Custom App for CRL downloads

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Custom App for CRL downloads

L1 Bithead

Hi,

I am trying to create a custom app that will match CRL downloads, to allow them without any questions ask. Shouldn't be too hard : on a previous web security gateway, I would match a pattern like the following: "http://([^/:])*crl.*\.crl"

When translated to an app signature, I already know I am looking for two patterns, on the following contexts:

  • http-req-host-header
  • http-req-uri-path

Now, my issue is that my pattern are incredibly simple. I am not able to reach the 7 bytes limit with this. One other way would be to match the MIME type in the RSP header I guess... application/pkix-crl or application/x-pkcs7-crl are fine. The issue is with the occasional misconfigured root CAs that still reports text/plain.

So....  Is there anything I can do with this to simply allow CRLs without creating a huge custom category?

Thank you.

1 ACCEPTED SOLUTION

Accepted Solutions

L1 Bithead

Well, I was able to create a working signature. I just needed to increase my "scope" ; I used the http-req-headers instead of http-req-uri-path. So, I can match on something like  ".*\.crl HTTP.*User\-Agent.*, and I looking into the http-rsp-headers for the transaction as well. Simple enough... I feel like it was a worthy exercise after all.

I attached the file for other people to use, and I submitted the request as well.

Thank you.

View solution in original post

20 REPLIES 20

L6 Presenter

Perhaps creating an URL-filter instead?

The URL-filter has severe limitations. Unless I maintain a huge custom category of all the common CRL distribution points on the Internet, I can't see how I would do this. I have two main issues with the URL-filter expressions.

1. matching hosts, the wildcard has to match a complete token. I can't use  *crl.*.* -> *crl is invalid because it is not the only character in the token. So, I can't restrict destination URLs based on the hostname.
2. The path is not predictable enough. All I know is that the file name is going to be .crl,, but it might a very long path (many subdirectories) or a short one. ex:  /*.crl, /*/*/*/*.crl ... Still, that's not so bad because I could create a dozen of URL filter expressions that would match the file up to 11 directories deep, as an exemple. And that would cover most case... But it still something that could be avoided with a custom appid, or less restrictive URL expressions.

Unless there is something I don't know yet about Pan-OS url-filter expressions...

Sounds like a case where you should contact the appid team and submit this as a request: http://researchcenter.paloaltonetworks.com/tools/

L1 Bithead

Well, I was able to create a working signature. I just needed to increase my "scope" ; I used the http-req-headers instead of http-req-uri-path. So, I can match on something like  ".*\.crl HTTP.*User\-Agent.*, and I looking into the http-rsp-headers for the transaction as well. Simple enough... I feel like it was a worthy exercise after all.

I attached the file for other people to use, and I submitted the request as well.

Thank you.

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