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Labs Note

DoS Tool: 403.php

January 17, 2020Luke Leal

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One of our analysts, Kaushal Bhavsar, found a malicious DoS file within a compromised website’s filesystem under the filename 403.php.

malicious DoS under the filename 403.php

The file attempts to conceal itself from casual viewers by loading a blank white page whenever a visitor loads the file directly in their browser.

To execute malicious code in the file, the visitor needs to pass two URL parameter values, time and host, through a submitted GET request. These two variables store modifiable values for the DoS attack, which the attacker can execute when they submit their request.

The time and host values let the PHP script know how long to run the attack (time) and against whom (host). Once the attacker knows the values they want to use, they can simply submit a GET request with the special parameter values.

Here is an example of a successful attack request shown in the browser, but this request could be sent from any HTTP client.

malicious DoS using GET request with special parameter values

A single website with this malicious file is not much of a threat to most servers and can be quickly blocked, but unprotected servers or clients could face issues if under attack from many different infected websites or host devices (e.g botnet).

The tool performs the attack by flooding UDP packets to the host through a port randomly chosen between 1-65000.

malicious DoS tool performs attack by flooding UDP packets to the host

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Categories: Sucuri Labs, Website SecurityTags: DDoS, Hacked Websites, Labs Note

About Luke Leal

Luke Leal is a member of the Malware Research team and joined the company in 2015. Luke's main responsibilities include threat research and malware analysis, which is used to improve our tools. His professional experience covers over eight years of deobfuscating malware code and using unique data from it to help in correlating patterns. When he’s not researching infosec issues or working on websites, you might find Luke traveling and learning about new things. Connect with him on Twitter.

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