ISA Error Code 502 Proxy Error (Full Version)

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Message -> ISA Error Code 502 Proxy Error (12.Jun.2007 1:46:22 AM)

Hello  all,

We are running ISA Server 2006 Enterprise.  All users use web proxy on port 8080 to browse the Internet from the Internal LAN.  Our users can succesfully browse the Internet except when they want to access one specific web site.  The web site is:

When they access the web site, the following ISA error is displayed:

Error Code: 502 Proxy Error. The HTTP message includes an unsupported header or an unsupported combination of headers. (12156)

Is there something we can do on our side to give access to the specific web site?

Thank you.

Yorgy -> RE: ISA Error Code 502 Proxy Error (13.Jun.2007 5:33:22 PM)

You may want to look over this KB from Microsoft and consult with them to see if there is a Hot Fix available for 2006. The KB only applies to 2004 with the issue you are experiencing.


Johan_A -> RE: ISA Error Code 502 Proxy Error (25.Jun.2007 7:22:21 AM)

We got the same error when we try to access
Running ISA 2006 STD.  dosent work for us either.
Yorgy have you fix the problem?


SFletchall -> RE: ISA Error Code 502 Proxy Error (27.Jun.2007 12:31:49 PM)

How about

How about  ?

Now, how about  (notice httpS)

Now try (notice httpS)

I believe you're experiencing this issue:
This is serious enough IMHO for M$ to release this in a patch all by itself.  Why are they waiting for the next Service Pack to add this hotfix?  Perhaps they're still testing it.  Anyway, you can call the technical support number at and receive this hotfix free of charge.  They will warn about using this in a production environment, so I would recommend caution... although I personally installed this hotfix w/o testing it. (We have a backup ISA server [2004] that I can quickly switch over too, so I wasn't concerned).

All right, now after installing this hotfix, nothing changed… at first.  A little while later and worked, but the other sites still do not ( and

Perhaps this is due to caching?  I even disabled caching, restarted the server and re-enabled caching and still no luck.  At any rate... my problems are solved as wellmark works :).  Good luck.  Please let me know how you fix it.
Here's an older thread with the same issue:
Also, one more thing.  I wonder if our firewall might be stripping certain information from these sites that isa2006 wants to see.  Perhaps I will test that and let you know.

Mylle -> RE: ISA Error Code 502 Proxy Error (29.Jun.2007 6:30:03 AM)


I have the exact same problem. Called Microsodt for the fix. installed it, but it is still not working :(

Any ideas?


tchescat -> RE: ISA Error Code 502 Proxy Error (31.Jul.2007 11:37:30 AM)

The hotfix worked for me!  All the sites that you've listed did not work for me before the hotfix and now they work.  The secret for me was the Reg entry that needs to be put in before the hotfix works!!!

[HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\RAT\Stingray\Debug\Web Filters]

I needed to type that reg in manually, but it worked!!!!!!!!!!!!!!!!!!!!!

Sauron76 -> RE: ISA Error Code 502 Proxy Error (16.Aug.2007 3:56:06 AM)

Dear Guys,
I want to replace my old ISA 2000 server with new ISA 2006. My ISA2006 joins an enterprise domain & all users will have to key-in their domain id in order to surf through proxy. But the problem is certain users when trying to surf, their is no authentication box appear instead of this error msg.

Error Code: 502 Proxy Error. The ISA Server denied the specified Uniform Resource Locator (URL). (12202)

 I tried install hotfix that were given in Microsoft websites/engineer but the problem still happens. Need your guys expertise to help me solve this problem. Currently i've revert back to my old ISA 2000 server which has never encountered this kind of problem.

jdmils -> RE: ISA Error Code 502 Proxy Error (14.Nov.2007 8:25:35 PM)

The error:

The HTTP message includes an unsupported header or an unsupported combination of headers

Is due to a new feature (???) which is introduced by ISA Server 2006.
This problem occurs if a Web server response contains an HTTP header that begins with a space or with a TAB character.

The intended resolution is to obtain the hotfix KB935882 which M$ will send to you. I tried installing the hotfix and kept getting the error:

Internal Error 2727. FBA_SET_ISA_XHTML_NLS_ZHTW

I found a post at which stated that the poster had the same error and found that it was due to kb939455 already being installed on the server.

I thus uninstalled kb939455 and installed KB935882 and it installed without error!

But the hotfix did NOT solve the problem. What I did find was this.....

I used the address:

to access the internal web server. The IE7 browser gives me the error:


X Network Access Message: The page cannot be displayed

Explanation: There is a problem with the page you are trying to reach and it cannot be displayed.

Technical Information (for support personnel) Error Code: 502 Proxy Error. The HTTP message includes an unsupported header or an unsupported combination of headers. (12156) IP Address: Date: 11/15/2007 12:47:31 AM [GMT] Server: Source: proxy

Using FireFox V2.0.0.9 works fine and gives me the page I'm after.

However, if I replace the IP "" with the host name "crs-cm", IE7 works fine! The same goes for IE6. And I have tested this on more than one workstation. This was not a problem when we were using ISA Server 2004.

This problem occurrs on our ISA Server CLA-ISA ( V5.0.5721.240. Interestingly, the problem is not apparent when the user is using our second ISA Server CRS-ISA ( V5.0.5721.240. The web server is local to the CRS-ISA server.

CLA-ISA & CRS-ISA Internal Network addressing:
quote: to to to

Anyone know what's happening here??

elmajdal -> RE: ISA Error Code 502 Proxy Error (16.Nov.2007 10:21:16 AM)

Hi Guys,

for the 502 error , install the ISA Server 2006 Supportability Update package

make sure to read the List of issues that this package fixes

tjcarst -> RE: ISA Error Code 502 Proxy Error (16.Oct.2008 10:13:50 AM)

I have the same problem with

I can access the site with Firefox, but not IE.

Error Code: 502 Proxy Error. The HTTP message includes an unsupported header or an unsupported combination of headers. (12156)

I have installed the supportability patch, no luck.

I've requested kb 935882 from MS, but haven't received yet.

tjcarst -> RE: ISA Error Code 502 Proxy Error (16.Oct.2008 1:32:21 PM)

935882 didn't fix the problem either...

tjcarst -> RE: ISA Error Code 502 Proxy Error (16.Oct.2008 5:17:15 PM)

Correction - If Firefox is set to use the ISA 2006 server, the site fails.  The client succeeds if the Firefox browser uses the Fireall Client to access the site when there are not proxy settings configured.

In IE7, the client uses the web proxy even though the site is configured to bypass the proxy client on the ISA server.

tjcarst -> RE: ISA Error Code 502 Proxy Error (16.Oct.2008 5:33:24 PM)

I've found a fix, though not pleased with it.

Install Firewall Client.  Do not enable web browser auto config. 

Set IE Tools, Internet Connections, LAN Settings, Advanced, Do not use proxy server for addresses beginning with *.<domain name> of problem site.

The Client does not pay any attention to the Web Proxy exclusion specified on ISA server for bypassying the domains in Config, Networks, Internal, Web Proxy, Directly Access these servers or domains.

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