diff --git a/CHANGELOG.md b/CHANGELOG.md
index 4ed85b630bbd4..f8182f0bd286d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -30,6 +30,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 - Fix wildcard query containing escaped character ([#15737](https://github.com/opensearch-project/OpenSearch/pull/15737))
 - Fix case-insensitive query on wildcard field ([#15882](https://github.com/opensearch-project/OpenSearch/pull/15882))
 - Add validation for the search backpressure cancellation settings ([#15501](https://github.com/opensearch-project/OpenSearch/pull/15501))
+- Avoid infinite loop when `flat_object` field contains invalid token ([#15985](https://github.com/opensearch-project/OpenSearch/pull/15985))
 
 ### Security
 
diff --git a/rest-api-spec/src/main/resources/rest-api-spec/test/index/90_flat_object.yml b/rest-api-spec/src/main/resources/rest-api-spec/test/index/90_flat_object.yml
index 83d3d273ebd93..b2c1e04d096aa 100644
--- a/rest-api-spec/src/main/resources/rest-api-spec/test/index/90_flat_object.yml
+++ b/rest-api-spec/src/main/resources/rest-api-spec/test/index/90_flat_object.yml
@@ -62,6 +62,17 @@ setup:
           },
           "required_matches": 1
         }
+  # The following document is invalid.
+  - do:
+      catch: /parsing_exception/
+      index:
+        index: test
+        id: 3
+        body: {
+          "ISBN13": "V12154942123242",
+          "catalog": ["Arrays in Action"],
+          "required_matches": 1
+        }
 
   # Do index refresh
   - do:
diff --git a/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java b/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java
index 2f60fc8f69f87..b9c0fa84f5ba7 100644
--- a/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java
+++ b/server/src/main/java/org/opensearch/common/xcontent/JsonToStringXContentParser.java
@@ -9,6 +9,7 @@
 package org.opensearch.common.xcontent;
 
 import org.opensearch.common.xcontent.json.JsonXContent;
+import org.opensearch.core.common.ParsingException;
 import org.opensearch.core.common.Strings;
 import org.opensearch.core.common.bytes.BytesReference;
 import org.opensearch.core.xcontent.AbstractXContentParser;
@@ -117,8 +118,6 @@ private boolean parseToken(Deque<String> path, String currentFieldName) throws I
                 isChildrenValueValid |= parseToken(path, currentFieldName);
             }
             this.parser.nextToken();
-        } else if (this.parser.currentToken() == Token.END_ARRAY) {
-            // skip
         } else if (this.parser.currentToken() == Token.START_OBJECT) {
             parser.nextToken();
             while (this.parser.currentToken() != Token.END_OBJECT) {
@@ -172,7 +171,7 @@ private String parseValue() throws IOException {
                 return this.parser.textOrNull();
             // Handle other token types as needed
             default:
-                throw new IOException("Unsupported value token type [" + parser.currentToken() + "]");
+                throw new ParsingException(parser.getTokenLocation(), "Unexpected value token type [" + parser.currentToken() + "]");
         }
     }
 
diff --git a/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java b/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java
index bf8f83e1b95df..738efcfafdca1 100644
--- a/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java
+++ b/server/src/main/java/org/opensearch/index/mapper/FlatObjectFieldMapper.java
@@ -30,6 +30,7 @@
 import org.opensearch.common.lucene.Lucene;
 import org.opensearch.common.lucene.search.AutomatonQueries;
 import org.opensearch.common.xcontent.JsonToStringXContentParser;
+import org.opensearch.core.common.ParsingException;
 import org.opensearch.core.xcontent.DeprecationHandler;
 import org.opensearch.core.xcontent.NamedXContentRegistry;
 import org.opensearch.core.xcontent.XContentParser;
@@ -568,31 +569,41 @@ protected void parseCreateField(ParseContext context) throws IOException {
         if (context.externalValueSet()) {
             String value = context.externalValue().toString();
             parseValueAddFields(context, value, fieldType().name());
-        } else if (context.parser().currentToken() != XContentParser.Token.VALUE_NULL) {
-            JsonToStringXContentParser jsonToStringParser = new JsonToStringXContentParser(
-                NamedXContentRegistry.EMPTY,
-                DeprecationHandler.IGNORE_DEPRECATIONS,
-                context.parser(),
-                fieldType().name()
-            );
-            /*
-              JsonToStringParser is the main parser class to transform JSON into stringFields in a XContentParser
-              It reads the JSON object and parsed to a list of string
-             */
-            XContentParser parser = jsonToStringParser.parseObject();
-
-            XContentParser.Token currentToken;
-            while ((currentToken = parser.nextToken()) != XContentParser.Token.END_OBJECT) {
-                switch (currentToken) {
-                    case FIELD_NAME:
-                        fieldName = parser.currentName();
-                        break;
-                    case VALUE_STRING:
-                        String value = parser.textOrNull();
-                        parseValueAddFields(context, value, fieldName);
-                        break;
+        } else {
+            XContentParser ctxParser = context.parser();
+            if (ctxParser.currentToken() != XContentParser.Token.VALUE_NULL) {
+                if (ctxParser.currentToken() != XContentParser.Token.START_OBJECT) {
+                    throw new ParsingException(
+                        ctxParser.getTokenLocation(),
+                        "[" + this.name() + "] unexpected token [" + ctxParser.currentToken() + "] in flat_object field value"
+                    );
                 }
 
+                JsonToStringXContentParser jsonToStringParser = new JsonToStringXContentParser(
+                    NamedXContentRegistry.EMPTY,
+                    DeprecationHandler.IGNORE_DEPRECATIONS,
+                    ctxParser,
+                    fieldType().name()
+                );
+                /*
+                  JsonToStringParser is the main parser class to transform JSON into stringFields in a XContentParser
+                  It reads the JSON object and parsed to a list of string
+                 */
+                XContentParser parser = jsonToStringParser.parseObject();
+
+                XContentParser.Token currentToken;
+                while ((currentToken = parser.nextToken()) != XContentParser.Token.END_OBJECT) {
+                    switch (currentToken) {
+                        case FIELD_NAME:
+                            fieldName = parser.currentName();
+                            break;
+                        case VALUE_STRING:
+                            String value = parser.textOrNull();
+                            parseValueAddFields(context, value, fieldName);
+                            break;
+                    }
+
+                }
             }
         }