From: "Anthony J. Bentley" <anthony@anjbe.name>
To: Ingo Schwarze <schwarze@usta.de>
Cc: tech@mandoc.bsd.lv
Subject: Re: Content-Security-Policy for man.cgi
Date: Sun, 10 Nov 2019 06:02:49 -0700 [thread overview]
Message-ID: <74937-1573390969.518612@LNmC.KNpy.68m_> (raw)
In-Reply-To: <20191110102234.GC53073@athene.usta.de>
Hi Ingo,
Ingo Schwarze writes:
> I tried to read the standard https://www.w3.org/TR/CSP/
> but miserably failed to understand anything because there
> is so much indirection: "to do what <other standard> defines
> in section 1.17.42.0 to the objects <yet another standard>
> defines in section 4.3.2.1, use the methods described in
> <some fourth standard> in section 36932451927, but only unless
> the conditions explained in sections 666.0b and <aaah!>
> apply." And when you follow the pointers, you only find
> more indirections to yet more places... :-[
Hm, I guess I'm just used to reading W3C standards at this point.
> Do you know a place where that stuff is explained in a more
> accessible reference-manual style?
https://content-security-policy.com/ perhaps?
> > Since man.openbsd.org hosts manuals from many sources, and there's
> > always danger of a bug in mandoc that allows dangerous HTML content
> > through, a policy of "default-src 'none'; style-src 'self'" would be
> > appropriate: this allows external stylesheets loaded from a URL on
> > the same domain, but prohibits external links
>
> You mean, prohibits embedding content (not sure whether that is the
> correct term) from other sites? Linking *to* other sites still
> appears to be permitted...
Sorry, that was jargon. I was referring to link elements here (as in:
"<link rel=stylesheet href=http://some.other.example.com/...>").
> > and inline CSS;
>
> I do understand why inline CSS is bad style and can harm accessibility
> and prevent formatting that is well adapted to the currently used
> browsing device - but i fail to understand why a *security*
> policy would worry about inline CSS. Isn't "inline" by definition
> the most secure source of content conceivable?
In short, because modern CSS is so featureful that it is a vector for
XSS as much as JavaScript. I don't have any examples off the top of my
head but I'm sure that's not such an unbelievable statement.
> > scripts are not allowed at all. (mandoc(1) no longer generates
> > inline styles at all, right?)
>
> In a very small number of places, it still does:
>
> mdoc_html.c, mdoc_it_pre(), LIST_tag, ROFFT_BODY:
> print_otag(h, TAG_DD, "s", "width", "auto");
> <dd style="width: auto">
>
> mdoc_html.c, mdoc_fn_pre():
> print_otag(h, TAG_VAR, "cs", "Fa", "white-space", "nowrap");
> <var class=Fa" style="white-space: nowrap">
>
> tbl_html.c, html_tblopen():
> h->tblt = print_otag(h, TAG_TABLE, "c?ss", "tbl", ...
> <tbl class="tbl" border=1
> style="border-style: solid; border-top-style: double">
>
> tbl_html.c, print_tbl():
> print_otag(h, TAG_TR, "ss",
> "border-left-style", lborder,
> "border-bottom-style", bborder);
> <tr style="border-left-style: solid; border-bottom-style: double">
>
> tbl_html.c, print_tbl():
> print_otag(h, TAG_TD, "??sss", ...
> <td colspan=3 rowspan=2 style="vertical-align: top; text-align;
> center; border-right-style: solid">
It might be worth replacing these with stylesheet references for the
sake of having a CSP strict enough to prevent malicious inline CSS in
the manual body. But if not, we'll have to broaden the policy. Even a
broad CSS policy is better because we still completely block JavaScript.
> I think the semicolon in "'self';\r\n" isn't needed, right?
It isn't needed.
> By the way, could you check whether the CSP in
>
> https://mandoc.bsd.lv/cgi-bin/cvsweb/cvsweb.cgi?cvsroot=cvsweb#rev4.10
>
> makes any sense?
Seems conceptually fine, though again "style-src 'self'" is strictly
better than "style-src 'unsafe-inline'" (but CVSWeb is not really
designed for that). You'll notice that in the log you linked, images
are blocked because they're from cvsweb.bsd.lv, not mandoc.bsd.lv, and
don't count as 'self'.
--
Anthony J. Bentley
--
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next prev parent reply other threads:[~2019-11-10 13:02 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-11-10 8:59 Anthony J. Bentley
2019-11-10 10:22 ` Ingo Schwarze
2019-11-10 13:02 ` Anthony J. Bentley [this message]
2019-11-10 17:47 ` Ingo Schwarze
2019-11-10 20:09 ` Anthony J. Bentley
2019-11-10 20:57 ` Ingo Schwarze
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