Firefox flaw exposes unpatched Tor Browser users

A high-severity flaw in Firefox’s JavaScript engine can allow attackers to compromise unpatched Tor Browser installations when a user visits a specially crafted webpage, security researchers have demonstrated.

The vulnerability, tracked as CVE-2026-10702, affects the just-in-time compiler used to improve JavaScript performance. A programming error can cause the compiler to generate unsafe machine code, giving an attacker the ability to execute commands within the browser’s content process without requiring a download, login or additional click.

Mozilla fixed the flaw in Firefox 151.0.3 on June 2 and classified its impact as high. Tor Browser is built on Mozilla’s Firefox codebase, meaning browser releases incorporating the vulnerable engine inherited the same weakness until the relevant security changes were integrated.

Nebula Security, the research company credited with discovering the vulnerability, has now shown that the bug can be exploited against Tor Browser. Its demonstration indicates that loading a malicious page can be enough to trigger code execution inside the browser’s sandboxed renderer.

The findings carry particular significance for Tor users because the browser is widely used by journalists, activists, researchers, whistleblowers and people attempting to bypass censorship or surveillance. Although Tor routes internet traffic through multiple encrypted relays, its anonymity protections cannot by themselves prevent exploitation of a vulnerability in the browser software running on a user’s device.

Successful exploitation of CVE-2026-10702 does not automatically reveal a user’s identity or provide complete control of the operating system. The initial access remains confined to Firefox’s sandbox, a security boundary designed to restrict what compromised web content can reach.

Attackers can, however, combine a browser flaw with a second vulnerability that escapes the sandbox or obtains elevated system privileges. Such exploit chains can potentially access local data, install malicious code, monitor activity or expose information capable of undermining a Tor user’s anonymity.

Nebula researchers incorporated the Firefox flaw into an exploit chain called IonStack, pairing browser-level code execution with a separate operating-system weakness. The demonstrated chain targeted an ARM64 device running Android 17 and was designed to progress from a malicious webpage to root-level control.

The full demonstration was tailored to a particular supported device build, limiting its immediate use across the wider Android ecosystem. Researchers said the underlying Firefox vulnerability was not restricted to ARM processors, however, leaving open the possibility of adaptation to other architectures and operating systems.

The disclosure illustrates the security tension surrounding just-in-time compilation. JIT technology accelerates web applications by translating frequently used JavaScript into native machine instructions while a page is running. Mistakes in that complex optimisation process can create memory-safety failures or incorrect assumptions about data types, allowing malicious scripts to manipulate browser behaviour.

Tor Browser applies additional privacy and security modifications to Firefox, including anti-fingerprinting measures, site isolation controls and restrictions on browser features that could expose identifying information. Those protections reduce several forms of tracking but do not make the browser immune to flaws inherited from its upstream code.

The Tor Project released Tor Browser 15.0.19 on July 21 with important Firefox security updates. Users running earlier builds have been advised to install the newest available version rather than relying solely on Tor’s network-level protections. Automatic updates should be allowed to complete, and the installed version can be checked through the browser’s settings or information menu.

People using portable copies, manually installed packages or devices that remain offline for extended periods face a greater chance of missing security releases. Organisations distributing Tor Browser internally must also ensure that packaged versions are replaced promptly when upstream Firefox fixes become available.

No confirmed campaign exploiting CVE-2026-10702 against Tor users has been publicly identified. The availability of technical details and demonstration material nevertheless increases the prospect that other researchers or threat actors could study the weakness and attempt to adapt it.

The episode also highlights the risk created by the interval between a vendor patch and widespread installation. Mozilla addressed the defect weeks before the Tor-focused demonstration attracted wider attention, but systems that were not updated remained exposed throughout that period.

Users facing heightened surveillance risks may further reduce exposure by selecting Tor Browser’s safer or safest security level, which restricts some active web features. Disabling JavaScript can obstruct many browser-based attacks, although it can also break websites and should not be treated as a substitute for installing security updates.