Regular expression in a WAF rule exhausts CPU on every edge server (Cloudflare, 2019)
A new WAF managed rule for XSS detection contained a regular expression that caused catastrophic backtracking and exhausted CPU.
What happened
- The rule was deployed globally through Cloudflare's configuration system and reached all data centers within seconds.Details of the Cloudflare outage on July 2, 2019
- The resulting outage lasted about 27 minutes.Details of the Cloudflare outage on July 2, 2019
The failure pattern
Watch the pattern: Global change without a staged rollout
Safeguards that address it
CollectSEA's engineering analysis of what would have prevented or contained this failure.
- Deploy rules through staged rolloutsRelease new rules to a small share of traffic first and watch CPU and error rates before going global.
- Bound rule execution costUse regex engines without catastrophic backtracking and enforce per-request CPU limits and a global kill switch.
Related standards
CollectSEA's mapping through this incident's failure pattern; not an official determination by the standards bodies.
- ASI08 Cascading Failures
- LLM06 Unbounded Consumption
- CM-3 Configuration Change Control
- CM-4 Impact Analyses
- SC-5 Denial-of-Service Protection
- RV.3.4 Update the development process
- MAP 5.1 Estimate impacts from past uses and public incident reports
- CWE-770 Allocation of Resources Without Limits or Throttling
Building something similar?
Describe your design and see every precedent that applies, with safeguards you can hand to your coding agent.
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