Can $_SERVER['REMOTE_ADDR'] be Spoofed?

Spoofing the $_SERVER['REMOTE_ADDR'] Variable

Question:

Is it possible to alter or manipulate the contents of the $_SERVER['REMOTE_ADDR'] variable, which typically represents the IP address of the client making the request?

Answer:

Remotely faking the $_SERVER['REMOTE_ADDR'] variable is indeed possible, depending on the extent of the manipulation desired. Let's delve into the options:

Socket-Level Forgery:

If receiving a response is not crucial, it's possible to establish a raw socket connection to the destination server and falsify the source IP address. This is because PHP lacks direct access to socket implementations below the TCP level.

Gateway Compromise:

By compromising the gateway associated with the target IP address, it's possible to impersonate the corresponding computer. As the server cannot distinguish between a genuine client and a spoofed one, the gateway compromise provides complete control.

Important Considerations:

When utilizing a framework to access the $_SERVER['REMOTE_ADDR'] variable, it's essential to verify whether it examines the X-HTTP-FORWARDED-FOR header. If not, it becomes trivial for an attacker to spoof their IP address by simply sending the header with a specific value.

Edit: A Relevant Example

In a recent blog post, the author highlighted a vulnerability in StackOverflow's application that exploited a mechanism similar to IP address spoofing. This serves as a practical demonstration of the relevance and potential impact of such manipulation.

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