PSIRT Advisory

Slow HTTP DoS Attacks Mitigation


An Uncontrolled Resource Consumption vulnerability in multiple products may allow an attacker to cause web service portal denial of service (DoS) via handling special crafted HTTP requests/responses in pieces slowly.

Slow HTTP attacks are denial-of-service (DoS) attacks in which the attacker sends HTTP requests in pieces slowly, one at a time to a Web server. If an HTTP request is not complete, or if the transfer rate is very low, the server keeps its resources busy waiting for the rest of the data. When the server’s concurrent connection pool reaches its maximum, this creates a DoS. Slow HTTP attacks are easy to execute because they require only minimal resources from the attacker.


Denial of service (DoS)

Affected Products

The admin webUI of following products/versions are impacted:

FortiOS versions 6.2.2 and below
FortiSwitch versions below 3.6.11, 6.0.6 and 6.2.2
FortiAnalyzer all versions below 6.2.3
FortiManager all versions below 6.2.3
FortiAP-S/W2 versions below 6.2.2


The following products/versions have implemented counter-measures:

Upgrade to FortiOS 6.2.3
Upgrade to FortiSwitch 3.6.11, 6.0.6 or 6.2.2
Upgrade to FortiAnalyzer 6.2.3
Upgrade to FortiManager 6.2.3
Upgrade to FortiAP-S/W2 6.2.2

When supported, configuring trust hosts for system administrators is a workaround, assuming those hosts are trusted to not initiate an attack.


Fortinet is pleased to thank Independent research team Denis Kolegov, Maxim Gorbunov, Nikita Oleksov and Anton Nikolaev for reporting this vulnerability under responsible disclosure.