Input validation and output escape are key security strategies developers must build into apps to protect against injection attacks, including cross-site scripting (XSS) — which has been identified as the number one software weakness in The Mitre Corporation’s 2024 Common Weakness Enumeration list. XSS attacks also often lead to data breaches, which cost small businesses $2.98 million on average. Input validation means all user data entered within your app is legit and can be used for its intended purpose, whereas output escape treats special characters — with the potential to turn malicious in certain context — as plain text, so there’s no risk of unauthorized code execution. Together, these two strategies work to prevent malicious attacks and consequential data breaches, and, ultimately, keep your app secure, your customers happy, and your professional reputation intact.
XSS attacks highlight need for tight security measures
If your app permits raw or unverified inputs from users, this is essentially an open invitation to cybercriminals to insert malicious script into your app. Through XSS attacks, a hacker’s HTML, JavaScript, or Java scripts are delivered to and executed by unsuspecting app users. This, for example, is a classic XSS attack script to look out for: “<script>alert(‘malicious code’)</script>”. In turn, once the code is injected and executed, hackers can infect the app or user device with malware, steal personal data, or take over user accounts.
XSS attacks are also a major vehicle for phishing: when hackers trick app users in order to steal their personal information and money. In fact, individual instances of phishing result in a financial loss of $50,000 on average. It’s therefore important for organizations to strengthen their technical defenses to keep cybercriminals out. In addition to secure app development, employees should learn how to spot phishing attempts, so threats are neutralized faster. In terms of phishing through XSS attacks, for example, hackers can create fake app login pages in an attempt to steal information. These pages may be characterized by poor design, bad grammar, or suspicious requests for personal information.
Use whitelists to validate inputs
So, to strengthen your app against malicious code injections, set whitelists that specify a correct format for each user input field — this includes username, password, and email. With whitelists, in order for data to be entered successfully, it must follow the permitted format, or else it’ll be rejected. This is why you often find usernames usually have to contain around six characters and contain a mix of alphanumeric characters (upper and lower case, letters, numbers, and punctuation marks). Similarly, passwords should be around six characters in length, and also contain alphanumeric characters. Emails should also be given in a standard format, which includes an “@” and a legitimate domain name.
You may have also heard of input blacklists — however, these aren’t as secure as whitelists and therefore not really a necessary addition. With a blacklist, harmful character inputs (such as, “<” and “>”) are blocked from app access altogether. But, hackers don’t have much trouble getting around these lists in order to insert malicious code. Whitelists, on the other hand, only accept safe inputs, and therefore provide a stronger defense.
Escape output to protect against malicious code
Escaping output is a sanitization strategy that stores user data exactly how it’s entered, but treats it all, including special characters, as plain text. So, that means the app doesn’t recognize the data as code to act upon. This boosts security as code can be fine and useful in one context, and malicious in another. For example, app users may need to input certain HTML characters, such as, “&” or apostrophes in sign-up fields or messages, but these characters can take on a malicious purpose in JavaScript, for instance. In particular, Htmlspecialchars() in PHP can be used to escape HTML output. As a result, the app will then process special characters as benign, risk-free data.
Input validation and escape output are key security strategies that can protect apps from XSS attacks. If you take time to learn about these methods, you’ll successfully cover all your bases and create a fully-functional app that prioritizes security and data protection.
With over a decade in the tech field, Bogdan blends technical expertise with insights on business innovation in technology.
A regular contributor to TMS Outsource's blog, where you'll find sharp analyses on software development, tech business strategies, and global tech dynamics.
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