Subject: [Acme] Guessable URLs and unprotected orders lists

From: Sophie Herold <[email-masked]>
Date: Thu, 11 Jan 2018 20:23:26 +0100

Hi,

challenge tokens "MUST have at least 128 bits of entropy", at the same
time it seems trivial to guess order and authorization URLs like the
ones used in the examples. It seems natural, that URLs MUST be generated
with the same amount of entropy. But I couldn't find that in the draft.

For account objects, GET request are not allowed:

   Servers SHOULD NOT respond to GET requests for account resources as
   these requests are not authenticated.

This suggests that all non-expiring URLs should be protected in this
way. At least for orders lists, this protection is missing.

Best,
Sophie

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From: Ilari Liusvaara <[email-masked]>
Date: Thu, 11 Jan 2018 21:36:12 +0200

> way. At least for orders lists, this protection is missing.

The token entropy requirement is to render those tokens unguessable
before the validation request is received. This is to protect against
careless servers. The token is not actually secret after it has been
generated.

Now, in some approved CA validation methods, the tokens actually are
secret, but none of those is used in ACME.

-Ilari

---

From: Niklas Keller <[email-masked]>
Date: Thu, 11 Jan 2018 21:25:43 +0100

> careless servers. The token is not actually secret after it has been

I don't think it is for careless servers, the token is provided in the
request, so any server can respond with it. Rather it's required to fulfill
the CA/Browser Forum rules.

Regards, Niklas

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