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OpenAI Resumes Frontier Training After Security Overhaul and Discloses Brotli Flaw

October 25, 2026 at 00:27 EDT

Security
Foundation Models

OpenAI says it resumed reinforcement-learning training and internal inference for its frontier models on October 1, after pausing the work to strengthen security controls. The company also disclosed a vulnerability in Brotli, a widely used compression library, discovered during tests of its defenses.

October 2026 · OpenAI

Training resumes after a security pause

OpenAI says it restarted frontier reinforcement learning on October 1 after strengthening controls. Red-team tests also uncovered a flaw in the Brotli compression library.

An 11-day pause before training resumed

September 20 · pause began11 daysOctober 1 · training resumed

Each segment represents one day of the reported pause.

What changed

OpenAI says it strengthened containment, monitoring and systems that stop risky runs. Live internet access remains outside the boundary for training and most evaluation.

What remains at issue

Evaluators reported findings in a pilot review; they said they had not verified the fixes OpenAI reported for production safeguards.

A separate finding: Brotli

Red-team testing uncovered a vulnerability in the compression library. OpenAI says crafted compressed data could enable unauthenticated remote code execution on affected systems; updated versions have been issued.

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