diff --git a/client/src/main/java/org/asynchttpclient/util/AuthenticatorUtils.java b/client/src/main/java/org/asynchttpclient/util/AuthenticatorUtils.java
index 444ef8503..bd9ee8bb2 100644
--- a/client/src/main/java/org/asynchttpclient/util/AuthenticatorUtils.java
+++ b/client/src/main/java/org/asynchttpclient/util/AuthenticatorUtils.java
@@ -491,13 +491,33 @@ private static String bufferAndHashBodyGenerator(BodyGenerator gen, String hashA
     private static void append(StringBuilder builder, String name, @Nullable String value, boolean quoted) {
         builder.append(name).append('=');
         if (quoted) {
-            builder.append('"').append(value).append('"');
+            builder.append('"');
+            appendQuotedStringContent(builder, value);
+            builder.append('"');
         } else {
             builder.append(value);
         }
         builder.append(", ");
     }
 
+    // RFC 7616 params like realm/nonce/opaque are echoed back from the server challenge into the
+    // quoted-string values of the Authorization header. Backslash-escape " and \ (RFC 7230 quoted-string)
+    // so a server-supplied value carrying a bare quote cannot close the value early and inject extra
+    // auth-params into the credentials the client emits.
+    private static void appendQuotedStringContent(StringBuilder builder, @Nullable String value) {
+        if (value == null) {
+            builder.append((String) null);
+            return;
+        }
+        for (int i = 0; i < value.length(); i++) {
+            char c = value.charAt(i);
+            if (c == '"' || c == '\\') {
+                builder.append('\\');
+            }
+            builder.append(c);
+        }
+    }
+
     public static @Nullable String perConnectionProxyAuthorizationHeader(Request request, @Nullable Realm proxyRealm) {
         String proxyAuthorization = null;
         if (proxyRealm != null && proxyRealm.isUsePreemptiveAuth()) {
diff --git a/client/src/test/java/org/asynchttpclient/util/AuthenticatorUtilsTest.java b/client/src/test/java/org/asynchttpclient/util/AuthenticatorUtilsTest.java
index 0cd3846c8..c98f19483 100644
--- a/client/src/test/java/org/asynchttpclient/util/AuthenticatorUtilsTest.java
+++ b/client/src/test/java/org/asynchttpclient/util/AuthenticatorUtilsTest.java
@@ -37,6 +37,7 @@
 import java.util.List;
 
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import static org.junit.jupiter.api.Assertions.assertThrows;
@@ -492,4 +493,29 @@ void computeRspAuth_basic() throws Exception {
         assertNotNull(rspauth);
         assertEquals(32, rspauth.length()); // MD5 hex is 32 chars
     }
+
+    // A server-controlled challenge value (nonce/realm/opaque) must be backslash-escaped when echoed
+    // into the quoted-string params of the outgoing Authorization header, so a bare " cannot close the
+    // value early and inject extra auth-params.
+    @Test
+    void digestChallengeValuesEscapedInAuthorizationHeader() {
+        Realm realm = new Realm.Builder("user", "pass")
+                .setScheme(Realm.AuthScheme.DIGEST)
+                .setUsePreemptiveAuth(true)
+                .setRealmName("testrealm")
+                .setNonce("abc\"x")
+                .setOpaque("op\\aque")
+                .setAlgorithm("MD5")
+                .build();
+        Request request = new RequestBuilder("GET")
+                .setUrl("http://example.com/secret")
+                .build();
+
+        String header = AuthenticatorUtils.perRequestAuthorizationHeader(request, realm);
+
+        assertNotNull(header);
+        assertTrue(header.contains("nonce=\"abc\\\"x\""), header);
+        assertTrue(header.contains("opaque=\"op\\\\aque\""), header);
+        assertFalse(header.contains("nonce=\"abc\"x\""), header);
+    }
 }