W3C home > Mailing lists > Public > public-webappsec@w3.org > November 2014

[CSP] Clarifications on nonces

From: Brian Smith <brian@briansmith.org>
Date: Wed, 5 Nov 2014 18:48:16 -0800
Message-ID: <CAFewVt4fqUuz-eQMUdSg_rG8xyShJXdAGmwaO=i=77S=y=e_og@mail.gmail.com>
To: "public-webappsec@w3.org" <public-webappsec@w3.org>
Hi,

I noticed that the current draft says this about nonces: "If the policy
contains a nonce-source expression, the server MUST generate a fresh value
for the nonce-value directive at random and independently each time it
transmits a policy. This requirement ensures that the nonce-value is
difficult for an attacker to predict."

I have one suggestion, one annoying comment, and one question regarding
this:

Suggestion: I think it would be useful to suggest that the nonce be
generated with a cryptographically secure random number generator, and that
the nonce be at least 128-bits long, before base64 encoding. Although
obvious to all of us, I suspect that many people trying to use nonces will
not have the background to create a proper nonce.

Annoying Comment: "nonce" is not a great name for this construct, because
technically the requirements for a CSP nonce are greater than the
requirements for a nonce in normal usage of that word. But, it seems to be
too late to change this.

Question: Are the current requirements on nonces sufficient to ensure safe
usage? I can think of three cases that make me think CSP nonce is not as
secure as I would expect, but I think I may be overlooking something
important:

Case 1. Imagine that the web page contains a CSP nonce of X. Further,
assume that the page uses XHR to retrieve HTML fragments from a server, and
then inserts those fragments into the document. Further, assume that an
attacker learns the value X (because CSP doesn't require X to be secret),
and then inserts <script nonce=X> into that fragment retrieved via XHR. It
seems like the attacker's XSS will succeed, despite CSP.

It seems to me that it would be good to restrict CSP nonce so that it only
works on static <script> elements, not on dynamically inserted ones, to
avoid this, but I don't see that in the spec.

Case 2: Imagine that the web page contains a CSP nonce of Y. Further,
imagine that the web page is dynamically generated and that the server
sends the headers and the first part of the page. Further, imagine that the
second part of the page is dynamically generated and contains
user-generated content from a database. If an attacker can learn the value
Y between the time it is generated and the time when all the dynamic
elements of the page have been built, it would be possible for the attacker
to use a stored XSS attack to insert <script nonce=Y> into the second part
of the page.

It seems like it is important to recommend that the server not transmit any
part of a response that uses script-nonce until the whole response page has
been generated, and more generally require the server to not otherwise
reveal the nonce value before the page is generated. Again, this is the
kind of requirement that is common for cryptographic protocols, but which
would be easy for a typical user of CSP to overlook, so explicitly
suggesting this would be useful for ensuring safe(r) use of this construct.

Case 3: A variation of Case 2. IIUC, it seems to me like violation reports
transmit the violated directive and the whole policy. It seems like the
nonce would be included at least once in the violation report. It would be
amusing if an attacker could force a violation report to be sent before the
page was finished loading, causing the browser to send the the violation
report containing the nonce to the server, causing the server to insert the
violation report into a database accessible to an attacker, where the
attacker read the violation report from the database and used it to build
an XSS that gets included in the page, because the page wasn't fully
generated before the browser sent the violation report.

Again, I'm not very familiar with CSP nonce, so I'm likely overlooking
something that invalidates the concerns in cases 1-3, but I cannot figure
out what it is. Help?

Thanks,
Brian
Received on Thursday, 6 November 2014 02:48:44 UTC

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