Cybersecurity researchers have discovered a new "0.0.0.0 Day" impacting all major web browsers that malicious websites could take advantage of to breach local networks.

The critical vulnerability "exposes a fundamental flaw in how browsers handle network requests, potentially granting malicious actors access to sensitive services running on local devices," Oligo Security researcher Avi Lumelsky said.

The Israeli application security company said the implications of the vulnerability are far-reaching, and that it stems from the inconsistent implementation of security mechanisms and a lack of standardization across different browsers.

As a result, a seemingly harmless IP address such as 0.0.0.0 could be weaponized to exploit local services, resulting in unauthorized access and remote code execution by attackers outside the network. The loophole is said to have been around since 2006.

0.0.0.0 Day impacts Google Chrome/Chromium, Mozilla Firefox, and Apple Safari that enables external websites to communicate with software that runs locally on MacOS and Linux. It does not affect Windows devices as Microsoft blocks the IP address at the operating system level.

Particularly, Oligo Security found that public websites using domains ending in ".com" are able to communicate with services running on the local network and execute arbitrary code on the visitor's host by using the address 0.0.0.0 as opposed to localhost/127.0.0.1.

It's also a bypass of Private Network Access (PNA), which is designed to prohibit public websites from directly accessing endpoints located within private networks.

Any application that runs on localhost and can be reached via 0.0.0.0 is likely susceptible to remote code execution, including local Selenium Grid instances by dispatching a POST request to 0.0.0[.]0:4444 with a crafted payload.

Put differently, the issue is as simple as a malicious web page sending requests to 0.0.0.0 and a port of its choosing that could then be processed by services running locally on that same port, leading to unintended consequences.

In response to the findings in April 2024, web browsers are expected to block access to 0.0.0.0 completely, thereby deprecating direct access to private network endpoints from public websites.

"When services use localhost, they assume a constrained environment," Lumelsky said. "This assumption, which can (as in the case of this vulnerability) be faulty, results in insecure server implementations."

"By using 0.0.0.0 together with mode 'no-cors,' attackers can use public domains to attack services running on localhost and even gain arbitrary code execution (RCE), all using a single HTTP request."