diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java
index d5d63c9c..f47ae091 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/challenge/TokenChallenge.java
@@ -23,6 +23,7 @@ import org.jose4j.jwk.PublicJsonWebKey;
 import org.jose4j.lang.JoseException;
 import org.shredzone.acme4j.Login;
 import org.shredzone.acme4j.exception.AcmeProtocolException;
+import org.shredzone.acme4j.toolbox.AcmeUtils;
 import org.shredzone.acme4j.toolbox.JSON;
 
 /**
@@ -51,7 +52,11 @@ public class TokenChallenge extends Challenge {
      * Gets the token.
      */
     protected String getToken() {
-        return getJSON().get(KEY_TOKEN).asString();
+        String token = getJSON().get(KEY_TOKEN).asString();
+        if (!AcmeUtils.isValidBase64Url(token)) {
+            throw new AcmeProtocolException("Invalid token: " + token);
+        }
+        return token;
     }
 
     /**
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java b/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
index c28b70ce..24307f39 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/connector/DefaultConnection.java
@@ -82,8 +82,6 @@ public class DefaultConnection implements Connection {
     private static final String MIME_JSON_PROBLEM = "application/problem+json";
     private static final String MIME_CERTIFICATE_CHAIN = "application/pem-certificate-chain";
 
-    private static final Pattern BASE64URL_PATTERN = Pattern.compile("[0-9A-Za-z_-]+");
-
     private static final URI BAD_NONCE_ERROR = URI.create("urn:ietf:params:acme:error:badNonce");
     private static final int MAX_ATTEMPTS = 10;
 
@@ -235,7 +233,7 @@ public class DefaultConnection implements Connection {
             return null;
         }
 
-        if (!BASE64URL_PATTERN.matcher(nonceHeader).matches()) {
+        if (!AcmeUtils.isValidBase64Url(nonceHeader)) {
             throw new AcmeProtocolException("Invalid replay nonce: " + nonceHeader);
         }
 
diff --git a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
index 368eb669..3a9c81b9 100644
--- a/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
+++ b/acme4j-client/src/main/java/org/shredzone/acme4j/toolbox/AcmeUtils.java
@@ -69,6 +69,8 @@ public final class AcmeUtils {
 
     private static final Pattern MAIL_PATTERN = Pattern.compile("\\?|@.*,");
 
+    private static final Pattern BASE64URL_PATTERN = Pattern.compile("[0-9A-Za-z_-]*");
+
     private static final Base64.Encoder PEM_ENCODER = Base64.getMimeEncoder(64,
                 "\n".getBytes(StandardCharsets.US_ASCII));
 
@@ -156,6 +158,20 @@ public final class AcmeUtils {
         return Base64Url.decode(base64);
     }
 
+    /**
+     * Validates that the given {@link String} is a valid base64url encoded value.
+     *
+     * @param base64
+     *            {@link String} to validate
+     * @return {@code true}: String contains a valid base64url encoded value.
+     *         {@code false} if the {@link String} was {@code null} or contained illegal
+     *         characters.
+     * @since 2.6
+     */
+    public static boolean isValidBase64Url(@Nullable String base64) {
+        return base64 != null && BASE64URL_PATTERN.matcher(base64).matches();
+    }
+
     /**
      * ASCII encodes a domain name.
      * <p>
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/challenge/TokenChallengeTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/challenge/TokenChallengeTest.java
new file mode 100644
index 00000000..1aedc93f
--- /dev/null
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/challenge/TokenChallengeTest.java
@@ -0,0 +1,56 @@
+/*
+ * acme4j - Java ACME client
+ *
+ * Copyright (C) 2019 Richard "Shred" Körber
+ *   http://acme4j.shredzone.org
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
+ */
+package org.shredzone.acme4j.challenge;
+
+import static org.junit.Assert.fail;
+
+import java.io.IOException;
+
+import org.junit.Test;
+import org.shredzone.acme4j.Login;
+import org.shredzone.acme4j.exception.AcmeProtocolException;
+import org.shredzone.acme4j.provider.TestableConnectionProvider;
+import org.shredzone.acme4j.toolbox.JSONBuilder;
+
+/**
+ * Unit tests for {@link TokenChallenge}.
+ */
+public class TokenChallengeTest {
+
+    /**
+     * Test that invalid tokens are detected.
+     */
+    @Test
+    public void testInvalidToken() throws IOException {
+        TestableConnectionProvider provider = new TestableConnectionProvider();
+        Login login = provider.createLogin();
+
+        JSONBuilder jb = new JSONBuilder();
+        jb.put("url", "https://example.com/acme/1234");
+        jb.put("type", "generic");
+        jb.put("token", "<script>someMaliciousCode()</script>");
+
+        TokenChallenge challenge = new TokenChallenge(login, jb.toJSON());
+
+        try {
+            challenge.getToken();
+            fail("Invalid token was accepted");
+        } catch (AcmeProtocolException ex) {
+            // expected
+        }
+
+        provider.close();
+    }
+
+}
diff --git a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
index 21271b3c..eac94010 100644
--- a/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
+++ b/acme4j-client/src/test/java/org/shredzone/acme4j/toolbox/AcmeUtilsTest.java
@@ -92,6 +92,22 @@ public class AcmeUtilsTest {
         assertThat(base64UrlDecode, is(sha256hash("foobar")));
     }
 
+    /**
+     * Test base64 URL validation.
+     */
+    @Test
+    public void testBase64UrlValidate() {
+        assertThat(isValidBase64Url(null), is(false));
+        assertThat(isValidBase64Url(""), is(true));
+        assertThat(isValidBase64Url("         "), is(false));
+        assertThat(isValidBase64Url("Zg"), is(true));
+        assertThat(isValidBase64Url("Zg="), is(false));
+        assertThat(isValidBase64Url("Zg=="), is(false));
+        assertThat(isValidBase64Url("Zm9v"), is(true));
+        assertThat(isValidBase64Url("   Zm9v   "), is(false));
+        assertThat(isValidBase64Url("<some>.illegal#Text"), is(false));
+    }
+
     /**
      * Test ACE conversion.
      */