Anonymity is valuable on the web. Geographical location and browsing destination details can be used to craft targeted marketing campaigns or steal user identities.

So the popularity of the Tor browser comes as no surprise, since its system of IP relays allows digital denizens to obfuscate their real IP address and physical location. However, even Tor isn’t perfect. As noted by Bleeping Computer, a new exploit known as TorMoil could allow cybercriminals to bypass browser protections and obtain actual IP information.

Talking TorMoil

The flaw was discovered and named by security researcher Filippo Cavallarin of Italian firm We Are Segment, who reported the issue to Mozilla on Oct. 26.

According to Ars Technica, the flaw only affects Mac and Linux versions of the Tor browser and occurs when users click on file:// links instead of HTTP or HTTPS variants. Using a custom-designed webpage as the destination, it’s possible for fraudsters to force direct Tor connections, bypassing all normal protections. As a result, attackers get access to real IP data and users aren’t aware they’ve been compromised.

Mozilla responded quickly to the discovery, creating a fix in the form of update 7.0.9 on Nov. 3 and then browser 7.5a7 the following day. Windows users remain unaffected, but all Linux and Mac users are encouraged to switch immediately.

It’s also worth noting that the current fix is a workaround. Using file URLs now requires users to drag links into the URL bar or a new browser tab.

Limited Release

According to the Tor Project, there’s no evidence that TorMoil has been active in the wild, but the public availability of patched code now makes it possible for threat actors to reverse engineer the problem and attack vulnerable browsers. Access to original IP details could lead to a host of problems, such as data theft and account takeover.

As noted by ZDNet, private exploit-selling company Zerodium has offered rewards of up to $1 million for Tor bugs, especially those that work with JavaScript blocked. While Zerodium’s intentions aren’t exactly noble — the company wants full rights to the bug discovery so it can resell the information — it’s clear that Tor troubles are of great interest to both Mozilla and motivated sellers alike.

An Illusion of Security

The bottom line is that nothing is perfect. Users can’t rely on any third-party technology to fully protect data against potential compromise. Tor is especially vulnerable, since it primarily acts as de facto security for users. If IP relay protection is rendered useless, formerly defended users may find their personal information up for grabs.

Individuals and enterprises alike can’t afford to single-track security. Tor is a great starting point, but avoiding leaky IPs means leveraging cloud-based antivirus solutions and following solid security hygiene practices. Ultimately, the cloak of secrecy offered by Tor does no good if fraudsters can peek behind the curtain.