diff --git a/server/src/main/java/org/elasticsearch/rest/RestRequest.java b/server/src/main/java/org/elasticsearch/rest/RestRequest.java
index 66ba0c743813e..c82666cfe3cb4 100644
--- a/server/src/main/java/org/elasticsearch/rest/RestRequest.java
+++ b/server/src/main/java/org/elasticsearch/rest/RestRequest.java
@@ -12,6 +12,7 @@
 import org.elasticsearch.ElasticsearchParseException;
 import org.elasticsearch.ElasticsearchStatusException;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.ValidationException;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.unit.ByteSizeValue;
@@ -297,11 +298,17 @@ public final BytesReference requiredContent() {
         if (hasContent() == false) {
             throw new ElasticsearchParseException("request body is required");
         } else if (xContentType.get() == null) {
-            throw new IllegalStateException("unknown content type");
+            throwValidationException("unknown content type");
         }
         return content();
     }
 
+    private static void throwValidationException(String msg) {
+        ValidationException unknownContentType = new ValidationException();
+        unknownContentType.addValidationError(msg);
+        throw unknownContentType;
+    }
+
     /**
      * Get the value of the header or {@code null} if not found. This method only retrieves the first header value if multiple values are
      * sent. Use of {@link #getAllHeaderValues(String)} should be preferred
@@ -561,12 +568,12 @@ public final Tuple<XContentType, BytesReference> contentOrSourceParam() {
         String source = param("source");
         String typeParam = param("source_content_type");
         if (source == null || typeParam == null) {
-            throw new IllegalStateException("source and source_content_type parameters are required");
+            throwValidationException("source and source_content_type parameters are required");
         }
         BytesArray bytes = new BytesArray(source);
         final XContentType xContentType = parseContentType(Collections.singletonList(typeParam));
         if (xContentType == null) {
-            throw new IllegalStateException("Unknown value for source_content_type [" + typeParam + "]");
+            throwValidationException("Unknown value for source_content_type [" + typeParam + "]");
         }
         return new Tuple<>(xContentType, bytes);
     }
diff --git a/server/src/test/java/org/elasticsearch/rest/RestRequestTests.java b/server/src/test/java/org/elasticsearch/rest/RestRequestTests.java
index bb06dbe5d09aa..81f357d1c69cd 100644
--- a/server/src/test/java/org/elasticsearch/rest/RestRequestTests.java
+++ b/server/src/test/java/org/elasticsearch/rest/RestRequestTests.java
@@ -9,6 +9,7 @@
 package org.elasticsearch.rest;
 
 import org.elasticsearch.ElasticsearchParseException;
+import org.elasticsearch.common.ValidationException;
 import org.elasticsearch.common.bytes.BytesArray;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.core.CheckedConsumer;
@@ -32,6 +33,9 @@
 import static java.util.Collections.emptyMap;
 import static java.util.Collections.singletonMap;
 import static org.elasticsearch.rest.RestRequest.PATH_RESTRICTED;
+import static org.elasticsearch.rest.RestRequest.OPERATOR_REQUEST;
+import static org.elasticsearch.rest.RestRequest.SERVERLESS_REQUEST;
+import static org.hamcrest.Matchers.containsString;
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
 import static org.hamcrest.Matchers.is;
@@ -129,8 +133,8 @@ public void testContentOrSourceParam() throws IOException {
                 .contentOrSourceParam()
                 .v2()
         );
-        e = expectThrows(IllegalStateException.class, () -> contentRestRequest("", Map.of("source", "stuff2")).contentOrSourceParam());
-        assertEquals("source and source_content_type parameters are required", e.getMessage());
+        e = expectThrows(ValidationException.class, () -> contentRestRequest("", Map.of("source", "stuff2")).contentOrSourceParam());
+        assertThat(e.getMessage(), containsString("source and source_content_type parameters are required"));
     }
 
     public void testHasContentOrSourceParam() throws IOException {
@@ -245,8 +249,8 @@ public void testRequiredContent() {
                 .requiredContent()
         );
         assertEquals("request body is required", e.getMessage());
-        e = expectThrows(IllegalStateException.class, () -> contentRestRequest("test", null, Collections.emptyMap()).requiredContent());
-        assertEquals("unknown content type", e.getMessage());
+        e = expectThrows(ValidationException.class, () -> contentRestRequest("test", null, Collections.emptyMap()).requiredContent());
+        assertThat(e.getMessage(), containsString("unknown content type"));
     }
 
     public void testMarkPathRestricted() {
diff --git a/x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/rest/RestRequestFilterTests.java b/x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/rest/RestRequestFilterTests.java
index 8a8bb19fe1796..37a70dccaa408 100644
--- a/x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/rest/RestRequestFilterTests.java
+++ b/x-pack/plugin/security/src/test/java/org/elasticsearch/xpack/security/rest/RestRequestFilterTests.java
@@ -115,7 +115,7 @@ public void testFilterUnknownContentTypeThrows() throws IOException {
         }
         RestRequestFilter filter = () -> Collections.singleton("root.second.third");
         RestRequest filtered = filter.getFilteredRequest(restRequest);
-        IllegalStateException e = expectThrows(IllegalStateException.class, () -> filtered.content());
+        Exception e = expectThrows(IllegalArgumentException.class, () -> filtered.content());
         assertThat(e.getMessage(), containsString("unknown content type"));
     }