K12636 : Slowloris denial-of-service attack vulnerability CVE-2007-6750

Security Advisory Description

The Apache HTTP Server 1.x and 2.x allows remote attackers to cause a denial of service (daemon outage) via partial HTTP requests, as demonstrated by Slowloris, related to the lack of the mod_reqtimeout module in versions before 2.2.15. (CVE-2007-6750)

Impact

The Slowloris attack is a type of denial-of-service (DoS) attack that targets threaded web servers. It attempts to monopolize all of the available request handling threads on the web server by sending HTTP requests that never complete. Because each request consumes a thread, the Slowloris attack eventually consumes all of the web server’s connection capacity, effectively denying access to legitimate users.

The HTTP protocol specification Internet Engineering Task Force (RFC 2616) states that a blank line must be used to indicate the end of the request headers and the beginning of the payload, if any. After the entire request is received, the web server may then respond.

Note: A blank line is created by sending two consecutive newlines:

<CR><LF><CR><LF>

The Slowloris attack operates by establishing multiple connections to the web server. On each connection, it sends an incomplete request that does not include the terminating newline sequence. The attacker sends additional header lines periodically to keep the connection alive, but never sends the terminating newline sequence. The web server keeps the connection open, expecting more information to complete the request. As the attack continues, the volume of long-standing Slowloris connections increases, eventually consuming all available web server connections, thus rendering the web server unavailable to respond to legitimate requests.