XSS is a cross-site scripting attack that steals cookies or hijacks sessions by inserting malicious scripts; htmlspecialchars() can escape special characters into HTML entities to prevent browser execution. 1. XSS often occurs when user input is directly displayed, such as comments, search boxes, etc.; 2. htmlspecialchars() avoids execution by escaping characters such as , &; 3. Correct use includes: always escape the output content, specify the encoding as UTF-8, not escape data before storage, and select processing methods according to the context; 4. The easy to ignore points include: quotation marks are required to be closed in HTML attributes, rich text needs to be used to use whitelist filters, and JavaScript should use json_encode() to process variables.
XSS (cross-site scripting attack) is a common type of vulnerability in Web security, and htmlspecialchars()
in PHP is one of the most commonly used and effective functions to prevent such attacks. Simply put, it can convert special characters into HTML entities to avoid the browser's mistakenly thinking it is executable code.

What is XSS? Why do you need htmlspecialchars?
XSS attacks usually occur when the user input is displayed directly on the page. For example, in comment areas, search boxes, user names, etc. If the content is not processed, the attacker can insert the <script></script>
tag to execute malicious scripts, steal cookies, hijack sessions, etc.
At this time, the role of htmlspecialchars()
comes: it will escape characters like , <code>>
, &
into a form that the browser will not execute, for example:

echo htmlspecialchars(&#39;<script>alert("xss")</script>&#39;); // Output: <script>alert("xss")</script>
This way the browser will not execute it as code.
How to use htmlspecialchars correctly?
There are several key points to note when using this function:

Always escape the output content
Regardless of whether the content is entered by the user or not, as long as it is finally displayed in the HTML page, it is best to pass this function once.Pay attention to encoding settings
It is recommended to specify the third parameter clearly as'UTF-8'
, otherwise there may be garbled or missession in some older versions of PHP.htmlspecialchars($input, ENT_QUOTES, &#39;UTF-8&#39;);
Do not escape storage data in advance
Escape should be done at the time of output, not before writing to the database. If you escape in advance, it may be troublesome to use the original data later.Different contexts are processed in different ways
htmlspecialchars()
is mainly for the escape of HTML content. If you want to output variables in JavaScript or URL, you need to use other methods to deal with them, such asjson_encode()
orurlencode()
.
What are the places that are easy to ignore?
Although htmlspecialchars()
is practical, it is not omnipotent. Some places will still be bypassed if you are not careful:
Output in HTML attributes
If you put user input in HTML attributes (such asvalue="<?php echo $input; ?>"
), usinghtmlspecialchars()
is not enough. You also need to make sure that the attribute itself is closed and quotes are added.Rich text content
If you allow users to submit rich text content (such as formatted comments), you cannot usehtmlspecialchars()
directly, otherwise all HTML will be escaped. At this time, it is necessary to cooperate with HTML whitelist filters, such asHTML Purifier
.Injection in JavaScript
If you embed PHP variables in your JS code, for example:echo "var name = &#39;{$name}&#39;;";
Even if
htmlspecialchars()
is used, injection problems may occur due to string splicing. In this case, it is recommended to usejson_encode()
to wrap variables:echo "var name = " . json_encode($name) . ";";
Let's summarize
PHP's
htmlspecialchars()
function is one of the most basic and critical tools for defending against XSS. As long as you output the content input by the user to a web page, you should use it to escape. But there are several key points to remember:- Escaped at output, not at storage
- Set the correct encoding (usually UTF-8)
- Different security policies should be used in different output environments
- Rich text and JS environments require additional processing
Basically all this is not complicated but easy to ignore.
The above is the detailed content of PHP htmlspecialchars to prevent XSS. For more information, please follow other related articles on the PHP Chinese website!

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