipeline pipeline = connection.channel().pipeline();
+        SslHandler ssl = settings.tls().newHandler(connection.channel().alloc());
+        pipeline.replace("smtp-lines", "smtp-lines", new LineBasedFrameDecoder(MAX_LINE, true, false));
+        pipeline.addFirst("smtp-tls", ssl);
+        return Mono.create(sink -> ssl.handshakeFuture().addListener(future -> {
+            if (future.isSuccess()) {
+                log.debug("SMTP {}: TLS {}", session.remoteIp, ssl.engine().getSession().getProtocol());
+                sink.success();
+            } else {
+                log.info("SMTP {}: TLS handshake failed: {}", session.remoteIp, future.cause().toString());
+                session.closing = true;
+                connection.dispose();
+                sink.success();
+            }
+        }));
     }
 
     public int port() {
@@ -99,6 +172,7 @@ public class SmtpServer {
 
     private final class Session {
         final String remoteIp;
+        boolean tls;
         State state = State.COMMAND;
         String helo;
         boolean extended;
@@ -111,12 +185,17 @@ public class SmtpServer {
         int badCommands;
         volatile boolean closing;
 
-        Session(String remoteIp) {
+        Session(String remoteIp, boolean tls) {
             this.remoteIp = remoteIp;
+            this.tls = tls;
         }
 
-        Mono<String> onLine(byte[] raw) {
+        Mono<String> onLine(Line input) {
-            byte[] line = stripCr(raw);
+            if (tls && !input.tls()) {
+                // Sent in plain text after STARTTLS, before the handshake: never execute it
+                return Mono.error(new IllegalStateException("plain-text input after STARTTLS"));
+            }
+            byte[] line = stripCr(input.bytes());
             return switch (state) {
                 case DATA -> onDataLine(line);
                 case AUTH_PLAIN -> authPlain(ascii(line));
@@ -142,7 +221,7 @@ public class SmtpServer {
                 }
                 case "NOOP" -> reply("250 2.0.0 OK");
                 case "VRFY" -> reply("252 2.5.0 Cannot VRFY user, but will accept message");
-                case "STARTTLS" -> reply("454 4.7.0 TLS not available");
+                case "STARTTLS" -> startTls(arg);
                 case "QUIT" -> {
                     closing = true;
                     yield reply("221 2.0.0 " + settings.hostname() + " Bye");
@@ -167,15 +246,41 @@ public class SmtpServer {
             sb.append("250-SIZE ").append(settings.maxMessageBytes()).append("\r\n");
             sb.append("250-8BITMIME\r\n");
             sb.append("250-PIPELINING\r\n");
-            if (settings.authWithoutTls()) {
+            if (settings.tls() != null && !tls) {
+                sb.append("250-STARTTLS\r\n");
+            }
+            if (authAllowed()) {
                 sb.append("250-AUTH PLAIN LOGIN\r\n");
             }
             sb.append("250 ENHANCEDSTATUSCODES\r\n");
             return Mono.just(sb.toString());
         }
 
+        private boolean authAllowed() {
+            return tls || settings.authWithoutTls();
+        }
+
+        private Mono<String> startTls(String arg) {
+            if (settings.tls() == null) {
+                return reply("454 4.7.0 TLS not available");
+            }
+            if (tls) {
+                return bad("503 5.5.1 TLS already active");
+            }
+            if (!arg.isEmpty()) {
+                return reply("501 5.5.4 Syntax: STARTTLS");
+            }
+            // RFC 3207 4.2: everything known about the client is forgotten, a new EHLO is expected
+            tls = true;
+            helo = null;
+            extended = false;
+            authenticatedUser = null;
+            resetTransaction();
+            return Mono.just(STARTTLS_READY);
+        }
+
         private Mono<String> auth(String arg) {
-            if (!settings.authWithoutTls()) {
+            if (!authAllowed()) {
                 return reply("538 5.7.11 Encryption required for requested authentication mechanism");
             }
             if (helo == null || !extended) {
@@ -369,7 +474,7 @@ public class SmtpServer {
 
         private byte[] withReceivedHeader(byte[] message) {
             String received = "Received: from " + helo + " ([" + remoteIp + "])\r\n"
-                    + "\tby " + settings.hostname() + " with " + (authenticatedUser != null ? "ESMTPA" : "ESMTP") + ";\r\n"
+                    + "\tby " + settings.hostname() + " with " + (tls ? "ESMTPS" : "ESMTP") + (authenticatedUser != null ? "A" : "") + ";\r\n"
                     + "\t" + DateTimeFormatter.RFC_1123_DATE_TIME.format(ZonedDateTime.now()) + "\r\n";
             byte[] header = received.getBytes(StandardCharsets.US_ASCII);
             byte[] all = new byte[header.length + message.length];

backend/src/main/java/com/mailserver/smtp/SmtpTls.java

@@ -0,0 +1,110 @@
+package com.mailserver.smtp;
+
+import io.netty.handler.ssl.SslContext;
+import io.netty.handler.ssl.SslContextBuilder;
+import org.bouncycastle.asn1.x500.X500Name;
+import org.bouncycastle.asn1.x509.Extension;
+import org.bouncycastle.asn1.x509.GeneralName;
+import org.bouncycastle.asn1.x509.GeneralNames;
+import org.bouncycastle.cert.jcajce.JcaX509CertificateConverter;
+import org.bouncycastle.cert.jcajce.JcaX509v3CertificateBuilder;
+import org.bouncycastle.operator.jcajce.JcaContentSignerBuilder;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+import java.io.IOException;
+import java.math.BigInteger;
+import java.nio.charset.StandardCharsets;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.nio.file.attribute.PosixFilePermissions;
+import java.security.KeyPair;
+import java.security.KeyPairGenerator;
+import java.security.SecureRandom;
+import java.security.cert.X509Certificate;
+import java.time.Duration;
+import java.time.Instant;
+import java.util.Base64;
+import java.util.Date;
+
+/**
+ * Certificate for SMTP TLS (STARTTLS and implicit TLS):
+ * <ul>
+ *   <li>{@code cert} and {@code key} given (PEM: certificate chain and PKCS#8 key, e.g. from Let's Encrypt) — used as is;</li>
+ *   <li>otherwise a self-signed certificate for the server hostname, created once in {@code dir} and reused —
+ *       enough for encryption between our own services; other servers deliver over it too (MX delivery does not
+ *       check certificates, RFC 7435 opportunistic TLS).</li>
+ * </ul>
+ */
+public final class SmtpTls {
+
+    private static final Logger log = LoggerFactory.getLogger(SmtpTls.class);
+    static final String CERT_FILE = "smtp-cert.pem";
+    static final String KEY_FILE = "smtp-key.pem";
+
+    private SmtpTls() {
+    }
+
+    public static SslContext serverContext(String certFile, String keyFile, Path dir, String hostname) throws Exception {
+        boolean givenCert = certFile != null && !certFile.isBlank();
+        boolean givenKey = keyFile != null && !keyFile.isBlank();
+        if (givenCert != givenKey) {
+            throw new IllegalStateException("TLS: set both the certificate and the key file, or neither");
+        }
+        Path cert;
+        Path key;
+        if (givenCert) {
+            cert = Path.of(certFile);
+            key = Path.of(keyFile);
+            log.info("SMTP TLS: certificate {}", cert);
+        } else {
+            cert = dir.resolve(CERT_FILE);
+            key = dir.resolve(KEY_FILE);
+            if (!Files.exists(cert) || !Files.exists(key)) {
+                selfSigned(hostname, cert, key);
+                log.warn("SMTP TLS: created a self-signed certificate for {} in {}", hostname, dir);
+            } else {
+                log.info("SMTP TLS: self-signed certificate {}", cert);
+            }
+        }
+        return SslContextBuilder.forServer(cert.toFile(), key.toFile())
+                .protocols("TLSv1.3", "TLSv1.2")
+                .build();
+    }
+
+    static void selfSigned(String hostname, Path certFile, Path keyFile) throws Exception {
+        KeyPairGenerator generator = KeyPairGenerator.getInstance("RSA");
+        generator.initialize(2048, new SecureRandom());
+        KeyPair pair = generator.generateKeyPair();
+        X500Name name = new X500Name("CN=" + hostname);
+        Instant now = Instant.now();
+        JcaX509v3CertificateBuilder builder = new JcaX509v3CertificateBuilder(name,
+                new BigInteger(64, new SecureRandom()), Date.from(now.minus(Duration.ofDays(1))),
+                Date.from(now.plus(Duration.ofDays(3650))), name, pair.getPublic());
+        builder.addExtension(Extension.subjectAlternativeName, false,
+                new GeneralNames(new GeneralName(GeneralName.dNSName, hostname)));
+        X509Certificate certificate = new JcaX509CertificateConverter()
+                .getCertificate(builder.build(new JcaContentSignerBuilder("SHA256withRSA").build(pair.getPrivate())));
+
+        Files.createDirectories(keyFile.toAbsolutePath().getParent());
+        write(keyFile, pem("PRIVATE KEY", pair.getPrivate().getEncoded()), true);
+        write(certFile, pem("CERTIFICATE", certificate.getEncoded()), false);
+    }
+
+    private static String pem(String type, byte[] der) {
+        return "-----BEGIN " + type + "-----\n"
+                + Base64.getMimeEncoder(64, new byte[]{'\n'}).encodeToString(der)
+                + "\n-----END " + type + "-----\n";
+    }
+
+    private static void write(Path file, String text, boolean secret) throws IOException {
+        Files.writeString(file, text, StandardCharsets.US_ASCII);
+        if (secret) {
+            try {
+                Files.setPosixFilePermissions(file, PosixFilePermissions.fromString("rw-------"));
+            } catch (UnsupportedOperationException ignored) {
+                // not a POSIX file system
+            }
+        }
+    }
+}

backend/src/main/resources/application.properties

@@ -41,6 +41,13 @@ mailserver.smtp.port=${SMTP_PORT:3025}
 mailserver.smtp.hostname=${MAIL_HOSTNAME:localhost}
 mailserver.smtp.auth-without-tls=${SMTP_AUTH_WITHOUT_TLS:false}
 mailserver.smtp.max-message-bytes=26214400
+# TLS (75.1): STARTTLS on the SMTP port; implicit TLS on SMTPS_PORT (465) if set. Certificate: MAIL_TLS_CERT + MAIL_TLS_KEY
+# (PEM, PKCS#8 key), otherwise a self-signed one created once in MAIL_TLS_DIR. MAIL_TLS=false — no TLS
+mailserver.tls.enabled=${MAIL_TLS:true}
+mailserver.tls.cert-file=${MAIL_TLS_CERT:}
+mailserver.tls.key-file=${MAIL_TLS_KEY:}
+mailserver.tls.dir=${MAIL_TLS_DIR:./tls}
+mailserver.smtps.port=${SMTPS_PORT:0}
 
 # Outbound: off | mx (straight to recipients' MX, port 25) | smarthost (via a provider, e.g. Resend)
 mailserver.outbound.mode=${OUTBOUND_MODE:off}

backend/src/test/java/com/mailserver/smtp/SmtpServerTest.java

@@ -8,11 +8,18 @@ import jakarta.mail.internet.MimeMessage;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
 
 import java.io.BufferedReader;
 import java.io.InputStreamReader;
 import java.io.OutputStream;
 import java.net.Socket;
+import java.nio.file.Path;
+import java.security.cert.X509Certificate;
+import javax.net.ssl.SSLContext;
+import javax.net.ssl.SSLSocket;
+import javax.net.ssl.TrustManager;
+import javax.net.ssl.X509TrustManager;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.util.ArrayList;
@@ -214,15 +221,131 @@ class SmtpServerTest {
         assertThat(SmtpServer.path("<a b@c>")).isNull();
     }
 
+    // ---------- TLS (75.1) ----------
+
+    @TempDir
+    Path tlsDir;
+
+    private SmtpServer tlsServer(boolean implicit) throws Exception {
+        server.stop();
+        server = new SmtpServer(new SmtpServer.Settings("127.0.0.1", 0, "mx.example.test", 2000, false,
+                Duration.ofSeconds(30), SmtpTls.serverContext(null, null, tlsDir, "mx.example.test"), implicit), backend);
+        server.start();
+        return server;
+    }
+
+    @Test
+    void startTlsThenAuthAndTheSessionStartsOver() throws Exception {
+        tlsServer(false);
+        try (Client client = new Client(server.port())) {
+            client.read();
+            client.send("EHLO laptop");
+            List<String> plain = client.readReplyLines();
+            assertThat(plain).contains("250-STARTTLS").noneMatch(line -> line.contains("AUTH"));
+            assertThat(client.send("AUTH PLAIN " + b64("\0alice@example.test\0secret"))).startsWith("538");
+            assertThat(client.send("MAIL FROM:<x@elsewhere.test>")).startsWith("250");
+
+            assertThat(client.send("STARTTLS")).isEqualTo("220 2.0.0 Ready to start TLS");
+            client.startTls();
+
+            // RFC 3207: MAIL FROM before STARTTLS is forgotten, EHLO again
+            assertThat(client.send("MAIL FROM:<x@elsewhere.test>")).startsWith("503");
+            client.send("EHLO laptop");
+            List<String> secure = client.readReplyLines();
+            assertThat(secure).contains("250-AUTH PLAIN LOGIN").noneMatch(line -> line.contains("STARTTLS"));
+            assertThat(client.send("STARTTLS")).startsWith("503");
+            assertThat(client.send("AUTH PLAIN " + b64("\0alice@example.test\0secret"))).startsWith("235");
+            client.send("MAIL FROM:<alice@example.test>");
+            client.send("RCPT TO:<friend@gmail.com>");
+            client.send("DATA");
+            assertThat(client.send("Subject: tls\r\n\r\nhi\r\n.")).startsWith("250");
+        }
+        assertThat(backend.messages.get(0)).contains("with ESMTPSA;");
+    }
+
+    // CVE-2011-0411: commands sent in plain text right after STARTTLS must not run inside the TLS session
+    @Test
+    void plainTextInjectedAfterStartTlsIsNotExecuted() throws Exception {
+        tlsServer(false);
+        try (Client client = new Client(server.port())) {
+            client.read();
+            client.send("EHLO laptop");
+            client.readReplyLines();
+            client.write("STARTTLS\r\nMAIL FROM:<x@elsewhere.test>\r\nRCPT TO:<bob@example.test>\r\n");
+            assertThat(client.read()).isEqualTo("220 2.0.0 Ready to start TLS");
+            client.startTls();
+            String reply = client.read();
+            assertThat(reply == null || reply.startsWith("421")).as(reply).isTrue();
+        }
+        assertThat(backend.envelopes).isEmpty();
+    }
+
+    @Test
+    void realClientRequiringStartTlsDelivers() throws Exception {
+        tlsServer(false);
+        Properties props = new Properties();
+        props.put("mail.smtp.host", "127.0.0.1");
+        props.put("mail.smtp.port", String.valueOf(server.port()));
+        props.put("mail.smtp.starttls.enable", "true");
+        props.put("mail.smtp.starttls.required", "true");
+        props.put("mail.smtp.ssl.trust", "*");
+        props.put("mail.smtp.ssl.checkserveridentity", "false"); // self-signed for mx.example.test, connecting to 127.0.0.1
+        props.put("mail.smtp.auth", "true");
+        Session session = Session.getInstance(props);
+        MimeMessage message = new MimeMessage(session);
+        message.setFrom(new InternetAddress("alice@example.test"));
+        message.setRecipient(Message.RecipientType.TO, new InternetAddress("bob@example.test"));
+        message.setSubject("Over STARTTLS");
+        message.setText("hello");
+
+        Transport.send(message, "alice@example.test", "secret");
+
+        assertThat(backend.envelopes).hasSize(1);
+        assertThat(backend.envelopes.get(0).authenticatedUser()).isEqualTo("alice@example.test");
+        assertThat(backend.messages.get(0)).contains("with ESMTPSA;");
+    }
+
+    @Test
+    void implicitTlsPortWorksWithoutStartTls() throws Exception {
+        tlsServer(true);
+        Properties props = new Properties();
+        props.put("mail.smtp.host", "127.0.0.1");
+        props.put("mail.smtp.port", String.valueOf(server.port()));
+        props.put("mail.smtp.ssl.enable", "true");
+        props.put("mail.smtp.ssl.trust", "*");
+        props.put("mail.smtp.ssl.checkserveridentity", "false"); // self-signed for mx.example.test, connecting to 127.0.0.1
+        props.put("mail.smtp.auth", "true");
+        Session session = Session.getInstance(props);
+        MimeMessage message = new MimeMessage(session);
+        message.setFrom(new InternetAddress("alice@example.test"));
+        message.setRecipient(Message.RecipientType.TO, new InternetAddress("bob@example.test"));
+        message.setSubject("Over 465");
+        message.setText("hello");
+
+        Transport.send(message, "alice@example.test", "secret");
+
+        assertThat(backend.messages.get(0)).contains("with ESMTPSA;");
+    }
+
+    @Test
+    void selfSignedCertificateIsCreatedOnceAndReused() throws Exception {
+        SmtpTls.serverContext(null, null, tlsDir, "mx.example.test");
+        String first = java.nio.file.Files.readString(tlsDir.resolve(SmtpTls.CERT_FILE));
+        SmtpTls.serverContext(null, null, tlsDir, "mx.example.test");
+
+        assertThat(java.nio.file.Files.readString(tlsDir.resolve(SmtpTls.CERT_FILE))).isEqualTo(first);
+        assertThat(java.nio.file.Files.readString(tlsDir.resolve(SmtpTls.KEY_FILE))).startsWith("-----BEGIN PRIVATE KEY-----");
+    }
+
     private static String b64(String text) {
         return Base64.getEncoder().encodeToString(text.getBytes(StandardCharsets.UTF_8));
     }
 
     /** Line-oriented test client. */
     static final class Client implements AutoCloseable {
-        private final Socket socket;
+        private Socket socket;
-        private final BufferedReader in;
+        private BufferedReader in;
-        private final OutputStream out;
+        private OutputStream out;
 
         Client(int port) throws Exception {
             socket = new Socket("127.0.0.1", port);
@@ -231,6 +354,28 @@ class SmtpServerTest {
             out = socket.getOutputStream();
         }
 
+        /** Upgrade the connection to TLS (any certificate is trusted — the test server's is self-signed). */
+        void startTls() throws Exception {
+            SSLContext context = SSLContext.getInstance("TLS");
+            context.init(null, new TrustManager[]{new X509TrustManager() {
+                public void checkClientTrusted(X509Certificate[] chain, String authType) {
+                }
+
+                public void checkServerTrusted(X509Certificate[] chain, String authType) {
+                }
+
+                public X509Certificate[] getAcceptedIssuers() {
+                    return new X509Certificate[0];
+                }
+            }}, null);
+            SSLSocket ssl = (SSLSocket) context.getSocketFactory().createSocket(socket, "127.0.0.1", socket.getPort(), true);
+            ssl.setUseClientMode(true);
+            ssl.startHandshake();
+            socket = ssl;
+            in = new BufferedReader(new InputStreamReader(ssl.getInputStream(), StandardCharsets.UTF_8));
+            out = ssl.getOutputStream();
+        }
+
         void write(String text) throws Exception {
             out.write(text.getBytes(StandardCharsets.UTF_8));
             out.flush();