F5 Networks BIG-IP : Slowloris denial-of-service attack vulnerability (K12636)

medium Nessus Plugin ID 97419
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The remote device is missing a vendor-supplied security patch.


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)


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 :


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.


Upgrade to one of the non-vulnerable versions listed in the F5 Solution K12636.

See Also



Plugin Details

Severity: Medium

ID: 97419

File Name: f5_bigip_SOL12636.nasl

Version: 3.8

Type: local

Published: 2/28/2017

Updated: 1/11/2021

Dependencies: f5_bigip_detect.nbin

Configuration: Enable paranoid mode

Risk Information


Risk Factor: Low

Score: 2.5


Risk Factor: Medium

Base Score: 5

Temporal Score: 3.7

Vector: AV:N/AC:L/Au:N/C:N/I:N/A:P

Temporal Vector: E:U/RL:OF/RC:C

Vulnerability Information

CPE: cpe:/a:f5:big-ip_access_policy_manager, cpe:/a:f5:big-ip_advanced_firewall_manager, cpe:/a:f5:big-ip_application_acceleration_manager, cpe:/a:f5:big-ip_application_security_manager, cpe:/a:f5:big-ip_application_visibility_and_reporting, cpe:/a:f5:big-ip_global_traffic_manager, cpe:/a:f5:big-ip_link_controller, cpe:/a:f5:big-ip_local_traffic_manager, cpe:/a:f5:big-ip_policy_enforcement_manager, cpe:/a:f5:big-ip_wan_optimization_manager, cpe:/a:f5:big-ip_webaccelerator, cpe:/h:f5:big-ip, cpe:/h:f5:big-ip_protocol_security_manager

Required KB Items: Host/local_checks_enabled, Host/BIG-IP/hotfix, Host/BIG-IP/modules, Host/BIG-IP/version, Settings/ParanoidReport

Exploit Ease: No known exploits are available

Patch Publication Date: 2/22/2011

Reference Information

CVE: CVE-2007-6750

BID: 21865