The next generation of “agentic” web browsers, which promise built-in AI-powered assistants and context-aware sidebars, may be quietly ushering in one of the most dangerous new attack surfaces on the internet. Security researchers have warned that while these tools aim to make browsing more efficient, they also create opportunities for sophisticated deception and credential theft that traditional defenses struggle to detect.
According to researchers from browser security firm SquareX, attackers can exploit the trust users place in AI sidebars through a simple but potent method. A benign-looking browser extension can overlay a counterfeit sidebar on top of the real one, intercepting every keystroke and action that passes through it. The fake interface can then feed malicious instructions back to the user, disguised as AI recommendations or system messages that appear completely legitimate.
This technique undermines one of the most fundamental assumptions about browsing: that the user can trust what they see inside their browser window. The counterfeit sidebar looks and behaves exactly like a genuine one. Because it does not modify the browser’s internal code, traditional antivirus software and browser permission systems are unlikely to detect the deception.
SquareX’s research shows how easily this can spiral into targeted attacks. The fake sidebars can redirect users to phishing pages that mimic popular services, capture OAuth tokens through forged file-sharing prompts, and even suggest commands that install remote access tools or scripts on a victim’s system. The danger multiplies when these fake assistants start handling account credentials or sensitive data inside automated workflows, where users may follow AI prompts without questioning their authenticity.
One of the biggest issues is how extensions are currently designed. Many require broad permissions, such as host access and local storage, which are often granted without scrutiny because they are common among productivity tools. This blurs the line between legitimate and malicious use, making permission analysis less effective as a defense. The result is a deceptive attack that operates entirely within the browser’s normal framework, bypassing security models built to detect direct code tampering or external injections.
Experts are particularly concerned because most major browser vendors — including Google, Microsoft, and Opera — are now experimenting with persistent AI sidebars or integrated assistants. As each vendor builds its own implementation, the overall attack surface grows across different platforms. That diversity makes it harder for security teams to create unified defenses, especially when each sidebar has unique interfaces and permission structures.
The real risk lies in how quickly users have begun to treat AI sidebars as trustworthy digital helpers. The illusion of legitimacy means many will follow their instructions without skepticism, particularly if the AI responds convincingly. When a malicious overlay can impersonate that same behavior, the potential for exploitation becomes significant. It is social engineering on a new level — the manipulation doesn’t come from an email or website, but from what appears to be your own browser assistant.
Researchers suggest users should consider all in-browser AI tools as experimental for now. They advise avoiding sensitive tasks such as entering passwords, handling financial transactions, or linking major accounts through these sidebars until vendors can verify their integrity. Sensitive workflows involving OAuth tokens, identity credentials, or document signing should remain outside these environments entirely.
For enterprises, the threat extends far beyond individual users. A compromised sidebar can provide a stealthy channel for harvesting credentials, gaining lateral access, or executing unauthorized commands inside a managed environment. Security teams are being urged to tighten extension governance policies, restrict permissions, and implement stricter endpoint controls. Monitoring for unusual OAuth behavior and enforcing multi-factor authentication on critical accounts are now considered essential defensive steps.
