PHP security vulnerabilities mainly include SQL injection, XSS, CSRF, and file upload vulnerabilities. 1. SQL injection tampers with database queries through malicious input. Prevention methods include using preprocessing statements, filtering inputs, and restricting database permissions. 2. XSS attacks harm user data by injecting malicious scripts. They should use htmlspecialchars to escape output, set CSP headers, and filter rich text content. 3. CSRF uses user identity to forge requests, preventive measures include using one-time tokens, verifying the Referer header, and setting the SameSite attribute of the cookie. 4. File upload vulnerability may cause the server to execute malicious scripts. The policy is to rename files and limit suffixes, prohibit uploading directories from executing scripts, and not place the upload path in the web directory. Good coding habits such as input verification, output escaping and minimum permission principles can effectively improve security.

PHP is a widely used server-side scripting language, which is very common in web development. But because of its widespread use, it has also become a common target for attackers. If you don't pay attention to security issues, the website may be easily hacked, leaked, or even hung up.

Below are some common PHP security vulnerabilities and corresponding prevention methods, which are easy to ignore but are very critical during the development process.

1. SQL Injection

SQL injection is an attack method that manipulates database queries by maliciously constructing input parameters. For example, when a user logs in, an attacker can enter something like ' OR '1'='1 in the user name or password box to bypass the verification logic.

How to prevent:

• Use prepared statements and parameterized queries such as PDO or MySQLi.
• Do not splice SQL query strings.
• Filter and verify user input, and use the whitelisting mechanism if necessary.
• Limit database account permissions and avoid using high-permission accounts to connect to the database.

For example, use PDO to execute a query:

 $stmt = $pdo->prepare('SELECT * FROM users WHERE id = ?');
$stmt->execute([$userId]);

This way, even if $userId is tampered with, it will not affect the original SQL structure.

2. XSS (cross-site scripting attack)

XSS refers to an attacker injecting a malicious script into a web page. When other users browse the page, the script will be executed in their browser, thereby stealing cookies, session information, etc.

Common scenarios:

• Embed the <script> tag in user comments
• The content submitted by the form is not escaped and displayed directly

Prevention method:

• Use the htmlspecialchars() function to escape special characters before output to the HTML page.
• Set the appropriate Content-Security-Policy (CSP) header to limit script loading sources.
• Strictly filter content in rich text editors (such as using the HTML Purifier library).

For example, when outputting the user's nickname:

 echo &#39;<div>&#39; . htmlspecialchars($userNickname, ENT_QUOTES, &#39;UTF-8&#39;) . &#39;</div>&#39;;

3. CSRF (cross-site request forgery)

CSRF attack refers to an attacker inducing a user to click on a link or access a page, thereby sending requests without the user's consent in the user's name, such as modifying passwords, transferring money, etc.

Precautionary advice:

• Use one-time tokens (such as CSRF tokens) in sensitive operations and verify their origin.
• Checking the HTTP Referer header, although it is not 100% reliable, it can add a layer of protection.
• Use the SameSite Cookie attribute to prevent cross-site requests from carrying cookies.

The basic process of token verification:

• A random token is generated when the page is loaded and saved in the Session.
• Bring this token when submitting the form.
• The backend compares whether the submitted Token and Session are consistent.

4. File upload vulnerability

Many websites allow users to upload images or other files, but if handled improperly, an attacker may upload malicious scripts (such as .php files) and execute them on the server.

FAQ:

• Directly use the file name uploaded by the user
• No file type checked
• Upload directory executable scripts

Coping strategies:

• Rename the upload file, using the random string whitelist suffix.
• Strictly limit the file types that are allowed to be uploaded (such as only jpg/png is allowed).
• Set the upload directory to non-executable scripts (such as .htaccess in Apache prohibits PHP execution).
• The storage path should not be placed in a web-accessible directory, but can be read through scripts.

Basically that's it. PHP security involves many points, but as long as you develop good coding habits, many problems can be avoided in advance. Practices like input verification, output escaping, and the minimum permission principle, although seemingly simple, are very important in actual projects.

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