support more base64 decode, for issue #3687

core/src/main/java/com/alibaba/fastjson2/JSONReader.java

@@ -1885,9 +1885,13 @@ public abstract class JSONReader
     public byte[] readBase64() {
         String str = readString();
         if (str != null) {
-            String prefix = "data:image/jpeg;base64,";
-            if (str.startsWith(prefix)) {
-                str = str.substring(prefix.length());
+            String prefix = "data:image/";
+            int p0, p1;
+            String base64 = "base64";
+            if (str.startsWith(prefix)
+                    && (p0 = str.indexOf(';', prefix.length() + 1)) != -1
+                    && (p1 = str.indexOf(',', p0 + 1)) != -1 && str.regionMatches(p0 + 1, base64, 0, base64.length())) {
+                str = str.substring(p1 + 1);
             }
         }
         if (str.isEmpty()) {

core/src/main/java/com/alibaba/fastjson2/JSONReaderUTF8.java

@@ -7463,16 +7463,13 @@ class JSONReaderUTF8
 
         byte[] decoded;
         if (index != offset) {
-            boolean hasPrefix = true;
-            String prefix = "data:image/jpeg;base64,";
-            for (int i = 0; i < prefix.length(); i++) {
-                if (bytes[offset + i] != prefix.charAt(i)) {
-                    hasPrefix = false;
-                    break;
-                }
-            }
-            if (hasPrefix) {
-                offset += prefix.length();
+            String prefix = "data:image/";
+            int p0, p1;
+            String base64 = "base64";
+            if (regionMatches(bytes, offset, prefix)
+                    && (p0 = IOUtils.indexOfChar(bytes, ';', prefix.length() + 1, index)) != -1
+                    && (p1 = IOUtils.indexOfChar(bytes, ',', p0 + 1, index)) != -1 && IOUtils.regionMatches(bytes, p0 + 1, base64)) {
+                offset = p1 + 1;
             }
 
             byte[] src = Arrays.copyOfRange(bytes, offset, index);

core/src/main/java/com/alibaba/fastjson2/util/IOUtils.java

@@ -1621,6 +1621,19 @@ public class IOUtils {
         return indexOfChar(buf, '\\', i, max);
     }
 
+    public static boolean regionMatches(byte[] bytes, int off, String prefix) {
+        int len = prefix.length();
+        if (off + len >= bytes.length) {
+            return false;
+        }
+        for (int i = 0; i < len; i++) {
+            if (bytes[off + i] != prefix.charAt(i)) {
+                return false;
+            }
+        }
+        return true;
+    }
+
     public static int indexOfChar(byte[] buf, int ch, int fromIndex, int max) {
         for (int i = fromIndex; i < max; i++) {
             if (buf[i] == ch) {