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Is it possible to fake or hijack a content of $_SERVER['REMOTE_ADDR'] variable?

I would like to fake a request with:


How could I do that with PHP? Can CURL do that somehow?



I assume that you mean faking it remotely. The short answer is yes you can. The long answer about how easy it is depends on how you want to fake it.

If you don't care about receiving a response, it's as trivial as opening a raw socket to the destination and forging the source IP address. I'm not sure if it's really easy to do in PHP since all of PHP's socket implementations are at or above the TCP level. But I'm sure it's possible. Now, since you're not in control of the network, the response will not go back to you. So that means that you cannot (reliably anyway) create a TCP connection via a trivial forged TCP header (since the syn-ack does prevent this by requiring two-way communication).

However, if you can compromise the gateway the IP is off of, you can do whatever you'd like. So if you compromise the wifi router a computer is connected to, you can pretend to be that computer, and the server won't tell the difference. If you compromise the ISP's outbound router, you can (in theory at least) pretend to be the computer and the server won't tell the difference.

For some more info, see these following links:

  • ServerFault Question
  • Symantec Article
  • Linux Security Article

However, you will only be able to forge the loopback address under TCP if you actually compromise the local machine/server. And at that point does it really matter?


If you're using a framework to access this information, be absolutely sure that it does not check the X-HTTP-FORWARDED-FOR header! Otherwise it's trivial to fake the IP address. For example, if you're using Zend Framework's Zend_Controller_Request_Http::getClientIp method, be absolutely sure that you pass false as the parameter! Otherwise someone just needs to send an HTTP header: X-Http-Forwarded-For: and they now appear to be local! This is one case where using a framework without understanding how it works in the backend can really be bad...

Edit: Relevant

I wrote a blog post recently about how I stumbled across a vulnerability in 's application. It's very relevant here, since it exploits a very similar mechanism to what this question is looking for (although the circumstances around it are somewhat narrow):

How I Hacked

Saturday, August 27, 2022

I applaud your efforts. You must, friendly community member, consider decoupling your operations.

1) Have one function/routine/class/method for filtering input (filter_input_array(), strip_tags(), str_ireplace(), trim(), etc ...). You may want to create functions that use loops to do filtering. Tricks such as double encoding, one-time-strip-spoofing, and more can defeat single usage of things like strip_tags().

Here is a strip_tags() wrapper method from my Sanitizer class. Notice how it compares the old value to the new value to see if they are equal. If they are not equal, it keeps on using strip_tags(). Although, there is quite of bit of preliminary INPUT_POST / $_POST checking done before this method is executed. Another version of this using trim() is actually executed before this one.

private function removeHtml(&$value)
    if (is_scalar($value)) {
        do {
            $old = $value;
            $value = strip_tags($value);

            if ($value === $old) {
        } while(1);
    } else if (is_array($value) && !empty($value)) {
        foreach ($value as $field => &$string) {
            do {
                $old = $string;
                $string = strip_tags($string);

                if ($string === $old) {
            } while (1);
    } else {
       throw new Exception('The data being HTML sanitized is neither scalar nor in an array.');


2) Have another one for validating input (filter_var_array(), preg_match(), mb_strlen, etc...)

Then, when your data needs to switch contexts ...

A) For databases, use prepared statements (PDO, preferably).

B) For returning / transmitting user input to the browser, escape the output with htmlentities() or htmlspecialchars accordingly.

In terms of magic quotes, the best thing to do is just disable that in the php.ini.

Now, with those various constructs having their own areas of responsibility, all you have to do is manage the flow of logic and data inside of your handler file. This includes providing error messages to the user (when necessary) and handling errors/exceptions.

There is no need to use htmlentities() or htmlspecialchars immediately if the data is going from the HTML form directly into the database. The point of escaping data is to prevent it from being interpreted as executable instructions inside a new context. There is no danger htmlentities() or htmlspecialchars can resolve when passing data to a SQL query engine (that is why you filter and validate the input, and use (PDO) prepared statements).

However, after the data is retrieved from database tables and is directly destined for the browser, ok, now use htmlentities() or htmlspecialchars. Create a function that uses a for or foreach loop to handle that scenario.

Here is a snippet from my Escaper class

public function superHtmlSpecialChars($html)
     return htmlspecialchars($html, ENT_QUOTES | ENT_HTML5, 'UTF-8', false);

public function superHtmlEntities(&$html)
    $html = htmlentities($html, ENT_QUOTES | ENT_HTML5, 'UTF-8', false);

public function htmlSpecialCharsArray(array &$html)
    foreach ($html as &$value) {
        $value = $this->superHtmlSpecialChars($value);


public function htmlEntitiesArray(array &$html)
    foreach ($html as &$value) {


You'll have to tailor your code to your own personal tastes and situation.

Note, if you plan on processing the data before sending it to the browser, do the processing first, then escape with your handy-dandy htmlentities() or htmlspecialchars looping function.

You can do it!

Wednesday, October 12, 2022

I see somebody else has already pointed to my old "assign and set" Cookbook recipe, which boils down in its simplest version to:

class Holder(object):
   def set(self, value):
     self.value = value
     return value
   def get(self):
     return self.value

h = Holder()


if h.set(isBig(y)): return h.get()

However, this was intended mostly to ease transliteration between Python and languages where assignment is directly supported in if or while. If you have "hundreds" of such check-and-return in a cascade, it's much better to do something completely different:

hundreds = isBig, isSmall, isJuicy, isBlah, ...

for predicate in hundreds:
  result = predicate(y)
  if result: return result

or even something like

return next(x for x in (f(y) for f in hundreds) if x)

if it's OK to get a StopIteration exception if no predicate is satisfied, or

return next((x for x in (f(y) for f in hundreds) if x)), None)

if None is the proper return value when no predicate is satisfied, etc.

Almost invariably, using (or even wishing for;-) the Holder trick/non-idiom is a "design smell" which suggests looking for a different and more Pythonic approach -- the one case where Holder is justified is exactly the special case for which I designed it, i.e., the case where you want to keep close correspondence between the Python code and some non-Python (you're transliterating a reference algorithm in Python and want it working first before refactoring it into a more Pythonic form, or you're writing Python as a prototype that will be transliterated into C++, C#, Java, etc, once it's working effectively).

Sunday, October 30, 2022

If you are talking about someone else's server, then the short answer is no. If third parties could read your PHP source code, that would be quite a security hole, since PHP files tend to contain database passwords, hash keys, proprietary algorithms and other goodies that you don't want falling in the wrong hands.

If you are talking about your own server (ie. that you yourself have access to), then there are simple scripts that you can put on the server, that allow you to specify a path to any file on the server and have it returned as plaintext. However, you NEVER EVER want to place such a script on a production server, for the reasons mentioned above.

Monday, November 21, 2022

As Karl ask, there could be different meanings of your question.
I try to give an answer for each possibility

@echo off
setlocal EnableDelayedExpansion
set EqS=Nope
set X=Eq

REM set Y1 to "EqS" 
set Y1=%X%S 

REM set Y2 to "Nope" (content of EqS)
set Y2=!%X%S!

REM set Y3 to "!EqS!"
set Y3=^^!%X%S^^!

echo %Y1%
echo %Y2%
echo %Y3%
set EqS=Something
echo Text %Y1%
echo Content %Y2%
echo Pointer %Y3%
Saturday, December 3, 2022
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