'No clicks, no permission prompts. Just visit a page, and an attacker completely controls your browser': Experts warn Claude Chrome extension could let hackers hijack your online browsing

Date:
Fri, 27 Mar 2026 17:55:00 +0000

Description:
Prompt injection attacks can now be carried out in browser extensions,
experts warn.

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now subscribed Your newsletter sign-up was successful Join the club Get full access to premium articles, exclusive features and a growing list of member rewards. Explore An account already exists for this email address, please log in. Subscribe to our newsletter Koi Security discovers ShadowPrompt
zero-click flaw in Claude Code Chrome extension Vulnerability let attackers exploit XSS on claude.ai subdomain to exfiltrate secrets without user interaction Anthropic patched issue in version 1.0.41; researchers warn AI browser assistants are high-value attack targets A Google Chrome extension
for Claude Code, one of the most popular AI tools around, was vulnerable to a zero-click attack which could have allowed malicious actors to exfiltrate sensitive data from the app with the user doing almost nothing risky.

Security researchers Koi Security found the bug, which they dubbed ShadowPrompt, which appears to have come from the browser extension trusting certain websites too much. It was designed to deem anything coming from claude.ai - including subdomains - as safe. However, one of the subdomains, a-cdn.claude[.]ai, had a cross-site scripting (XSS) bug that allowed
attackers to run their own code on it. Article continues below You may like Claude desktop extension can be hijacked to send out malware by a simple Google Calendar event Three high-risk AI vulnerabilities discovered in Claude.ai end-to-end attack chain exfiltrates sensitive info without user knowing 'The attack requires no exploit, no user clicks, and no explicit request for
sensitive actions': Experts say Perplexity's AI Comet browser can be hijacked to steal your passwords How prompt injection gets used So, in theory, a
threat actor could load a malicious prompt into this website, and through social engineering, trick the victim into visiting it. Since the site is hosted on claude.ai, the extension would see it as safe. If it is set up to scan all the sites the user visits, it could end up executing the malicious prompt without the user ever knowing.

In practice, the victim could visit a simple blog that is, in fact, running hidden code in the background. The code sends a prompt to the Claude Chrome extension such as summarize the users recent conversations and extract any
API keys or passwords. The extension thinks this was a user request and processes it, sending valuable secrets to the attackers.

"No clicks, no permission prompts. Just visit a page, and an attacker completely controls your browser, Koi Security researcher Oren Yomtov said.

Anthropic has since patched the bug. Therefore, if youre running the Claude extension for Chrome, make sure youre using at least version 1.0.41 that enforces strict origin checks that require an exact match to the domain. Are you a pro? Subscribe to our newsletter Sign up to the TechRadar Pro
newsletter to get all the top news, opinion, features and guidance your business needs to succeed! Contact me with news and offers from other Future brands Receive email from us on behalf of our trusted partners or sponsors By submitting your information you agree to the Terms & Conditions and Privacy Policy and are aged 16 or over.

Arkose Labs, whose CAPTCHA component had the DOM-based XSS vulnerability, has since also fixed the XSS flaw bug on its end.

"The more capable AI browser assistants become, the more valuable they are as attack targets," Koi said. "An extension that can navigate your browser, read your credentials, and send emails on your behalf is an autonomous agent. And the security of that agent is only as strong as the weakest origin in its trust boundary."

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