Vulnerable products : Linksys EA2700, EA3500, E4200, EA4500 using
lighttpd 1.4.28 and Utopia on  Linux 2.6.22

Firmware Version: 1.0.14  EA2700
Firmware Version: 1.0.30  EA3500
Firmware Version: 2.0.36  E4200
Firmware Version: 2.0.36  EA4500

Impact: - Major

Timeline: - Still awaiting word back from Linksys support. Partial
disclosure at the present due to the impact; Full disclosure in near
future if warranted.

Vulnerabilities:
- Unauthenticated remote access to all pages of the router
administration GUI, bypassing any credential prompts under certain
common configurations (see below)
- Direct access to several other critical files, unauthenticated as well

Vulnerability Conditions seen in all variations:

- Remote Management - Disabled
- UPnP - Enabled
- IPv4 SPI Firewall Protection - Disabled

Although not the same symptoms as the bug that plagues most ASUS
routers that are AiCloud enabled with WebDav, the utilization of both
UPnP and SSL on lighttpd v 1.4.28 appears to be an extremely
problematic combination, exposing certain vulnerabilities to the WAN
side of the router.

Recommendations-

- Disable UPnP
- Enable at minimum the built in IPv4 SPI firewall
- Oddly, in some instances, resetting the password and doing a full
power down reboot has shown to close the vulnerability, but not always
- Disallow remote access from the WAN side - both http and https
- Changing the default user name and password won't help in this case,
but it always bears repeating
- Since an attacker has access to enable FTP service, USB drives
mounted in the router should be removed until a patch is out, or the
full scope of the issue is known

Testing additional firmware is ongoing.