Vulnerability in Claude Extension for Chrome Exposes AI Agent to Takeover

A vulnerability in the Claude extension for Chrome could allow attackers to take over the AI agent and abuse it for information theft, cybersecurity firm LayerX reports.

The flaw, dubbed ClaudeBleed, is a combination of lax permissions, where any Chrome extension can run commands in Claude in Chrome, and poorly implemented trust in the origin of the command, not the execution context.

According to LayerX, the main issue is that the Claude extension allows interaction with any script running in the origin browser, without verifying its owner.

“As a result, any extension can invoke a content script (which does not require any special permissions) and issue commands to the Claude extension,” the company explains.

Claude in Chrome, it says, trusts the origin of the execution, which is claude.ai, and not the execution context, thus allowing any JavaScript running in the origin to issue privileged commands.

This allows an attacker to create an extension with a declared content script and configured to run in the Main world, thus ensuring the script is executed as part of the page, and send a message to the Claude extension, which trusts the sender because it runs in claude.ai.

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Because a message handler in Claude in Chrome accepts and forwards arbitrary prompts, the attacker can perform remote prompt injection and control the AI agent’s actions.

While Claude enforces user confirmation for sensitive actions, as well as policies that prevent certain actions, and makes decisions based on certain inputs, LayerX discovered that the attacker’s script could bypass these protections.

The company was able to forge user approval by repeatedly sending a confirmation message and relied on Document Object Model (DOM) manipulation to dynamically modify UI elements and alter Claude’s perception of the actions.

It was also able to gain visibility into command execution through repeated triggering of the action and by observing the effects.

“This vulnerability effectively breaks Chrome’s extension security model by allowing a zero-permission extension to inherit the capabilities of a trusted AI assistant,” LayerX says.

This attack chain, the company says, allows an attacker to weaponize Claude to exfiltrate data from Gmail, GitHub, or Google Drive, as well as to send emails, delete data, and share documents on behalf of the user.

When notified of the issue, Anthropic told LayerX it was working on a patch, but the fix only partially addressed the underlying vulnerability, through “internal security checks to prevent extensions running in ‘standard’ mode from executing remote commands”.

Because the root cause of the weakness was not addressed, an attacker can simply switch the extension to ‘privileged’ mode and bypass the fix. The user is never notified or asked to approve the switch, LayerX says.

Originally published by SecurityWeek

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