Skip to main content

CRE-2025-0075

Nginx Upstream Failure Cascade CrisisCritical
Impact: 10/10
Mitigation: 6/10

CRE-2025-0075View on GitHub

Description

Detects critical Nginx upstream failure cascades that lead to complete service unavailability.

This advanced rule identifies comprehensive upstream failure patterns including DNS resolution

failures, connection timeouts, SSL/TLS handshake errors, protocol violations, and server

unavailability, followed by HTTP 5xx error responses within a 60-second window.


The rule uses optimized regex patterns for maximum detection coverage while maintaining

high performance and low false-positive rates. It captures both the root cause (upstream

failures) and the user-facing impact (HTTP errors) to provide complete incident context.


Cause

ROOT CAUSES:

  • Complete backend server cluster failure (crash, hang, resource exhaustion)
  • Critical database outages affecting all application instances
  • Network partitions isolating nginx from backend infrastructure
  • DNS resolution failures for backend service discovery
  • SSL/TLS certificate expiration or misconfiguration
  • Load balancer configuration errors or upstream pool exhaustion
  • Infrastructure failures (cloud provider outages, data center issues)
  • Cascading microservice dependencies causing widespread failures
  • Memory/CPU exhaustion across entire backend server fleet
  • Container orchestration failures (Kubernetes, Docker Swarm)
  • Security policy changes blocking backend communication

Mitigation

IMMEDIATE TRIAGE (0-5 minutes):

  • Check nginx error logs: `tail -f /var/log/nginx/error.log | grep -E "upstream|backend"`
  • Verify backend service health: `curl -f http://backend-server:port/health`
  • Test network connectivity: `ping backend-servers && telnet backend-server port`
  • Check nginx configuration: `nginx -t && nginx -s reload`
  • Monitor system resources: `top`, `free -h`, `df -h`
  • Review recent deployments and configuration changes

EMERGENCY RESPONSE (5-15 minutes):

  1. Activate incident response team and communication channels
  2. Implement emergency maintenance page to inform users
  3. Scale up healthy backend instances if available
  4. Restart failed backend services with health checks
  5. Consider nginx failover to backup upstream pools
  6. Enable nginx static content serving for critical pages
  7. Implement traffic throttling to reduce backend load

RECOVERY ACTIONS (15-60 minutes):

  1. Identify and resolve root cause (database, network, resources)
  2. Gradually restore backend services with canary deployments
  3. Monitor error rates and response times during recovery
  4. Validate end-to-end functionality before full restoration
  5. Update monitoring thresholds based on incident learnings
  6. Document incident timeline and resolution steps

PREVENTION STRATEGIES:

  • Implement comprehensive health checks with circuit breakers
  • Configure nginx upstream backup servers and failover logic
  • Set up multi-region deployment with automatic failover
  • Implement graceful degradation and static content fallbacks
  • Monitor backend dependencies and cascade failure risks
  • Establish automated scaling policies for traffic spikes
  • Regular disaster recovery testing and runbook validation

References