What Happened During the Cloudflare Outage on November 18, 2025

The outage on November 18, 2025 hit more than 1.2 billion users worldwide. Major e‑commerce sites, streaming services, and financial platforms reported sudden downtime. Developers and sysadmins wondered: why did a leader in CDN fail, and what can we learn?

Incident Overview

The outage began at 02:15 UTC and lasted roughly 3 hours. Cloudflare reported that a mis‑configured load balancer caused a cascading failure across three data centers. Traffic was rerouted to secondary nodes, but those were overloaded and could not serve requests.

Key Impact Points

• E‑commerce: 7 % of global sales dropped during the outage.
• Streaming: 15 % of users experienced buffering for 90 minutes.
• Finance: Several trading platforms lost connectivity for 45 minutes.

Root Cause and Response

Cloudflare’s engineering team traced the issue to a software bug in the traffic routing module. The bug triggered an infinite loop that consumed CPU resources and blocked new connections.

Response Actions

1. Rollback the latest deployment that introduced the buggy code.
2. Throttle traffic to remaining healthy nodes to prevent overload.
3. Deploy a hotfix within 30 minutes of detecting the loop.
4. Notify customers through status pages and social media.

What the Team Learned

• Automated tests missed the edge case due to insufficient load scenarios.
• Monitoring alerts were delayed because of a mis‑configured threshold.
• Communication with stakeholders was slower than ideal, causing confusion.

Takeaways for Developers

Developers can mitigate similar disruptions by adopting the following practices.

1. Build Robust Testing

• Simulate traffic spikes in staging environments.
• Include edge‑case scenarios for routing logic.
• Automate regression tests for critical modules.

2. Strengthen Monitoring

• Set real‑time alerts for CPU and memory thresholds.
• Use distributed tracing to pinpoint bottlenecks.
• Create a run‑book for common failure scenarios.

3. Improve Incident Communication

• Draft a pre‑defined communication plan for outages.
• Use a single source of truth (status page) for updates.
• Schedule post‑mortem meetings within 24 hours.

4. Design for Fail‑over

• Deploy services across multiple availability zones.
• Use canary releases to roll out changes gradually.
• Implement rate limiting to protect downstream systems.

Quick Check‑list (Code Block)

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결론

The Cloudflare outage on November 18, 2025 showed how a single software bug can cascade into global downtime. By reinforcing testing, monitoring, communication, and fail‑over design, developers can reduce the impact of future incidents. Start today by reviewing your routing logic and setting up a health‑check pipeline.

Share your own outage experiences or mitigation strategies in the comments below.