diff --git a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilConfig.java b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilConfig.java
index 21ce6313e..136436dc2 100644
--- a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilConfig.java
+++ b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilConfig.java
@@ -3,6 +3,8 @@
 import com.facebook.react.bridge.ReadableArray;
 import com.facebook.react.bridge.ReadableMap;
 
+import java.util.Locale;
+
 class ReactNativeBlobUtilConfig {
 
     public Boolean fileCache;
@@ -33,7 +35,7 @@ class ReactNativeBlobUtilConfig {
         }
         if (options.hasKey("binaryContentTypes"))
             this.binaryContentTypes = options.getArray("binaryContentTypes");
-        if (this.path != null && path.toLowerCase().contains("?append=true")) {
+        if (this.path != null && path.toLowerCase(Locale.ROOT).contains("?append=true")) {
             this.overwrite = false;
         }
         if (options.hasKey("overwrite"))
diff --git a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilFS.java b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilFS.java
index 1add27a95..43808e8a1 100644
--- a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilFS.java
+++ b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilFS.java
@@ -25,6 +25,7 @@
 import java.security.MessageDigest;
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.Locale;
 import java.util.Map;
 import java.util.UUID;
 
@@ -209,7 +210,7 @@ else if (resolved == null) {
                 return;
             }
 
-            switch (encoding.toLowerCase()) {
+            switch (encoding.toLowerCase(Locale.ROOT)) {
                 case "base64":
                     promise.resolve(Base64.encodeToString(bytes, Base64.NO_WRAP));
                     break;
@@ -1095,7 +1096,7 @@ protected Integer doInBackground(ReadableArray... paths) {
     private static byte[] stringToBytes(String data, String encoding) {
         if (encoding.equalsIgnoreCase("ascii")) {
             return data.getBytes(Charset.forName("US-ASCII"));
-        } else if (encoding.toLowerCase().contains("base64")) {
+        } else if (encoding.toLowerCase(Locale.ROOT).contains("base64")) {
             return Base64.decode(data, Base64.NO_WRAP);
 
         } else if (encoding.equalsIgnoreCase("utf8")) {
diff --git a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilReq.java b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilReq.java
index 5b40b1286..2d763183a 100644
--- a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilReq.java
+++ b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilReq.java
@@ -54,6 +54,7 @@
 import java.util.List;
 import java.util.HashMap;
 
+import java.util.Locale;
 import java.util.concurrent.TimeUnit;
 
 import javax.net.ssl.SSLSocketFactory;
@@ -120,7 +121,7 @@ enum ResponseFormat {
     OkHttpClient client;
 
     public ReactNativeBlobUtilReq(ReadableMap options, String taskId, String method, String url, ReadableMap headers, String body, ReadableArray arrayBody, OkHttpClient client, final Callback callback) {
-        this.method = method.toUpperCase();
+        this.method = method.toUpperCase(Locale.ROOT);
         this.options = new ReactNativeBlobUtilConfig(options);
         this.taskId = taskId;
         this.url = url;
@@ -311,14 +312,14 @@ else if (this.options.fileCache)
                         else if (value.equalsIgnoreCase("utf8"))
                             responseFormat = ResponseFormat.UTF8;
                     } else {
-                        builder.header(key.toLowerCase(), value);
-                        mheaders.put(key.toLowerCase(), value);
+                        builder.header(key.toLowerCase(Locale.ROOT), value);
+                        mheaders.put(key.toLowerCase(Locale.ROOT), value);
                     }
                 }
             }
 
             if (method.equalsIgnoreCase("post") || method.equalsIgnoreCase("put") || method.equalsIgnoreCase("patch")) {
-                String cType = getHeaderIgnoreCases(mheaders, "Content-Type").toLowerCase();
+                String cType = getHeaderIgnoreCases(mheaders, "Content-Type").toLowerCase(Locale.ROOT);
 
                 if (rawRequestBodyArray != null) {
                     requestType = RequestType.Form;
@@ -332,7 +333,7 @@ else if (value.equalsIgnoreCase("utf8"))
                     if (rawRequestBody.startsWith(ReactNativeBlobUtilConst.FILE_PREFIX)
                             || rawRequestBody.startsWith(ReactNativeBlobUtilConst.CONTENT_PREFIX)) {
                         requestType = RequestType.SingleFile;
-                    } else if (cType.toLowerCase().contains(";base64") || cType.toLowerCase().startsWith("application/octet")) {
+                    } else if (cType.toLowerCase(Locale.ROOT).contains(";base64") || cType.toLowerCase(Locale.ROOT).startsWith("application/octet")) {
                         cType = cType.replace(";base64", "").replace(";BASE64", "");
                         if (mheaders.containsKey("content-type"))
                             mheaders.put("content-type", cType);
@@ -718,7 +719,7 @@ private boolean isBlobResponse(Response resp) {
         boolean isCustomBinary = false;
         if (options.binaryContentTypes != null) {
             for (int i = 0; i < options.binaryContentTypes.size(); i++) {
-                if (ctype.toLowerCase().contains(options.binaryContentTypes.getString(i).toLowerCase())) {
+                if (ctype.toLowerCase(Locale.ROOT).contains(options.binaryContentTypes.getString(i).toLowerCase(Locale.ROOT))) {
                     isCustomBinary = true;
                     break;
                 }
@@ -730,13 +731,13 @@ private boolean isBlobResponse(Response resp) {
     private String getHeaderIgnoreCases(Headers headers, String field) {
         String val = headers.get(field);
         if (val != null) return val;
-        return headers.get(field.toLowerCase()) == null ? "" : headers.get(field.toLowerCase());
+        return headers.get(field.toLowerCase(Locale.ROOT)) == null ? "" : headers.get(field.toLowerCase(Locale.ROOT));
     }
 
     private String getHeaderIgnoreCases(HashMap<String, String> headers, String field) {
         String val = headers.get(field);
         if (val != null) return val;
-        String lowerCasedValue = headers.get(field.toLowerCase());
+        String lowerCasedValue = headers.get(field.toLowerCase(Locale.ROOT));
         return lowerCasedValue == null ? "" : lowerCasedValue;
     }
 
diff --git a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilUtils.java b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilUtils.java
index 456b9e804..28f002242 100644
--- a/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilUtils.java
+++ b/android/src/main/java/com/ReactNativeBlobUtil/ReactNativeBlobUtilUtils.java
@@ -6,6 +6,7 @@
 
 import java.security.MessageDigest;
 import java.security.cert.CertificateException;
+import java.util.Locale;
 
 import javax.net.ssl.HostnameVerifier;
 import javax.net.ssl.SSLContext;
@@ -30,7 +31,7 @@ public static String getMD5(String input) {
             StringBuilder sb = new StringBuilder();
 
             for (byte b : digest) {
-                sb.append(String.format("%02x", b & 0xff));
+                sb.append(String.format(Locale.ROOT, "%02x", b & 0xff));
             }
 
             result = sb.toString();