# Anthropic Denies System Breach in Claude Code Extension Incident Malicious code targeting Cursor users was not linked to Anthropic's internal infrastructure. - **Source:** [www.theregister.com - Articles](https://brifing.si/source/2c34ea50-d341-4a10-925d-12c012a9ad10) - **Published:** 2026-10-30 - **Topic:** [Software & Engineering](https://brifing.si/c/engineering) - **Original article:** - **On Brifing:** https://brifing.si/s/e1550627-05b5-4826-b929-4e9f0445291b ## Marketplace Security Incident A malicious version of the Claude Code extension was uploaded to the Cursor marketplace, impacting approximately 5,000 developers. Anthropic clarified that its internal systems and employee devices remained secure throughout the event. The company explicitly stated that it did not build or publish the compromised extension version found on the platform. ## Technical Nature of Attack The compromised extension, identified as version 2.1.313, contained a persistent SSH backdoor compatible with Windows, macOS, and Linux. EDR vendor Phorion first detected the threat, which remained available for download for roughly 11 hours. The malicious script targeted host information, including usernames, configured Git emails, and public IP addresses of affected users. ## Execution and Failure Points The malicious script attempted to establish a reverse SSH tunnel to an attacker-controlled server. However, the mechanism required administrative privileges on macOS and Windows to function correctly. In most instances, the script failed to gain these necessary rights, though it still successfully exfiltrated host data and installed a persistence mechanism. ## Remediation and Cleanup Simply updating the extension to the latest version is insufficient to remove the threat. Developers must actively hunt for the persistence mechanism and use specific scripts provided by Cursor to wipe the infection. The incident highlights the critical need for manual verification when dealing with third-party marketplace extensions and software updates. ## Cursor's Response Protocol Upon detecting the malicious package, Cursor removed the extension and blocked further distribution. The company performed a full shutdown of the VSCode Marketplace as a precautionary measure. Furthermore, Cursor rotated its internal keys and collaborated with a cloud hosting provider to terminate the connection to the attacker's command and control server. ## Verification of Scope Cursor conducted extensive checks across its platform to ensure no other extensions were compromised by the same vector. While the investigation confirmed the scope of the Claude Code incident, the specific mechanism used by the attacker to gain access to the marketplace for the upload remains undisclosed by both involved parties. ## Anthropic's Official Stance Felix Rieseberg, a member of Anthropic’s technical staff, confirmed the company is working closely with Cursor to address the incident. Anthropic maintains that the breach was isolated to the Cursor marketplace environment. Official channels for Claude Code remain unaffected, and users are advised to rely solely on verified, official distribution sources. ## Broader Security Implications This event underscores the risks inherent in software supply chains where marketplace trust is exploited. Even when primary vendors like Anthropic are not breached, the downstream impact on developers can be severe. Organizations must prioritize endpoint security and rigorous monitoring of third-party tools to mitigate risks from similar future marketplace compromises. ## Sources covering this story - https://www.theregister.com/cyber-crime/2026/10/30/anthropic-that-claude-code-extension-compromise-couldnt-be-us/5306682 — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._