PHP does not filter HTML tags - brings more risks and challenges

With the continuous development of Internet applications, Web development has become a widely used skill. In web development, the server-side scripting language PHP is favored by developers because of its open source, easy to learn and use, and powerful functions. However, in actual applications, some PHP developers lack understanding of security, resulting in various security risks on the website. Among them, not filtering HTML tags is a very common security risk.

HTML tags are the basis for web page presentation and an essential part of web development and design. However, HTML tags are also a common means used by attackers to conduct XSS (cross-site scripting attacks). XSS attacks generally refer to attackers injecting specific HTML or JavaScript code, causing users to execute malicious scripts constructed by the attacker when accessing the attacked page, thereby achieving the purpose of the attack.

As a server-side scripting language, PHP can perform various processing on data input by users, including filtering HTML tags. For some developers, perhaps for the sake of development efficiency, they will not process the data input by the user too much, or even filter the HTML tags, and output the content input by the user to the page intact. superior. Although this approach improves development efficiency, it also brings more risks and challenges to the website.

The risks and challenges caused by not filtering HTML tags are mainly reflected in the following aspects:

1. XSS attack: By injecting HTML or JavaScript code, the attacker can steal the user's Information, Cookies, etc.
2. SQL injection attack: Injecting HTML tags containing special SQL statements allows attackers to directly obtain sensitive information in the database.
3. Script injection attack: Injecting HTML tags containing special scripts allows attackers to execute malicious scripts through the browser to attack the server.
4. CSRF attack: Attackers can inject special links in HTML tags and launch CSRF attacks when users visit the attacked page.

In order to prevent security risks of HTML tags, developers need to filter user-entered data as much as possible. This filtering includes not only filtering HTML tags, but also filtering other suspicious scripts, special characters, etc. Common HTML tag filtering methods include whitelist method and blacklist method.

The whitelist method is a method of filtering by retaining some legal HTML tags and rejecting all tags that are not in the whitelist. This method is suitable for websites that have strict requirements on user input content, such as financial, government, etc. The whitelist method can effectively prevent XSS attacks and reduce the rate of false positives.

The blacklist method is a way of filtering by defining some unsafe HTML tags and rejecting all content containing such tags. This method is suitable for websites that do not have strict requirements on user input content, such as news, entertainment, etc. The blacklist method can prevent some simple XSS attacks, but its defense capabilities are weak against complex XSS attacks.

In addition to filtering HTML tags, there are other methods to prevent security risks, such as using the HTTPOnly attribute to prohibit JavaScript from operating cookies; using CSP (Content-Security-Policy) to limit the resources loaded by the website .

In short, not filtering HTML tags will bring immeasurable security risks and challenges to the website. As PHP developers, we need to pay attention to security issues in software development, filter and process user-entered data as much as possible, and improve the security of the website.

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