Handle a false positive

A false positive is an incorrect identification. In the case of DDoS protection, there is a false positive when legitimate traffic is mistakenly classified as attack traffic. This can occur when legacy applications, Internet services, or faulty client applications generate legitimate traffic that appears suspicious, has odd traffic patterns, deviates from best practices, or violates protocols.

In these cases, Cloudflare’s DDoS Protection systems may flag that traffic as malicious and apply mitigation actions. If the traffic is in fact legitimate and not part of an attack, the mitigation actions can cause service disruptions and outages to your Internet properties.

To remedy a false positive:

1. Log in to the Cloudflare dashboard ↗ and select your account.
2. Go to the analytics dashboard and apply filters to the displayed data.

For WAF/CDN customers

3. Select the zone that is experiencing DDoS attack false positives.
4. Go to Security > Events.
5. Select Add filter and filter by Service equals HTTP DDoS.

For Magic Transit and Spectrum customers

6. Go to Account Home > Analytics & Logs > Network Analytics.
7. Identify the legitimate traffic that is causing the false positives. Use the Attack ID number included in the DDoS alert (if you received one), or apply dashboard filters such as destination IP address and port.

8. Scroll down to Top events by source > HTTP DDoS rules.
9. Copy the rule name.
10. Go to your zone > Security > DDoS and select Deploy a DDoS override. If you cannot deploy any additional overrides, edit an existing override to adjust rule configuration.
11. Select Browse rules and paste the rule name in the search field.
12. Decrease the rule’s Sensitivity Level to Essentially Off or change the rule action to Log (if supported by your current plan and subscriptions).
13. Select Next and then select Save.

Once saved, the rule takes effect within one or two minutes. The rule adjustment should provide immediate remedy, which you can view in the analytics dashboard.

Update the adjusted rules later

Later, you can change the sensitivity level of the rule causing the false positives to avoid future issues, and change the rule action back to its default value.

Recommendation: Enable DDoS alerts

Cloudflare recommends that you create notifications for DDoS alerts to get real-time notifications on detected and mitigated attacks automatically performed by Cloudflare’s systems. When you receive these notifications, you can review if it is in fact a real DDoS attack, or if it is a false positive, and then take action to remedy it.

Use cases

Avoid false positives while retaining protection and visibility

To see what DDoS Managed Rules do in a high sensitivity level while remaining protected by blocking attacks at a low sensitivity level, Advanced DDoS protection customers can create a first override that blocks attacks at a low sensitivity and a second override to log at a high sensitivity.

The overrides must be set in that order. Otherwise, it will not work. This is because overrides are evaluated in order and will stop at the first override that matches both expression and sensitivity. Setting the overrides in the wrong order would cause the Log override at a high sensitivity to match all instances. As a result, Cloudflare will never evaluate the Block override that would be placed behind it, causing all rules to be set in Log mode.

If an override without an expression matches, Cloudflare will not evaluate the expressions that follow it.