Rndom internet sites redirected to page with error about being blokced by firewall

We have about 70 users. Every now and then some users have reported that the get redirected to webpage that says your Firewall is blocking this site. I have attached an image of the browser page. When I looked at the attached screen shoot it seems like it is being directed to blocked-wescreenshootbsite.com and the error is fake. I am not sure if all users are effected or not since the users that have reported this tell me that it does not happen all the time. This primary happens on knows sites such as youtube, facebook.....
I wonder if there is a common software on their computer that has caused this.
We have Viper antivirus and have a Cisco asa 5505 firewall. I wonder how I could to see if there are others with the same issue. Only 4 people have reported within the last month.
I am going to scan one of the computers using malwarebytes,. Your suggestions are appreciated.
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netcompAsked:
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RafaelConnect With a Mentor Commented:
I noticed from your screen shot that you're using OPEN DNS. You need to log into the Open DNS portal for your company and see if you're blocking or controlling these sites through DNS.
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Sam Simon NasserCONSULTANT/DIRECTORCommented:
i suggest to run a malware scan on the reported computers.
question: what is your blocking policy or software? i.e. SQUID or TMG? cause the website redirect to www.blocked-website.com .. i tried accessing it but it gives an error as well
blocked.jpg
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Mohammed HamadaSenior IT ConsultantCommented:
It might be a spyware, try to check your host file if there's anything there or run malwarebaytes.
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netcompAuthor Commented:
You were right. Our DNS servers were pointing to Open DNS. For some reason the old IT admin had are DNS forwarders pointed to Open DNS. I am not sure why he had done that. I replaced it with the ones from our ISP and the issue looks resolved.
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