The negotiation process of the SSL encryption uses much more resources on the server than on the client. If the client can initiate the renegotiation process, an attacker can render the server unavailable with a Denial of Service attack.
Security Assessment
CVSS Vector: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Vulnerability Information
The negotiation process of the SSL encryption uses significantly more resources on the server than on the client. If the client can initiate the renegotiation process, an attacker can render the server unavailable with a Denial of Service attack.
How to Prevent Insecure Client-Initiated SSL Renegotiation
This problem has been fixed in recent webserver versions. E.g. the latest versions of Apache and nginx do not allow client-initiated SSL renegotiation.
Therefore make sure that your webserver is up-to-date.
E.g. run:
apt-get update; apt-get upgrade # Debian / Ubuntu
yum update # RHeL / CentOS
pacman -Syu # Arch Linux
If your webserver does not prevent this by default, you need to ensure to disable the Client Initiated SSL Renegotiation. Only the Server should be allowed to initiate a renegotiation of the SSL/TLS connection.
If you have any questions about SSL/TLS implementation or have any challenges, please let us know and we are happy to provide solutions on your personal challenge.
To test if your specific application is vulnerable for client-initiated SSL Renegotiation, run a non-invasive scan in our Vulnerability Testing Software for free.
