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5 Common PHP Vulnerabilities And How To Address Them

WRITTEN BY
Timothy Shim
UPDATED
June 17, 2024

 

PHP is a widely-used scripting language commonly used to create dynamic web pages and applications. However, like all web technologies, PHP is also vulnerable to attacks. These attacks can cause significant damage to a website, including data theft, data loss, and even complete website shutdown!

 

 

This article will explore the most common types of PHP vulnerabilities, how to mitigate them, and best practices for securing PHP applications.

 

Understanding and addressing PHP vulnerabilities is crucial for maintaining the security of your website and protecting your users’ data. By gaining a deeper understanding of these vulnerabilities, you can better protect your website from potential attacks and maintain the trust of your users.

 

So – here are some of the most common PHP vulnerabilities and how to mitigate them!

 

1. SQL Injection Attacks

 

SQL injection attacks

SQL injection attacks allow malicious code to be added to PHP applications. (Credit: One.com)

 

SQL injection attacks are web application security vulnerabilities that allow attackers to inject malicious SQL code into a web application’s database. This can result in data theft, data corruption, and even complete system compromise.

 

How SQL injection attacks work

 

SQL injection attacks exploit vulnerabilities in web applications that allow an attacker to insert malicious SQL code into a database query. This is typically done by inserting SQL code into user input fields, such as search boxes, login forms, or comment sections.

 

For example, consider a login form that accepts a username and password. The PHP code for this form might look something like this:

 

$username = $_POST[‘username’];

 

$password = $_POST[‘password’];

 

$query = “SELECT * FROM users WHERE username=’$username’ AND password=’$password'”;

 

In this example, the username and password submitted by the user are directly inserted into the SQL query without any sanitization or validation. An attacker could exploit this vulnerability by submitting a specially crafted username or password with malicious SQL code.

 

For example, consider the following username:

 


admin’–

 

This username includes the SQL comment symbol, effectively commenting on the rest of the SQL query. The resulting SQL query would look like this:

 


SELECT * FROM users WHERE username=’admin’–‘ AND password=’password’

 

In this case, the password check is bypassed entirely, and the attacker gains access to the application as the administrator.

 

Mitigating SQL injection attacks

 

To prevent SQL injection attacks in PHP, you should follow these best practices:

 

  • Use prepared statements or parameterized queries
    One of the most effective ways to prevent SQL injection attacks is to use prepared statements or parameterized queries. Prepared statements allow you to define a SQL query template with placeholders for user input.
     
    The user input is then passed as parameters to the prepared statement, which is automatically sanitized and validated.
  • Sanitize user input
    Even when using prepared statements, sanitizing user input is essential to prevent other types of attacks, such as cross-site scripting (XSS) attacks. Sanitizing user input involves removing unwanted or malicious characters from the input data.
     
    There are several functions available in PHP for sanitizing user input, including:

    • htmlspecialchars()
    • filter_var()
    • stripslashes()
    • trim()
  • Use stored procedures
    Stored procedures are precompiled SQL statements that are stored in a database server. Using stored procedures can help prevent SQL injection attacks by separating the SQL code from the application code and providing an additional layer of security.
     
    To use stored procedures in PHP, you must define the stored procedure in the database server and then call it from your PHP code.
  • Limit user privileges
    Another way to prevent SQL injection attacks is to limit the privileges of the user account used by the web application. In particular, the account should be limited to only those privileges required by the application, and it should not have administrative privileges.
     
    If an attacker can compromise the web application and gain access to the database, limiting the user account’s privileges can help minimize the damage they can do.

 

 

2. Cross-site Scripting Attacks

 

XSS attacks

An XSS attack helps exploit web application vulnerabilities. (Credit: Cloudflare)

 

Cross-site scripting (XSS) is another common web application security vulnerability affecting PHP applications. In this attack, an attacker injects malicious scripts into a web page viewed by other users, typically by exploiting a vulnerability in the web application.

 

XSS attacks can be used to steal sensitive user information, such as login credentials or credit card numbers, or to perform actions on behalf of the user, such as making unauthorized purchases or posting spam.

 

Mitigating XSS attacks

 

Several best-practice approaches can help prevent XSS attacks on PHP applications. These include;

 

  • Validate and sanitize user input
    As with SQL injection attacks, one of the most important steps you can take to prevent XSS attacks is to validate and sanitize all user input to your PHP application. This includes input submitted via web forms and input received from sources such as query strings and cookies.
     
    To validate user input, you should check that it conforms to the expected format by checking that an email address contains an “@” symbol. To sanitize user input, remove any potentially dangerous characters, such as HTML tags, from the input before it is displayed on a web page.
     
    To sanitize user input, you can use PHP’s built-in functions, such as htmlspecialchars() and strip_tags().
  • Use Content Security Policy
    Another effective way to prevent XSS attacks is to use Content Security Policy (CSP) headers. CSP is a web security standard that allows you to specify the sources from which content can be loaded on a web page, such as images, scripts, and stylesheets.
     
    By using CSP, you can limit the sources from which scripts can be loaded, thereby preventing attackers from injecting malicious scripts into your web pages.
  • Use HttpOnly cookies
    XSS attacks can also be used to steal user session cookies, which can then be used to impersonate the user and perform unauthorized actions on their behalf. To prevent this type of attack, you should use HttpOnly cookies for session management.
     
    HttpOnly cookies can only be accessed by the web browser and cannot be read or modified by JavaScript. This makes it much more difficult for an attacker to steal a user’s session cookie using an XSS attack.
  • Use input filtering libraries
    Another way to prevent XSS attacks is to use input filtering libraries, such as HTMLPurifier, to sanitize user input automatically. Input filtering libraries can remove potentially dangerous HTML tags and attributes while preserving safe content.

 

 

3. Cross-Site Request Forgery Attacks

 

CSRF attack

CSRF attacks dupe browsers into executing unauthorized or bad actions. (Credit: Security Souls)

 

Cross-Site Request Forgery (CSRF) is another common type of web application security vulnerability that can affect PHP applications. In this attack, an attacker tricks a user into acting on a web application without their knowledge or consent, typically by exploiting a vulnerability.

 

For example, an attacker could craft a malicious URL that, when clicked by a user currently authenticated to a web application, performs an unauthorized action on their behalf, such as transferring funds or changing their account details.

 

Mitigating CSRF attacks

 

Fortunately, there are several steps you can take to mitigate the risk of CSRF attacks in your PHP web application. Here are some best practices to follow:

 

  • Use anti-CSRF tokens
    One of the most effective ways to prevent CSRF attacks is to use anti-CSRF tokens in your web application. An anti-CSRF token is a random value generated by the web application and included in each form submission or link request. The server then validates the token to ensure that it matches the expected value.
     
    By using anti-CSRF tokens, you can ensure that only authorized requests are accepted by your web application and that malicious requests are rejected.
  • Use SameSite cookies
    Another effective way to prevent CSRF attacks is to use SameSite cookies. SameSite cookies are a relatively new web security standard that allows you to specify whether a cookie can be sent in a cross-site request.
     
    By using SameSite cookies, you can prevent cookies from being sent in CSRF attacks, thereby limiting the impact of these attacks.
  • Use HTTP verbs correctly
    CSRF attacks typically rely on the fact that many web applications use the HTTP POST verb to perform actions that modify server state, such as creating new resources or updating existing ones. By default, HTML forms use the POST method when submitting data, which makes them vulnerable to CSRF attacks.
     
    To prevent CSRF attacks, you should ensure that your web application uses the correct HTTP verb for each action, and that form submissions use the appropriate verb. For example, creating a new resource should use the HTTP POST verb, while deleting a resource should use the HTTP DELETE verb.

 

 

4. Remote Code Execution Attacks

 

RCE attack

RCE attacks let hackers execute malicious code from anywhere in the world. (Credit: Baeldung)

 

Remote Code Execution (RCE) is a severe security vulnerability that can affect PHP applications. In this type of attack, an attacker can execute arbitrary code on a server by exploiting a vulnerability in the application.

 

For example, an attacker could exploit an RCE vulnerability in a PHP application to execute a shell command on the server, which could allow them to gain access to sensitive data or perform other malicious actions.

 

Mitigating Remote Code Execution attacks

 

To mitigate the risk of RCE attacks in your PHP web application, there are several best practices to follow:

 

  • Keep PHP updated
    One of the most effective ways to prevent RCE attacks is to keep your PHP installation up-to-date. PHP is constantly being updated to address security vulnerabilities and other issues, and it is important to ensure that you are running the latest version.
     
    In addition to updating PHP, you should also keep any third-party PHP libraries and extensions that you are using up-to-date, as these can also be vulnerable to RCE attacks.
  • Avoid using eval() and exec()
    One of the most common causes of RCE vulnerabilities in PHP applications is the use of the eval() and exec() functions. These functions allow you to execute arbitrary code on the server and can be easily exploited by attackers.
     
    To prevent RCE attacks, you should avoid using eval() and exec() in your PHP code whenever possible. Instead, use built-in PHP functions and libraries to accomplish the same task.
  • Use file permissions and user permissions
    Another effective way to prevent RCE attacks is to use file permissions and user permissions to limit the actions that can be performed by the web server process.
     
    By setting the appropriate file permissions and user permissions, you can ensure that the web server process only has access to the files and directories that it needs to function and that it cannot execute arbitrary code or access sensitive data.

 

 

5. File Inclusion Attacks

 

file inclusion attack

File Inclusion Attacks can happen if user input isn’t properly validated. (Credit: Cobalt)

 

PHP File Inclusion attacks, also known as Local File Inclusion (LFI) or Remote File Inclusion (RFI), occur when an attacker injects code into a PHP application that allows them to access and execute files on the server.

 

This type of attack is possible when the application does not properly sanitize user input or does not properly restrict file system access.

 

Mitigating file inclusion attacks

 

The consequences of a file inclusion attack can be severe. Attackers can use this vulnerability to execute arbitrary code, steal sensitive information, or even take complete control of the server. Therefore, it is essential to take measures to mitigate these types of attacks.

 

  • Use absolute paths
    Always use absolute paths when including files in PHP applications. This prevents attackers from using relative paths to access files outside the intended directory. Use the DIR magic constant to specify the absolute path of the file you want to include.
  • Restrict file system access
    Restrict file system access to the minimum necessary for the application to function. This means setting appropriate file permissions and limiting the directories that PHP can access. Avoid using the wildcard (*) character in file paths as this can allow attackers to access files outside the intended directory.
  • Disable remote file inclusion
    Disable remote file inclusion in the PHP configuration file (php.ini). This prevents attackers from including files from a remote server, which is a common technique used in RFI attacks.

 

 

Preventing PHP Attacks Are Vital to Web Application Operations

 

The rise in popularity of PHP has also led to an increase in security threats. It is crucial for developers to understand and be able to mitigate common PHP attacks in order to protect their applications and their users properly.

 

In addition to the methods listed above, you should also stay up-to-date with the latest security threats and implement security measures such as SSL, firewalls, and intrusion detection systems. Regular security audits can also help identify vulnerabilities and potential threats.

 

Most reputable and reliable web hosts offer a formidable array of security measures that can help you safeguard PHP applications.