Cloudflare Outage Explained — What Happened on 18 November 2025
On 18 November 2025, Cloudflare — a major internet infrastructure provider — experienced a significant service outage that caused many popular websites and apps to return errors or become unreachable for several hours. This article explains, in plain language, what happened, why it affected so many services, how long it lasted, and the key lessons for website operators and end users.
For a full breakdown, watch the video on YouTube: Cloudflare Outage Explained — One File Broke the Internet
1. Who is Cloudflare (short)
Cloudflare provides CDN, DNS, reverse-proxy, DDoS protection, and other network/security services that sit in front of millions of websites and applications. Because many sites route traffic through Cloudflare, a major disruption there can cause a large portion of the web to show errors or fail to load.
2. Timeline & symptoms
- ~11:20 UTC, 18 Nov 2025: Cloudflare detection systems noted an internal service degradation and the first user reports of HTTP 5xx errors began to appear.
- Users worldwide reported websites returning 5xx errors or failing to load; many popular services were affected temporarily (examples reported: ChatGPT, X, Spotify, Canva, and more).
- Cloudflare engineers investigated and deployed mitigations; services progressively returned to normal later the same day.
3. Root cause (short version)
Cloudflare’s official post-mortem identifies the root cause as a bug in the generation logic for a Bot Management “feature file.” A change to a database system permission caused multiple unexpected entries to be output into the feature file, which then doubled in size. That larger-than-expected file propagated across Cloudflare’s fleet; certain software components had size limits and began to fail when they received the oversized data, causing cascading errors across the network.
4. Why the outage affected so many sites
Cloudflare is an infrastructure provider used by a large share of websites for DNS, CDN, reverse proxying and security. When a shared infrastructure component fails, all customers relying on that component can see degraded service simultaneously — which is why a single failure at Cloudflare produced a widely-visible outage.
5. Impact & duration
- Scope: Global — many websites and services experienced error pages or interruptions.
- Peak reports: Downdetector and other monitoring tools showed a rapid spike in incident reports shortly after the issue started.
- Duration: The incident started at ~11:20 UTC and Cloudflare declared core traffic delivery restored within a few hours; residual issues (dashboard/API access, etc.) were monitored after that. Many sources and the Cloudflare post-mortem place the main outage window at roughly 3–4 hours for broad restoration, with fixes deployed the same day.
6. Lessons learned
- For operators: Avoid single points of failure — consider multi-CDN, separate DNS providers, and disaster runbooks for third-party outages.
- For vendors: Defensive limits and careful handling of unexpected large inputs are critical; propagation controls can limit blast radius.
- For users: When a service goes down, it may be due to an upstream infrastructure provider rather than the app itself.
- Systemic risk: As the internet consolidates around large providers, a problem at one provider has outsized effects — transparency and robust post-mortems help the community learn and improve.
7. Official post-mortem & further reading
Cloudflare published a detailed post-mortem explaining the timeline and technical root cause. For technical readers who want the primary source, see Cloudflare’s post: Cloudflare outage on November 18, 2025 (post-mortem).
Other reputable coverage: Reuters, The Guardian and major tech outlets also reported on the outage and its impact. Useful reads:
- Reuters: Cloudflare outage coverage
- The Guardian: Cloudflare outage causes error messages across the internet
- Washington Post: Cloudflare outage briefly disrupts ChatGPT, X and dozens of apps
- Cloudflare official post-mortem (primary source)
8. Quick checklist for site owners (what to do now)
- Confirm whether your site uses Cloudflare for DNS/CDN/WAF or other services.
- Ensure you have an incident runbook: how to switch DNS or temporarily bypass an edge provider if needed.
- Consider multi-vendor architecture for critical services (e.g., multi-CDN, separate DNS provider).
- Review logging/alerts to ensure you detect provider outages quickly and distinguish them from application issues.
9. Final thoughts
The 18 November 2025 outage is a timely reminder that despite mature engineering and high reliability, large distributed systems can fail in surprising ways. The most useful responses are improved observability, careful input handling, propagation controls, and well-rehearsed incident plans.
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