PHP form prevents repeated submission (anti-csrf vulnerability)

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Token A brief discussion

Token is a token. Its biggest feature is randomness and unpredictability. Normal hackers or software cannot guess it.

So, what is the function of Token? What is the principle?

Token is generally used in two places - to prevent repeated form submissions and anti-csrf attacks (cross-site request forgery).

Both are implemented in principle through session tokens. When the client requests a page, the server will generate a random number Token, place the Token in the session, and then send the Token to the client (usually by constructing a hidden form). The next time the client submits a request, the Token will be submitted to the server along with the form.

Then, if applied to "anti csrf attack", the server will verify the Token value to determine whether it is equal to the Token value in the session. If equal, the request can be proven Valid, not fake.

However, if it is used to "prevent repeated submission of forms", after the first verification of the same, the server will update the Token value in the session. If the user submits repeatedly, the The secondary verification judgment will fail because the Token in the form submitted by the user has not changed, but the Token in the server-side session has changed.

The above session application is relatively safe, but it is also cumbersome. At the same time, when there are multiple pages and multiple requests, the method of generating multiple Tokens at the same time must be used, which will occupy more resources and reduce the execution efficiency. reduce. Therefore, the method of storing verification information in cookies can also be used instead of session Token. For example, when dealing with "repeated submissions", the submitted information will be written to the cookie after the first submission. When submitted for the second time, since the cookie already has a submission record, the second submission will fail.

However, cookie storage has a fatal weakness. If the cookie is hijacked (XSS attacks can easily obtain user cookies), then there will be another gameover. Hackers will directly implement CSRF attacks.

1. Prevent xss attacks first

2. Verify referrer

3. Important cookie settings are https only, such as token

4. Use Signature, token

5. Get is only used to query information

6. Use post

to submit the form 7. Use cross-text with caution Script injection

So, safety and efficiency are relative. Let’s deal with specific issues in detail.

Add Token to the php form to prevent repeated submissions

The principle is to generate a random string and put it in the session Here, verifying this string after submitting the form can prevent others from writing the form themselves to cheat the submission, repeat submission or double-click submission.

The simple code implemented in php is as follows:

##The above method is simpler, and the following code is safer.

Token.php

',
    'e',
    '2',
    'f',
    'P',
    'g',
    ')',
    '?',
    'H',
    'i',
    'X',
    'U',
    'J',
    'k',
    'r',
    'l',
    '3',
    't',
    'M',
    'n',
    '=',
    'o',
    '+',
    'p',
    'F',
    'q',
    '!',
    'K',
    'R',
    's',
    'c',
    'm',
    'T',
    'v',
    'j',
    'u',
    'V',
    'w',
    ',',
    'x',
    'I',
    '$',
    'Y',
    'z',
    '*'
  );
  # Array indice friendly number of chars;
  $numChars = count($chars) - 1;
  $token = '';
  # Create random token at the specified length
  for ($i = 0; $i < $len; $i++)
    $token .= $chars[mt_rand(0, $numChars)];
  # Should token be run through md5?
  if ($md5) {
    # Number of 32 char chunks
    $chunks = ceil(strlen($token) / 32);
    $md5token = '';
    # Run each chunk through md5
    for ($i = 1; $i <= $chunks; $i++)
      $md5token .= md5(substr($token, $i * 32 - 32, 32));
    # Trim the token
    $token = substr($md5token, 0, $len);
  }
  return $token;
}
?>

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