W3C home > Mailing lists > Public > public-webappsec@w3.org > February 2015

Re: Signed CSP

From: Scott Arciszewski <kobrasrealm@gmail.com>
Date: Sun, 15 Feb 2015 21:10:10 -0500
Message-ID: <CAPKwhws-fZ5swRYX+xNpRyPoZ0P1gnA+-n+qhsnzWA2LO7CePw@mail.gmail.com>
To: Crispin Cowan <crispin@microsoft.com>
Cc: "public-webappsec@w3.org" <public-webappsec@w3.org>
CSP + HSTS, excuse me. (But TACK is nice too! :D)

On Sun, Feb 15, 2015 at 8:58 PM, Scott Arciszewski <kobrasrealm@gmail.com>
wrote:

> If the service operator loses control over their server to a malicious
> operator (NSA, GCHQ, bored ex-zf0/PHC/dikline/HTP member, etc), this
> contains the compromise to the server. Clients will see that hashes don't
> match, or signatures don't match, so decloaking visitors becomes an
> incredibly difficult problem. Unless you steal the airgapped signing key as
> well.
>
> It solves the problem of "once the FBI gains control over Tor hidden
> services, they can implant malware to decloak users". How does TACK + HSTS
> stop the feds from identifying clients if they gain complete access to the
> correct server with a valid private key?
>
> On Sun, Feb 15, 2015 at 8:48 PM, Crispin Cowan <crispin@microsoft.com>
> wrote:
>
>>  What does the Wired article tell us that helps? I get that it is an
>> important problem, I just don't see how your proposes addresses that
>> problem.
>>
>> Sent from my Windows Phone
>>  ------------------------------
>> From: Scott Arciszewski <kobrasrealm@gmail.com>
>> Sent: ‎2/‎15/‎2015 2:27 PM
>>
>> To: Crispin Cowan <crispin@microsoft.com>
>> Cc: public-webappsec@w3.org
>> Subject: Re: Signed CSP
>>
>>  http://www.wired.com/2013/09/freedom-hosting-fbi/
>>
>> On Sun, Feb 15, 2015 at 5:15 PM, Crispin Cowan <crispin@microsoft.com>
>> wrote:
>>
>>>  So the client trusts the offline signing key, but not the server.
>>>
>>>
>>>
>>> An attacker can compromise everything about the server, except it’s CSP:
>>> JS, content, etc.
>>>
>>>
>>>
>>> So the attacker can load this server full of lies and such, but can’t
>>> change its CSP. What threats does that defend the client against?
>>>
>>> ·         This site can’t be XSS’d by another site: don’t care, this
>>> site is already completely p0wned.
>>>
>>> ·         Other sites can’t be XSS’d by this site: I don’t think that
>>> this site’s CSP assures that, and don’t really care, the attacker will just
>>> XSS you from a different place.
>>>
>>>
>>>
>>> So I’m still not seeing a real threat that this really mitigates.
>>>
>>>
>>>
>>> *From:* Scott Arciszewski [mailto:kobrasrealm@gmail.com]
>>> *Sent:* Sunday, February 15, 2015 1:23 PM
>>> *To:* Crispin Cowan
>>> *Cc:* public-webappsec@w3.org
>>> *Subject:* Re: Signed CSP
>>>
>>>
>>>
>>> What value does this proposal deliver, that you do not get by combining
>>> HSTS pinning with CSP?
>>>
>>>
>>> The signing key remains offline, so an attacker cannot forge signatures.
>>> HSTS + CSP does not achieve this since the SSL private key must remain
>>> accessible to the server.
>>>
>>> The goal here is to disrupt the US government's malware campaigns on the
>>> Tor network, but it could also be advantageous against less sophisticated
>>> threats.
>>>
>>> On Sun, Feb 15, 2015 at 3:35 PM, Crispin Cowan <crispin@microsoft.com>
>>> wrote:
>>>
>>>  What value does this proposal deliver, that you do not get by
>>> combining HSTS pinning with CSP?
>>>
>>>
>>>
>>> In particular, since the threat you are trying to defend against is a
>>> compromised server, the most the client can do is ensure that this is still
>>> the server you think it is. Even that is dubious, because a compromised
>>> server gives the SSL private key to the attacker. Doing more than that
>>> would seem to prevent the legitimate site admin from changing policy.
>>>
>>>
>>>
>>> *From:* Scott Arciszewski [mailto:kobrasrealm@gmail.com]
>>> *Sent:* Sunday, February 15, 2015 7:28 AM
>>> *To:* public-webappsec@w3.org
>>> *Subject:* Signed CSP
>>>
>>>
>>>
>>> I love Content-Security-Policy headers, but I feel that they could do
>>> more to protect end-users from malicious Javascript especially if the
>>> entire host web server gets compromised and attackers are able to tamper
>>> with headers at will.
>>>
>>>
>>>
>>> I would like to propose an extension to the Content-Security-Policy
>>> specification to mitigate the risk of a hacked server distributing malware,
>>> similar to what happened during the Freedom Hosting incident in 2013.
>>>
>>> The new proposed header looks like this:
>>>
>>> Signed-Content-Security-Policy: /some_request_uri publicKeyA,
>>> [publicKeyB, ... ]
>>>
>>> WHEREBY:
>>> --------
>>>
>>> * /some_request_uri is a message signed with one of the public keys
>>> specified in the header
>>>
>>> * /some_request_uri contains a full CSP definition with one caveat:
>>> hashes of script src files are required!
>>>
>>> * The proposed signing mechanism is EdDSA, possibly Ed25519 (depending
>>> on CFRG's final recommendation to the TLS working group)
>>>
>>> * At least one public key is required, but multiple are allowed (more on
>>> this below)
>>>
>>> With this mechanism in place on the client and the server, if you were
>>> to compromise a server (say, a Tor Hidden Service), you would not be able
>>> to tamper with the Javascript to deliver malware onto the client machines
>>> without access to the EdDSA secret key (or a hash collision in the CSP
>>> definition) or fooling the client into accepting a bad public key.
>>>
>>> Server Implementation:
>>>
>>>
>>>
>>> Let's say I wish to publish a Tor Hidden Service that hosts, say, the
>>> unredacted Snowden files. These are the steps I would need to take to
>>> prevent malware deployment:
>>>
>>> 1. Generate N EdDSA secret/public key pairs (N > 2).
>>>
>>> 2. Put all of the public keys in the SCSP header.
>>>
>>> 3. Use only one secret key for signing from an airgapped machine
>>> whenever a website update is required. The rest should remain on encrypted
>>> thumb drives which are in hidden caches.
>>>
>>> Client Implementation:
>>>
>>> Upon accessing a website with a CSP header, render the fingerprints and
>>> ask the user if they trust this series of hexits. If someone attempts to
>>> add/replace any of the public keys, immediately disable Javascript and
>>> panic to the user. This is basically SSH model of trust, but in the event
>>> of a signing key compromise, the other keys can be used and the untrusted
>>> public key can be removed without causing a ruckus to the end user.
>>>
>>> Users' trust decisions should be stored in a separate file than
>>> cert8.db, and users should be able to tell their browser where to store it.
>>> In "private browsing" modes, this file should be cloned into memory and
>>> never written back to disk without explicit user action (e.g. for Tor
>>> Browser Bundle users).
>>>
>>>
>>>
>>> This is obviously a very rough draft, but I would love to get feedback
>>> on it and, if everyone approves, move forward with developing it into
>>> something greater. (Browser extension? Internet Standard? Not my place to
>>> say :)
>>>
>>> Scott Arciszewski
>>>
>>>
>>>
>>
>>
>
Received on Monday, 16 February 2015 02:10:39 UTC

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