Anthropic reveals security flaw in biological weapons filter that lasted nearly a year

Anthropic disclosed a security issue affecting its systems designed to block requests related to chemical and biological weapons. The safety filters responsible for preventing this type of content failed between May 2025 and April 2026, leaving approximately 133 million conversations unverified. The problem involved external contractors who provided feedback for training the Claude model. Around 50,000 of these contractors generated the unfiltered conversations during the affected period. The company attributes the incident to inadequate review processes for external vendors. According to Anthropic, an internal investigation found no evidence that these conversations were used for malicious purposes. In response, the company implemented stricter requirements for managing external contractors. This is one component of Anthropic's security measures, which include protection against risks related to chemical, biological, radiological, and nuclear weapons. The company also acknowledges that overly restrictive classifiers can harm legitimate research. To address this, it redirects requests in these sensitive areas to older model versions while working on improvements to its protections.