diff --git a/scram-client/src/test/java/com/ongres/scram/client/ScramBuilderTest.java b/scram-client/src/test/java/com/ongres/scram/client/ScramBuilderTest.java
index 82019af..f5a230d 100644
--- a/scram-client/src/test/java/com/ongres/scram/client/ScramBuilderTest.java
+++ b/scram-client/src/test/java/com/ongres/scram/client/ScramBuilderTest.java
@@ -13,8 +13,6 @@
 
 import java.util.Arrays;
 import java.util.Base64;
-import java.util.Collection;
-import java.util.Collections;
 
 import com.ongres.scram.common.ClientFinalMessage;
 import com.ongres.scram.common.ClientFirstMessage;
diff --git a/scram-common/src/main/java/com/ongres/scram/common/ServerFirstMessage.java b/scram-common/src/main/java/com/ongres/scram/common/ServerFirstMessage.java
index 53c8f56..0573194 100644
--- a/scram-common/src/main/java/com/ongres/scram/common/ServerFirstMessage.java
+++ b/scram-common/src/main/java/com/ongres/scram/common/ServerFirstMessage.java
@@ -150,6 +150,9 @@ public int getIterationCount() {
       throw new ScramParseException(
           "nonce must be the 1st element of the server-first-message");
     }
+    if (!isValidNonce(nonce.getValue())) {
+      throw new ScramParseException("nonce contains invalid characters");
+    }
     if (!nonce.getValue().startsWith(clientNonce)) {
       throw new ScramParseException("parsed nonce does not start with client nonce");
     }
@@ -176,6 +179,17 @@ public int getIterationCount() {
         salt.getValue(), iterationInt);
   }
 
+  // RFC 5802 Section 7: printable = %x21-2B / %x2D-7E (printable ASCII excluding comma)
+  private static boolean isValidNonce(String nonce) {
+    for (int i = 0; i < nonce.length(); i++) {
+      char c = nonce.charAt(i);
+      if (c < 0x21 || c > 0x7E || c == ',') {
+        return false;
+      }
+    }
+    return true;
+  }
+
   @Override
   StringBuilder writeTo(StringBuilder sb) {
     return StringWritableCsv.writeTo(
diff --git a/scram-common/src/test/java/com/ongres/scram/common/ServerFirstMessageTest.java b/scram-common/src/test/java/com/ongres/scram/common/ServerFirstMessageTest.java
index 142c54a..3980385 100644
--- a/scram-common/src/test/java/com/ongres/scram/common/ServerFirstMessageTest.java
+++ b/scram-common/src/test/java/com/ongres/scram/common/ServerFirstMessageTest.java
@@ -8,9 +8,14 @@
 import static com.ongres.scram.common.RfcExampleSha1.CLIENT_NONCE;
 import static com.ongres.scram.common.RfcExampleSha1.SERVER_FIRST_MESSAGE;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.util.stream.Stream;
 
 import com.ongres.scram.common.exception.ScramParseException;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.MethodSource;
 
 class ServerFirstMessageTest {
   @Test
@@ -31,4 +36,20 @@ void validParseFrom() throws ScramParseException {
 
     assertEquals(SERVER_FIRST_MESSAGE, serverFirstMessage.toString());
   }
+
+  static Stream<String> invalidServerNonceSuffixes() {
+    return Stream.of(
+      "server,nonce", // printable ASCII except ","
+        "server nonce", // space (0x20) is below the printable ASCII range
+        "servernonce" + (char) 0x7F // DEL (0x7F) is above the printable ASCII range
+    );
+  }
+
+  @ParameterizedTest(name = "nonce with invalid char rejected")
+  @MethodSource("invalidServerNonceSuffixes")
+  void invalidNonceCharactersRejected(String serverNonceSuffix) {
+    String message = "r=" + CLIENT_NONCE + serverNonceSuffix + ",s=QSXCR+Q6sek8bf92,i=4096";
+    assertThrows(ScramParseException.class,
+        () -> ServerFirstMessage.parseFrom(message, CLIENT_NONCE));
+  }
 }