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Blank page displayed in Firefox when requesting websites behind corporate firewall

sb3k | Last updated: Jan 18, 2017 04:24PM UTC

Hi, As the topic says, the browser (Firefox) simply shows a blank page when requesting websites hosted behind our corporate firewall (but not on same network). The behavior is quite strange considering the following: 1) "No proxy for" is empty. 2) This can be reproduced on several workstations in the company. 3) Burp proxy in response intercept mode ON correctly receives the HTML response from the server. The browser turns into a blank page once we press the forward button. 4) The favicon, pictures and JS scripts referenced in the response homepage are loaded in subsequent requests, indicating that the response is received and parsed by Firefox. 5) The websites display correctly if the proxy is disabled, indicating that they do not contain return broken code. If anyone already witnessed this issue, any help would be greatly appreciated. sb3k ps: version is 1.7.16, fully licensed.

Liam, PortSwigger Agent | Last updated: Jan 20, 2017 04:32PM UTC

Hi Thanks for your message. Have you installed the Burp CA certificate in your browser? - https://support.portswigger.net/customer/en/portal/articles/1783075-installing-burp-s-ca-certificate-in-your-browser Are you able to visit any HTTPS websites while proxying through Burp?

Burp User | Last updated: Jan 23, 2017 01:34PM UTC

Hi Liam, thank you for replying! Yes, the CA is installed, and no issues to reach TLS protected websites hosted outside our network. But the issue I am referring to affects websites exposed through http, not https. Would these two things be related? kind regards, sb3k

Liam, PortSwigger Agent | Last updated: Jan 23, 2017 01:42PM UTC

Hi sbk3 Have you tried restoring Burp's default settings? Additionally, you could try unchecking all the options in Proxy > Options > Miscellaneous. When you view the page in the Firefox Dev console, do you see any warnings?

Burp User | Last updated: Jan 24, 2017 09:00AM UTC

Liam, you solved it! I followed your recommendation and disabled all options under proxy->miscellaneous, all pages were displaying fine again, so yes, you were right about an blocking option. So I re-enabled 'default options' for the proxy tab and started isolating the option that was causing problems, all work but one: "Strip Accept-Encoding headers in incoming requests". It must be disabled for requests to websites hosted behind the corporate firewall to work. A huge thank you for your help!!! sb3k PS: I don't see a link to edit the title of this post, so I guess I can't mark it as solved...?

Liam, PortSwigger Agent | Last updated: Jan 24, 2017 09:18AM UTC