Security Misconfiguration

M6: Insufficient Input/Output Validation is a category that covers various types of flaws that allow attackers to exploit or manipulate the data or messages that are entered or displayed by mobile applications. Input is the data or messages that are entered by the user or received by the app from other sources, such as the network, the device, or other apps. Output is the data or messages that are displayed by the app to the user or sent by the app to other destinations, such as the network, the device, or other apps. For example, when you use a chat app to send or receive messages, you are entering input and viewing output.

Insufficient input/output validation can happen when mobile applications do not check or filter the input or output data or messages for any errors, anomalies, or malicious content. For example, some common flaws are:

• Buffer overflow: Some applications do not check if the input data or messages are longer than the expected or allowed size, and they try to store them in a fixed-size memory space (buffer) that cannot hold them. This causes the excess data or messages to overwrite the adjacent memory space (overflow) and corrupt or modify other data or code. For example, an app that accepts a username might not check if the username is longer than 10 characters, and it might try to store it in a 10-byte buffer. If the user enters a username that is 20 characters long, the extra 10 characters might overwrite the next 10 bytes of memory and change some important values or instructions.

• Format string vulnerability: Some applications use format strings to specify how to display or print output data or messages. Format strings are special characters that indicate how to format the output, such as %s for string, %d for decimal, %x for hexadecimal, etc. However, some applications do not validate if the output data or messages match the format strings, and they blindly pass them to functions that interpret and execute them. This allows attackers to inject malicious format strings that can read or write arbitrary memory locations. For example, an app that displays a message might use a format string like “Hello %s” to print the user’s name. If the user enters a name that contains a malicious format string like “%x%x%x”, the app might pass it to a function that will print the hexadecimal values of three memory locations instead of the name.

• SQL injection: Some applications use SQL (Structured Query Language) statements to interact with databases and perform operations such as querying, inserting, updating, or deleting data. However, some applications do not sanitize or escape the input data or messages that are used in SQL statements, and they directly concatenate them with SQL keywords. This allows attackers to inject malicious SQL commands that can manipulate or compromise the database. For example, an app that searches for products might use a SQL statement like “SELECT * FROM products WHERE name LIKE ‘%input%’” to find products that match the user’s input. If the user enters an input that contains a malicious SQL command like “’ OR 1=1 --”, the app might concatenate it with the SQL statement and execute it as “SELECT * FROM products WHERE name LIKE ‘%’ OR 1=1 --'”. This will return all products from the database instead of matching ones, and it will also comment out (ignore) the rest of the SQL statement.

• Cross-site scripting (XSS): Some applications use HTML (HyperText Markup Language) tags to format and display output data or messages on web pages or web views. HTML tags are special characters that indicate how to structure and style the output, such as <b> for bold, <i> for italic, <a> for link, etc. However, some applications do not encode or filter the output data or messages that contain HTML tags, and they directly insert them into HTML documents. This allows attackers to inject malicious HTML tags that can execute JavaScript code (a scripting language that can manipulate web pages) on the web page or web view. For example, an app that displays a message might use an HTML tag like “<p>Message: output</p>” to print the user’s message. If the user enters a message that contains a malicious HTML tag like “<script>alert(‘XSS’)</script>”, the app might insert it into the HTML document and execute it as “<p>Message: <script>alert(‘XSS’)</script></p>”. This will display an alert box with the text “XSS” on the web page or web view.

These flaws can lead to serious consequences, such as data corruption, data injection, data leakage, code execution, etc. Therefore, it is very important for mobile applications to implement and enforce sufficient input/output validation mechanisms, such as:

• Buffer limit: Applications should check if the input data or messages are longer than the expected or allowed size, and they should reject or truncate them if they exceed it. They should also use dynamic memory allocation (such as malloc or calloc) instead of static memory allocation (such as char array[10]) to avoid fixed-size buffers. For example, an app that accepts a username should check if the username is longer than 10 characters, and it should reject it or cut it to 10 characters if it is. It should also use a pointer (such as char *username) instead of an array (such as char username[10]) to store the username in memory.

• Format string validation: Applications should validate if the output data or messages match the format strings, and they should escape or encode any format strings that are part of the output. They should also use secure functions (such as snprintf or printf_s) instead of insecure functions (such as sprintf or printf) to handle format strings. For example, an app that displays a message should validate if the user’s name contains any format strings, and it should escape or encode them if it does. It should also use a function like snprintf to print the user’s name with the format string “Hello %s”.

• SQL sanitization: Applications should sanitize or escape the input data or messages that are used in SQL statements, and they should use parameterized queries (such as prepared statements or stored procedures) instead of direct concatenation. For example, an app that searches for products should sanitize or escape the user’s input, and it should use a parameterized query like “SELECT * FROM products WHERE name LIKE ?” to find products that match the user’s input.

• HTML encoding: Applications should encode or filter the output data or messages that contain HTML tags, and they should use secure libraries or frameworks (such as DOMPurify or AngularJS) to handle HTML output. For example, an app that displays a message should encode or filter the user’s message, and it should use a library like DOMPurify to sanitize and display the user’s message with the HTML tag “<p>Message: output</p>”.

I hope this explanation helps you understand what M6: Insufficient Input/Output Validation is and why it is important. If you want to learn more about this category and how to prevent it, you can check out these resources:

• [M6-Insufficient Input/Output Validation | OWASP Foundation]: The official website of the OWASP Mobile Top 10 project. It contains detailed information about this category and links to other useful resources.

• [Mobile App Security: How To Secure Your Mobile App Input/Output]: A blog post that provides an overview of the best practices for mobile app input/output security.

• [Mobile App Security Testing | Cybrary]: A course that teaches you how to test and secure mobile applications against various security risks.