On Sunday 4 April 2021 at 12:52 UTC, a cache-busting query flood targeted a tax authority portal customer in the Nordics. The attack peaked at 45.4 million requests per second and lasted 42 minutes. Traffic originated from 3630 autonomous systems in 105 countries, predominantly a Mirai-derived IoT botnet.
| Vector | cache-busting query flood |
| Peak | 45.4 million requests per second |
| Duration | 42 min |
| Time to mitigation | 0.344 s |
| Attack traffic reaching origin | 0.043% |
| Legitimate traffic challenged | 0.11% |
Timeline
The attack was preceded by an extortion email received two days earlier. Request rates on the targeted routes exceeded their hourly baseline by a factor of 414 within 11 seconds. The risk score of participating clients crossed the challenge threshold automatically and proof-of-work difficulty rose with origin load.
What the customer saw
Checkout conversion was unchanged compared with the same hour of the previous week.
Recommendations
- Enable log streaming to your SIEM for faster correlation.
- Keep origin IPs out of public DNS history.
- Add a dedicated rate limit for the targeted route.
The billing model is what got our finance team on board, honestly.
We had the exact false-positive issue with CGNAT carriers. The weighting change makes sense.