The gist: Invisible HTML comments in GitHub Issues could trick Claude Code AI into reading protected environment variables like ANTHROPIC_API_KEY due to insufficient restrictions on the Read tool.
Microsoft security researchers identified a vulnerability in Anthropic’s Claude Code GitHub Action that allowed attackers to extract confidential CI/CD credentials through manipulated prompts. Anthropic released a security update on May 5 (version 2.1.128).
The Microsoft Threat Intelligence team observed a surge in prompt-injection attacks in public code repositories and identified a critical security flaw in Anthropic’s Claude Code GitHub Action. Attackers embedded malicious commands in HTML comments within GitHub Issues that were invisible to humans in the browser but readable in raw format by the AI model. In automated workflows, this was sufficient to control the AI without system privileges.
The core problem lay in asymmetric security measures: while Anthropic had blocked Bash execution in isolated environments (sandboxes), the integrated Read tool for accessing files was not subject to comparable restrictions. In a controlled test environment, the researchers manipulated a prompt as an alleged compliance check to bypass the internal security filters. The assistant then accessed the system file /proc/self/environ and disclosed the unencrypted ANTHROPIC_API_KEY as well as additional GitHub runtime credentials.
Microsoft notified Anthropic of the vulnerability on April 29. Anthropic implemented restrictive access controls for the file access tool in version 2.1.128 (released May 5) that unconditionally block access to /proc/ directories. This prevents environment variables and secrets from being disclosed via the AI assistant.
Source: www.it-daily.net · Published June 9, 2026
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