Introduction
On November 18, 2025, the internet experienced a significant disruption as Cloudflare, one of the web’s most pervasive infrastructure providers, suffered a widespread outage. Websites and applications relying on Cloudflare’s global network returned error pages, causing frustration for millions of users and critical service interruptions for businesses and organizations worldwide. This blog provides a detailed breakdown of the outage, what caused it, how the Cloudflare team resolved it, and what IT professionals can learn from this incident to enhance resilience and operational reliability.
What Happened: Timeline of the Outage
At 11:20 UTC on November 18th, Cloudflare’s highly interconnected network began showing abnormal failures to deliver core traffic. End users encountered a surge in HTTP 5xx error responses when accessing sites protected or accelerated by Cloudflare. Initially, the Cloudflare engineering and incident response teams suspected the symptoms were due to an external, high powered DDoS attack. However, as investigation progressed, it became clear that the disruption was internal and tied to recent configuration changes in their own software environment.
Diagnosis: Root Cause Analysis
Cloudflare’s investigation pinpointed a permissions change in a ClickHouse database that produced malformed configuration data. A feature file used by the Bot Management system doubled in size due to duplicate data and exceeded a hard limit in the bot module. When the software encountered the oversized file it triggered Rust panic errors, leading to cascading failures in traffic routing processes and HTTP 5xx errors at the edge. Because the feature file was regenerated every five minutes with mixed good and bad data, the platform showed alternating periods of recovery and failure, complicating diagnosis.
Impacted Services and Functional Scope
The outage affected far more than standard websites. Key Cloudflare services impacted included:
- Core CDN and security proxying, which produced large volumes of HTTP 5xx error codes.
- Turnstile, preventing users from completing challenges and logging in to protected sites.
- Workers KV, whose gateway returned elevated error rates.
- The Cloudflare dashboard, where most admins could not sign in.
- Access and Zero Trust authentication systems, blocking reachability of internal apps.
- Email security functions, which briefly lost full IP reputation access.
For many users this translated to timeouts, slow responses, or complete inability to reach services fronted by Cloudflare.
Resolution and Technical Steps Taken
Once the root cause was confirmed, Cloudflare engineers moved through a focused mitigation and recovery plan:
- Stopping generation of malformed configuration files and inserting a known good version into the distribution pipeline.
- Restarting core proxy services across the global network to clear bad state and reload valid configuration.
- Progressively restoring affected services until error rates normalized.
- Routing Workers KV and Access traffic through fallback paths to reduce downstream impact.
- Scaling dashboard backends to handle the backlog of administrator login attempts.
Detailed Technical Breakdown
The incident involved multiple layers of automation and data access:
- Metadata duplication in ClickHouse after an update that expanded visibility for automated queries.
- An unfiltered query in the feature generator that ingested duplicate columns from system tables.
- Static feature limits in the bot management runtime that were exceeded by the enlarged data set.
- Cross‑service impact because bot management logic is embedded throughout Cloudflare’s proxy and access stack.
- A coincidental failure of Cloudflare’s external status page, adding confusion during the early investigation.
Key Lessons and Preventive Measures
The outage shared several important lessons for IT and DevOps teams:
- Configuration validation is essential, even for internal, automated files.
- Defensive limits and feature flags must fail gracefully and support quick kill switches.
- Fault tolerance must consider software and automation errors as much as hardware failures.
- Observability tooling needs to be tuned so it does not overload platforms during an incident.
- Incident communication benefits from clear playbooks and disciplined feedback loops.
- Rollback and recovery testing for configuration and software should be routine.
What’s New: Cloudflare’s Post Outage Improvements
In its post incident report, Cloudflare outlined several platform improvements, including hardened configuration ingestion, expanded kill switch capabilities, refined crash handling logic, broader architectural reviews for hidden single points of failure, and continued transparency through detailed public postmortems.
Analysis: Broader Implications for the Internet Ecosystem
The November 2025 outage highlights how central a handful of infrastructure providers have become to the internet. Redundancy and multi cloud strategies help, but they are not enough if a single provider’s control plane can still disrupt vast portions of the web. Modern services should regularly test the ability to fail over, degrade gracefully, and, when necessary, switch providers quickly.
For security and reliability teams, the incident is a reminder that internal configuration and automation changes can carry external blast radius comparable to large scale attacks. Investing in chaos engineering, separation of data and control planes, and human review for high impact automation are increasingly important practices.
Final Thoughts
Cloudflare’s response to the November 2025 outage offers a case study in both the power and fragility of large scale internet infrastructure. Automation cannot replace oversight, resilience must be built into every critical path, and no platform is immune to the consequences of a single misstep.
— Ravi Jay, QuantumThread Blog (Nov 2025)
