Yii2 XSS attack prevention strategy analysis

This article describes the XSS attack prevention strategy of Yii2 with examples. Share it with everyone for your reference, the details are as follows:

XSS vulnerability repair

Principle: Do not trust the data entered by the customer
Note: The attack code is not necessarily in <script></script>

① Change important cookies Marked as http only, in this case, the document.cookie statement in Javascript cannot obtain the cookie.
② Only allow users to enter the data we expect. For example: In the age textbox, users are only allowed to enter numbers. Characters other than numbers are filtered out.
③ Html Encode processing of data
④ Filter or remove special Html tags, such as: script, iframe, < for <, > for >, " for
⑤ Filter JavaScript event tags. For example, "onclick = "," Onfocus "and so on .

XSS prevention

<?php echo CHtml::encode($user->name) ?>

The source code of this method:

/**
* Encodes special characters into HTML entities.
* The [[\yii\base\Application::charset|application charset]] will be used for encoding.
* @param string $content the content to be encoded
* @param boolean $doubleEncode whether to encode HTML entities in `$content`. If false,
* HTML entities in `$content` will not be further encoded.
* @return string the encoded content
* @see decode()
* @see http://www.php.net/manual/en/function.htmlspecialchars.php
*/
public static function encode($content, $doubleEncode = true)
{
  return htmlspecialchars($content, ENT_QUOTES | ENT_SUBSTITUTE, Yii::$app->charset, $doubleEncode);
}

Http:// /php.net/manual/zh/function.htmlspecialchars.php
http://php.net/manual/zh/function.htmlentities.php

Available flags constants
Constant Name Description
ENT_COMPAT Will convert double-quotes and leave single-quotes alone.
ENT_QUOTES Will convert both double and single quotes.
ENT_NOQUOTES Will leave both double and single quotes unconverted.
ENT_IGNORE Silently discard invalid code unit sequences instead of returning an empty string. Using this flag is discouraged as it » may have security implications.
ENT_SUBSTITUTE Replace invalid code unit sequences with a Unicode Replacement Character U+FFFD (UTF-8) or &#FFFD; ( otherwise) instead of returning an empty string.
ENT_DISALLOWED Replace invalid code points for the given document type with a Unicode Replacement Character U+FFFD (UTF-8) or &#FFFD; (otherwise) instead of leaving them as is. This may be useful, for instance, to ensure the well-formedness of XML documents with embedded external content.
ENT_HTML401 Handle code as HTML 4.01.
ENT_XML1 Handle code as XML 1.
ENT_XHTML Handle code as XHTML.

string htmlspecialchars (
      string $string
      [, int $flags = ENT_COMPAT | ENT_HTML401
      [, string $encoding = ini_get("default_charset")
      [, bool $double_encode = true ]
    ]
  ]
)

& (ampersand) becomes &
" (double quote) becomes " when ENT_NOQUOTES is not set.
' (single quote) becomes ' (or ') only when ENT_QUOTES is set.
< (less than) becomes <

<?php
$new = htmlspecialchars("<a href=&#39;test&#39;>Test</a>", ENT_QUOTES);
echo $new; // <a href=&#39;test&#39;>Test</a>
?>

string htmlentities (
      string $string
      [, int $flags = ENT_COMPAT | ENT_HTML401
      [, string $encoding = ini_get("default_charset")
      [, bool $double_encode = true ]
    ]
  ]
)

<?php
$str = "A &#39;quote&#39; is <b>bold</b>";
// Outputs: A &#39;quote&#39; is <b>bold</b>
echo htmlentities($str);
// Outputs: A &#39;quote&#39; is <b>bold</b>
echo htmlentities($str, ENT_QUOTES);
?>

For example, search for "test Chinese characters" in Baidu. The URL will become

The so-called URL encoding is: Non-alphanumeric characters will be replaced with a percent sign (%) followed by two hexadecimal digits, and spaces are encoded as plus signs (+)

<?php
echo &#39;<a href="mycgi?foo=&#39;, urlencode($userinput), &#39;">&#39;;
?>

<?php
$query_string = &#39;foo=&#39; . urlencode($foo) . &#39;&bar=&#39; . urlencode($bar);
echo &#39;<a href="mycgi?&#39; . htmlentities($query_string) . &#39;">&#39;;
?>