Patchwork hgweb: support Content Security Policy

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Submitter Gregory Szorc
Date Jan. 11, 2017, 7:41 a.m.
Message ID <113293954736e020d29e.1484120470@ubuntu-vm-main>
Download mbox | patch
Permalink /patch/18181/
State Accepted
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Gregory Szorc - Jan. 11, 2017, 7:41 a.m.
# HG changeset patch
# User Gregory Szorc <gregory.szorc@gmail.com>
# Date 1484120228 28800
#      Tue Jan 10 23:37:08 2017 -0800
# Node ID 113293954736e020d29e8e48aa3e01657ec853f3
# Parent  79314c9a79b3aa033b6f79d066b97d7157ecac33
hgweb: support Content Security Policy

Content-Security-Policy (CSP) is a web security feature that allows
servers to declare what loaded content is allowed to do. For example,
a policy can prevent loading of images, JavaScript, CSS, etc unless
the source of that content is whitelisted (by hostname, URI scheme,
hashes of content, etc). It's a nifty security feature that provides
extra mitigation against some attacks, notably XSS.

Mitigation against these attacks is important for Mercurial because
hgweb renders repository data, which is commonly untrusted. While we
make attempts to escape things, etc, there's the possibility that
malicious data could be injected into the site content. If this happens
today, the full power of the web browser is available to that
malicious content. A restrictive CSP policy (defined by the server
operator and sent in an HTTP header which is outside the control of
malicious content), could restrict browser capabilities and mitigate
security problems posed by malicious data.

CSP works by emitting an HTTP header declaring the policy that browsers
should apply. Ideally, this header would be emitted by a layer above
Mercurial (likely the HTTP server doing the WSGI "proxying"). This
works for some CSP policies, but not all.

For example, policies to allow inline JavaScript may require setting
a "nonce" attribute on <script>. This attribute value must be unique
and non-guessable. And, the value must be present in the HTTP header
and the HTML body. This means that coordinating the value between
Mercurial and another HTTP server could be difficult: it is much
easier to generate and emit the nonce in a central location.

This commit introduces support for emitting a
Content-Security-Policy header from hgweb. A config option defines
the header value. If present, the header is emitted. A special
"%nonce%" syntax in the value triggers generation of a nonce and
inclusion in <script> elements in templates. The inclusion of a
nonce does not occur unless "%nonce%" is present. This makes this
commit completely backwards compatible and the feature opt-in.

The nonce is a type 4 UUID, which is the flavor that is randomly
generated. It has 122 random bits, which should be plenty to satisfy
the guarantees of a nonce.
Sean Farley - Jan. 11, 2017, 6:53 p.m.
Gregory Szorc <gregory.szorc@gmail.com> writes:

> # HG changeset patch
> # User Gregory Szorc <gregory.szorc@gmail.com>
> # Date 1484120228 28800
> #      Tue Jan 10 23:37:08 2017 -0800
> # Node ID 113293954736e020d29e8e48aa3e01657ec853f3
> # Parent  79314c9a79b3aa033b6f79d066b97d7157ecac33
> hgweb: support Content Security Policy
>
> Content-Security-Policy (CSP) is a web security feature that allows
> servers to declare what loaded content is allowed to do. For example,
> a policy can prevent loading of images, JavaScript, CSS, etc unless
> the source of that content is whitelisted (by hostname, URI scheme,
> hashes of content, etc). It's a nifty security feature that provides
> extra mitigation against some attacks, notably XSS.
>
> Mitigation against these attacks is important for Mercurial because
> hgweb renders repository data, which is commonly untrusted. While we
> make attempts to escape things, etc, there's the possibility that
> malicious data could be injected into the site content. If this happens
> today, the full power of the web browser is available to that
> malicious content. A restrictive CSP policy (defined by the server
> operator and sent in an HTTP header which is outside the control of
> malicious content), could restrict browser capabilities and mitigate
> security problems posed by malicious data.
>
> CSP works by emitting an HTTP header declaring the policy that browsers
> should apply. Ideally, this header would be emitted by a layer above
> Mercurial (likely the HTTP server doing the WSGI "proxying"). This
> works for some CSP policies, but not all.
>
> For example, policies to allow inline JavaScript may require setting
> a "nonce" attribute on <script>. This attribute value must be unique
> and non-guessable. And, the value must be present in the HTTP header
> and the HTML body. This means that coordinating the value between
> Mercurial and another HTTP server could be difficult: it is much
> easier to generate and emit the nonce in a central location.
>
> This commit introduces support for emitting a
> Content-Security-Policy header from hgweb. A config option defines
> the header value. If present, the header is emitted. A special
> "%nonce%" syntax in the value triggers generation of a nonce and
> inclusion in <script> elements in templates. The inclusion of a
> nonce does not occur unless "%nonce%" is present. This makes this
> commit completely backwards compatible and the feature opt-in.
>
> The nonce is a type 4 UUID, which is the flavor that is randomly
> generated. It has 122 random bits, which should be plenty to satisfy
> the guarantees of a nonce.

Looks pretty good to me. I'd appreciate if someone else with web server
experience to gave it a look over. One small nit below.

> diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
> --- a/mercurial/help/config.txt
> +++ b/mercurial/help/config.txt
> @@ -2084,6 +2084,20 @@ The full set of options is:
>      Name or email address of the person in charge of the repository.
>      (default: ui.username or ``$EMAIL`` or "unknown" if unset or empty)
>  
> +``csp``
> +    Send a ``Content-Security-Policy`` HTTP header with this value.
> +
> +    The value may contain a special string ``%nonce%``, which will be replaced
> +    by a randomly-generated one-time use value. If the value contains
> +    ``%nonce%``, ``web.cache`` will be disabled, as caching undermines the
> +    one-time property of the nonce. This nonce will also be inserted into
> +    ``<script>`` elements containing inline JavaScript.
> +
> +    Note: lots of HTML content sent by the server is derived from repository
> +    data. Please consider the potential for malicious repository data to
> +    "inject" itself into generated HTML content as part of your security
> +    threat model.
> +
>  ``deny_push``
>      Whether to deny pushing to the repository. If empty or not set,
>      push is not denied. If the special value ``*``, all remote users are
> diff --git a/mercurial/hgweb/common.py b/mercurial/hgweb/common.py
> --- a/mercurial/hgweb/common.py
> +++ b/mercurial/hgweb/common.py
> @@ -8,9 +8,11 @@
>  
>  from __future__ import absolute_import
>  
> +import base64
>  import errno
>  import mimetypes
>  import os
> +import uuid
>  
>  from .. import (
>      encoding,
> @@ -199,3 +201,22 @@ def caching(web, req):
>      if req.env.get('HTTP_IF_NONE_MATCH') == tag:
>          raise ErrorResponse(HTTP_NOT_MODIFIED)
>      req.headers.append(('ETag', tag))
> +
> +def cspvalues(ui):
> +    """Obtain the Content-Security-Policy header and nonce value.
> +
> +    Returns a 2-tuple of the CSP header value and the nonce value.
> +
> +    First value is ``None`` if CSP isn't enabled. Second value is ``None``
> +    if CSP isn't enabled or if the CSP header doesn't need a nonce.
> +    """
> +    # Don't allow untrusted CSP setting since it be disable protections
> +    # from a trusted/global source.
> +    csp = ui.config('web', 'csp', untrusted=False)
> +    nonce = None
> +
> +    if csp and '%nonce%' in csp:

Since we just talked about this recently, should we test 'if csp is not
None'?
Gregory Szorc - Jan. 12, 2017, 12:18 a.m.
On Wed, Jan 11, 2017 at 10:53 AM, Sean Farley <sean@farley.io> wrote:

> Gregory Szorc <gregory.szorc@gmail.com> writes:
>
> > # HG changeset patch
> > # User Gregory Szorc <gregory.szorc@gmail.com>
> > # Date 1484120228 28800
> > #      Tue Jan 10 23:37:08 2017 -0800
> > # Node ID 113293954736e020d29e8e48aa3e01657ec853f3
> > # Parent  79314c9a79b3aa033b6f79d066b97d7157ecac33
> > hgweb: support Content Security Policy
> >
> > Content-Security-Policy (CSP) is a web security feature that allows
> > servers to declare what loaded content is allowed to do. For example,
> > a policy can prevent loading of images, JavaScript, CSS, etc unless
> > the source of that content is whitelisted (by hostname, URI scheme,
> > hashes of content, etc). It's a nifty security feature that provides
> > extra mitigation against some attacks, notably XSS.
> >
> > Mitigation against these attacks is important for Mercurial because
> > hgweb renders repository data, which is commonly untrusted. While we
> > make attempts to escape things, etc, there's the possibility that
> > malicious data could be injected into the site content. If this happens
> > today, the full power of the web browser is available to that
> > malicious content. A restrictive CSP policy (defined by the server
> > operator and sent in an HTTP header which is outside the control of
> > malicious content), could restrict browser capabilities and mitigate
> > security problems posed by malicious data.
> >
> > CSP works by emitting an HTTP header declaring the policy that browsers
> > should apply. Ideally, this header would be emitted by a layer above
> > Mercurial (likely the HTTP server doing the WSGI "proxying"). This
> > works for some CSP policies, but not all.
> >
> > For example, policies to allow inline JavaScript may require setting
> > a "nonce" attribute on <script>. This attribute value must be unique
> > and non-guessable. And, the value must be present in the HTTP header
> > and the HTML body. This means that coordinating the value between
> > Mercurial and another HTTP server could be difficult: it is much
> > easier to generate and emit the nonce in a central location.
> >
> > This commit introduces support for emitting a
> > Content-Security-Policy header from hgweb. A config option defines
> > the header value. If present, the header is emitted. A special
> > "%nonce%" syntax in the value triggers generation of a nonce and
> > inclusion in <script> elements in templates. The inclusion of a
> > nonce does not occur unless "%nonce%" is present. This makes this
> > commit completely backwards compatible and the feature opt-in.
> >
> > The nonce is a type 4 UUID, which is the flavor that is randomly
> > generated. It has 122 random bits, which should be plenty to satisfy
> > the guarantees of a nonce.
>
> Looks pretty good to me. I'd appreciate if someone else with web server
> experience to gave it a look over. One small nit below.
>
> > diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
> > --- a/mercurial/help/config.txt
> > +++ b/mercurial/help/config.txt
> > @@ -2084,6 +2084,20 @@ The full set of options is:
> >      Name or email address of the person in charge of the repository.
> >      (default: ui.username or ``$EMAIL`` or "unknown" if unset or empty)
> >
> > +``csp``
> > +    Send a ``Content-Security-Policy`` HTTP header with this value.
> > +
> > +    The value may contain a special string ``%nonce%``, which will be
> replaced
> > +    by a randomly-generated one-time use value. If the value contains
> > +    ``%nonce%``, ``web.cache`` will be disabled, as caching undermines
> the
> > +    one-time property of the nonce. This nonce will also be inserted
> into
> > +    ``<script>`` elements containing inline JavaScript.
> > +
> > +    Note: lots of HTML content sent by the server is derived from
> repository
> > +    data. Please consider the potential for malicious repository data to
> > +    "inject" itself into generated HTML content as part of your security
> > +    threat model.
> > +
> >  ``deny_push``
> >      Whether to deny pushing to the repository. If empty or not set,
> >      push is not denied. If the special value ``*``, all remote users are
> > diff --git a/mercurial/hgweb/common.py b/mercurial/hgweb/common.py
> > --- a/mercurial/hgweb/common.py
> > +++ b/mercurial/hgweb/common.py
> > @@ -8,9 +8,11 @@
> >
> >  from __future__ import absolute_import
> >
> > +import base64
> >  import errno
> >  import mimetypes
> >  import os
> > +import uuid
> >
> >  from .. import (
> >      encoding,
> > @@ -199,3 +201,22 @@ def caching(web, req):
> >      if req.env.get('HTTP_IF_NONE_MATCH') == tag:
> >          raise ErrorResponse(HTTP_NOT_MODIFIED)
> >      req.headers.append(('ETag', tag))
> > +
> > +def cspvalues(ui):
> > +    """Obtain the Content-Security-Policy header and nonce value.
> > +
> > +    Returns a 2-tuple of the CSP header value and the nonce value.
> > +
> > +    First value is ``None`` if CSP isn't enabled. Second value is
> ``None``
> > +    if CSP isn't enabled or if the CSP header doesn't need a nonce.
> > +    """
> > +    # Don't allow untrusted CSP setting since it be disable protections
> > +    # from a trusted/global source.
> > +    csp = ui.config('web', 'csp', untrusted=False)
> > +    nonce = None
> > +
> > +    if csp and '%nonce%' in csp:
>
> Since we just talked about this recently, should we test 'if csp is not
> None'?
>

I'm not sure exactly which conversation you are referring to, but
ui.config() will return a string or None. A Python string can only be true
if it is non-empty. So an explicit identity check against None should not
be necessary (and is unPythonic).

(Or am I missing a scenario where ui.config() returns something other than
a string or None?)
Augie Fackler - Jan. 12, 2017, 3:05 a.m.
On Tue, Jan 10, 2017 at 11:41:10PM -0800, Gregory Szorc wrote:
> # HG changeset patch
> # User Gregory Szorc <gregory.szorc@gmail.com>
> # Date 1484120228 28800
> #      Tue Jan 10 23:37:08 2017 -0800
> # Node ID 113293954736e020d29e8e48aa3e01657ec853f3
> # Parent  79314c9a79b3aa033b6f79d066b97d7157ecac33
> hgweb: support Content Security Policy

Queued this, thanks. Sometimes I wonder if http and html need to die
when I see stuff like this.
Pierre-Yves David - Jan. 12, 2017, 11:45 a.m.
On 01/12/2017 01:18 AM, Gregory Szorc wrote:
> On Wed, Jan 11, 2017 at 10:53 AM, Sean Farley <sean@farley.io
> <mailto:sean@farley.io>> wrote:
>
>     Gregory Szorc <gregory.szorc@gmail.com
>     <mailto:gregory.szorc@gmail.com>> writes:
>
>     > # HG changeset patch
>     > # User Gregory Szorc <gregory.szorc@gmail.com
>     <mailto:gregory.szorc@gmail.com>>
>     > # Date 1484120228 28800
>     > #      Tue Jan 10 23:37:08 2017 -0800
>     > # Node ID 113293954736e020d29e8e48aa3e01657ec853f3
>     > # Parent  79314c9a79b3aa033b6f79d066b97d7157ecac33
>     > hgweb: support Content Security Policy
>     >
>     > Content-Security-Policy (CSP) is a web security feature that allows
>     > servers to declare what loaded content is allowed to do. For example,
>     > a policy can prevent loading of images, JavaScript, CSS, etc unless
>     > the source of that content is whitelisted (by hostname, URI scheme,
>     > hashes of content, etc). It's a nifty security feature that provides
>     > extra mitigation against some attacks, notably XSS.
>     >
>     > Mitigation against these attacks is important for Mercurial because
>     > hgweb renders repository data, which is commonly untrusted. While we
>     > make attempts to escape things, etc, there's the possibility that
>     > malicious data could be injected into the site content. If this
>     happens
>     > today, the full power of the web browser is available to that
>     > malicious content. A restrictive CSP policy (defined by the server
>     > operator and sent in an HTTP header which is outside the control of
>     > malicious content), could restrict browser capabilities and mitigate
>     > security problems posed by malicious data.
>     >
>     > CSP works by emitting an HTTP header declaring the policy that
>     browsers
>     > should apply. Ideally, this header would be emitted by a layer above
>     > Mercurial (likely the HTTP server doing the WSGI "proxying"). This
>     > works for some CSP policies, but not all.
>     >
>     > For example, policies to allow inline JavaScript may require setting
>     > a "nonce" attribute on <script>. This attribute value must be unique
>     > and non-guessable. And, the value must be present in the HTTP header
>     > and the HTML body. This means that coordinating the value between
>     > Mercurial and another HTTP server could be difficult: it is much
>     > easier to generate and emit the nonce in a central location.
>     >
>     > This commit introduces support for emitting a
>     > Content-Security-Policy header from hgweb. A config option defines
>     > the header value. If present, the header is emitted. A special
>     > "%nonce%" syntax in the value triggers generation of a nonce and
>     > inclusion in <script> elements in templates. The inclusion of a
>     > nonce does not occur unless "%nonce%" is present. This makes this
>     > commit completely backwards compatible and the feature opt-in.
>     >
>     > The nonce is a type 4 UUID, which is the flavor that is randomly
>     > generated. It has 122 random bits, which should be plenty to satisfy
>     > the guarantees of a nonce.
>
>     Looks pretty good to me. I'd appreciate if someone else with web server
>     experience to gave it a look over. One small nit below.
>
>     > diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
>     > --- a/mercurial/help/config.txt
>     > +++ b/mercurial/help/config.txt
>     > @@ -2084,6 +2084,20 @@ The full set of options is:
>     >      Name or email address of the person in charge of the repository.
>     >      (default: ui.username or ``$EMAIL`` or "unknown" if unset or
>     empty)
>     >
>     > +``csp``
>     > +    Send a ``Content-Security-Policy`` HTTP header with this value.
>     > +
>     > +    The value may contain a special string ``%nonce%``, which
>     will be replaced
>     > +    by a randomly-generated one-time use value. If the value contains
>     > +    ``%nonce%``, ``web.cache`` will be disabled, as caching
>     undermines the
>     > +    one-time property of the nonce. This nonce will also be
>     inserted into
>     > +    ``<script>`` elements containing inline JavaScript.
>     > +
>     > +    Note: lots of HTML content sent by the server is derived from
>     repository
>     > +    data. Please consider the potential for malicious repository
>     data to
>     > +    "inject" itself into generated HTML content as part of your
>     security
>     > +    threat model.
>     > +
>     >  ``deny_push``
>     >      Whether to deny pushing to the repository. If empty or not set,
>     >      push is not denied. If the special value ``*``, all remote
>     users are
>     > diff --git a/mercurial/hgweb/common.py b/mercurial/hgweb/common.py
>     > --- a/mercurial/hgweb/common.py
>     > +++ b/mercurial/hgweb/common.py
>     > @@ -8,9 +8,11 @@
>     >
>     >  from __future__ import absolute_import
>     >
>     > +import base64
>     >  import errno
>     >  import mimetypes
>     >  import os
>     > +import uuid
>     >
>     >  from .. import (
>     >      encoding,
>     > @@ -199,3 +201,22 @@ def caching(web, req):
>     >      if req.env.get('HTTP_IF_NONE_MATCH') == tag:
>     >          raise ErrorResponse(HTTP_NOT_MODIFIED)
>     >      req.headers.append(('ETag', tag))
>     > +
>     > +def cspvalues(ui):
>     > +    """Obtain the Content-Security-Policy header and nonce value.
>     > +
>     > +    Returns a 2-tuple of the CSP header value and the nonce value.
>     > +
>     > +    First value is ``None`` if CSP isn't enabled. Second value is
>     ``None``
>     > +    if CSP isn't enabled or if the CSP header doesn't need a nonce.
>     > +    """
>     > +    # Don't allow untrusted CSP setting since it be disable
>     protections
>     > +    # from a trusted/global source.
>     > +    csp = ui.config('web', 'csp', untrusted=False)
>     > +    nonce = None
>     > +
>     > +    if csp and '%nonce%' in csp:
>
>     Since we just talked about this recently, should we test 'if csp is not
>     None'?
>
>
> I'm not sure exactly which conversation you are referring to, but
> ui.config() will return a string or None. A Python string can only be
> true if it is non-empty. So an explicit identity check against None
> should not be necessary

If '' (empty string) is a valid value, equivalent to None, boolean 
checks seems fine.
However if the string is empty, the 'in' check will fails, so it might 
be useful to switch the first check to explicit None to make the 
possibility of a None clear.

> (and is unPythonic).

I'm not certain what you exactly meant here, but for clarity, let me 
remind that explicit identity check is the Pythonic way to check for 
None. Doing otherwise might lead to bugs. this was brought up in a 
couple of discussion recently including your series about mutable 
default value.

Quoting PEP8: https://www.python.org/dev/peps/pep-0008/

>> Comparisons to singletons like None should always be done with is or is
>> not , never the equality operators.
>>
>> Also, beware of writing if x when you really mean if x is not None --
>> e.g. when testing whether a variable or argument that defaults to None
>> was set to some other value. The other value might have a type (such as
>> a container) that could be false in a boolean context!


> (Or am I missing a scenario where ui.config() returns something other
> than a string or None?)

Cheers,

Patch

diff --git a/mercurial/help/config.txt b/mercurial/help/config.txt
--- a/mercurial/help/config.txt
+++ b/mercurial/help/config.txt
@@ -2084,6 +2084,20 @@  The full set of options is:
     Name or email address of the person in charge of the repository.
     (default: ui.username or ``$EMAIL`` or "unknown" if unset or empty)
 
+``csp``
+    Send a ``Content-Security-Policy`` HTTP header with this value.
+
+    The value may contain a special string ``%nonce%``, which will be replaced
+    by a randomly-generated one-time use value. If the value contains
+    ``%nonce%``, ``web.cache`` will be disabled, as caching undermines the
+    one-time property of the nonce. This nonce will also be inserted into
+    ``<script>`` elements containing inline JavaScript.
+
+    Note: lots of HTML content sent by the server is derived from repository
+    data. Please consider the potential for malicious repository data to
+    "inject" itself into generated HTML content as part of your security
+    threat model.
+
 ``deny_push``
     Whether to deny pushing to the repository. If empty or not set,
     push is not denied. If the special value ``*``, all remote users are
diff --git a/mercurial/hgweb/common.py b/mercurial/hgweb/common.py
--- a/mercurial/hgweb/common.py
+++ b/mercurial/hgweb/common.py
@@ -8,9 +8,11 @@ 
 
 from __future__ import absolute_import
 
+import base64
 import errno
 import mimetypes
 import os
+import uuid
 
 from .. import (
     encoding,
@@ -199,3 +201,22 @@  def caching(web, req):
     if req.env.get('HTTP_IF_NONE_MATCH') == tag:
         raise ErrorResponse(HTTP_NOT_MODIFIED)
     req.headers.append(('ETag', tag))
+
+def cspvalues(ui):
+    """Obtain the Content-Security-Policy header and nonce value.
+
+    Returns a 2-tuple of the CSP header value and the nonce value.
+
+    First value is ``None`` if CSP isn't enabled. Second value is ``None``
+    if CSP isn't enabled or if the CSP header doesn't need a nonce.
+    """
+    # Don't allow untrusted CSP setting since it be disable protections
+    # from a trusted/global source.
+    csp = ui.config('web', 'csp', untrusted=False)
+    nonce = None
+
+    if csp and '%nonce%' in csp:
+        nonce = base64.urlsafe_b64encode(uuid.uuid4().bytes).rstrip('=')
+        csp = csp.replace('%nonce%', nonce)
+
+    return csp, nonce
diff --git a/mercurial/hgweb/hgweb_mod.py b/mercurial/hgweb/hgweb_mod.py
--- a/mercurial/hgweb/hgweb_mod.py
+++ b/mercurial/hgweb/hgweb_mod.py
@@ -19,6 +19,7 @@  from .common import (
     HTTP_OK,
     HTTP_SERVER_ERROR,
     caching,
+    cspvalues,
     permhooks,
 )
 from .request import wsgirequest
@@ -109,6 +110,8 @@  class requestcontext(object):
         # of the request.
         self.websubtable = app.websubtable
 
+        self.csp, self.nonce = cspvalues(self.repo.ui)
+
     # Trust the settings from the .hg/hgrc files by default.
     def config(self, section, name, default=None, untrusted=True):
         return self.repo.ui.config(section, name, default,
@@ -202,6 +205,7 @@  class requestcontext(object):
             'sessionvars': sessionvars,
             'pathdef': makebreadcrumb(req.url),
             'style': style,
+            'nonce': self.nonce,
         }
         tmpl = templater.templater.frommapfile(mapfile,
                                                filters={'websub': websubfilter},
@@ -319,6 +323,13 @@  class hgweb(object):
         encoding.encoding = rctx.config('web', 'encoding', encoding.encoding)
         rctx.repo.ui.environ = req.env
 
+        if rctx.csp:
+            # hgwebdir may have added CSP header. Since we generate our own,
+            # replace it.
+            req.headers = [h for h in req.headers
+                           if h[0] != 'Content-Security-Policy']
+            req.headers.append(('Content-Security-Policy', rctx.csp))
+
         # work with CGI variables to create coherent structure
         # use SCRIPT_NAME, PATH_INFO and QUERY_STRING as well as our REPO_NAME
 
@@ -415,7 +426,9 @@  class hgweb(object):
                 req.form['cmd'] = [tmpl.cache['default']]
                 cmd = req.form['cmd'][0]
 
-            if rctx.configbool('web', 'cache', True):
+            # Don't enable caching if using a CSP nonce because then it wouldn't
+            # be a nonce.
+            if rctx.configbool('web', 'cache', True) and not rctx.nonce:
                 caching(self, req) # sets ETag header or raises NOT_MODIFIED
             if cmd not in webcommands.__all__:
                 msg = 'no such method: %s' % cmd
diff --git a/mercurial/hgweb/hgwebdir_mod.py b/mercurial/hgweb/hgwebdir_mod.py
--- a/mercurial/hgweb/hgwebdir_mod.py
+++ b/mercurial/hgweb/hgwebdir_mod.py
@@ -19,6 +19,7 @@  from .common import (
     HTTP_NOT_FOUND,
     HTTP_OK,
     HTTP_SERVER_ERROR,
+    cspvalues,
     get_contact,
     get_mtime,
     ismember,
@@ -227,8 +228,12 @@  class hgwebdir(object):
         try:
             self.refresh()
 
+            csp, nonce = cspvalues(self.ui)
+            if csp:
+                req.headers.append(('Content-Security-Policy', csp))
+
             virtual = req.env.get("PATH_INFO", "").strip('/')
-            tmpl = self.templater(req)
+            tmpl = self.templater(req, nonce)
             ctype = tmpl('mimetype', encoding=encoding.encoding)
             ctype = templater.stringify(ctype)
 
@@ -466,7 +471,7 @@  class hgwebdir(object):
                     sortcolumn=sortcolumn, descending=descending,
                     **dict(sort))
 
-    def templater(self, req):
+    def templater(self, req, nonce):
 
         def motd(**map):
             if self.motd is not None:
@@ -510,6 +515,7 @@  class hgwebdir(object):
             "staticurl": staticurl,
             "sessionvars": sessionvars,
             "style": style,
+            "nonce": nonce,
         }
         tmpl = templater.templater.frommapfile(mapfile, defaults=defaults)
         return tmpl
diff --git a/mercurial/templates/gitweb/graph.tmpl b/mercurial/templates/gitweb/graph.tmpl
--- a/mercurial/templates/gitweb/graph.tmpl
+++ b/mercurial/templates/gitweb/graph.tmpl
@@ -45,7 +45,7 @@  graph |
 <ul id="graphnodes"></ul>
 </div>
 
-<script>
+<script{if(nonce, ' nonce="{nonce}"')}>
 <!-- hide script content
 
 var data = {jsdata|json};
@@ -108,7 +108,7 @@  graph.render(data);
 | {changenav%navgraph}
 </div>
 
-<script type="text/javascript">
+<script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
     ajaxScrollInit(
             '{url|urlescape}graph/{rev}?revcount=%next%&style={style}',
             {revcount}+60,
diff --git a/mercurial/templates/gitweb/shortlog.tmpl b/mercurial/templates/gitweb/shortlog.tmpl
--- a/mercurial/templates/gitweb/shortlog.tmpl
+++ b/mercurial/templates/gitweb/shortlog.tmpl
@@ -40,7 +40,7 @@  shortlog |
 {changenav%navshort}
 </div>
 
-<script type="text/javascript">
+<script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
     ajaxScrollInit(
             '{url|urlescape}shortlog/%next%{sessionvars%urlparameter}',
             '{nextentry%"{node}"}', <!-- NEXTHASH
diff --git a/mercurial/templates/monoblue/graph.tmpl b/mercurial/templates/monoblue/graph.tmpl
--- a/mercurial/templates/monoblue/graph.tmpl
+++ b/mercurial/templates/monoblue/graph.tmpl
@@ -40,7 +40,7 @@ 
         <ul id="graphnodes"></ul>
     </div>
 
-    <script>
+    <script{if(nonce, ' nonce="{nonce}"')}>
     <!-- hide script content
 
     document.getElementById('noscript').style.display = 'none';
@@ -104,7 +104,7 @@ 
         | {changenav%navgraph}
     </div>
 
-    <script type="text/javascript">
+    <script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
     ajaxScrollInit(
             '{url|urlescape}graph/{rev}?revcount=%next%&style={style}',
             {revcount}+60,
diff --git a/mercurial/templates/monoblue/shortlog.tmpl b/mercurial/templates/monoblue/shortlog.tmpl
--- a/mercurial/templates/monoblue/shortlog.tmpl
+++ b/mercurial/templates/monoblue/shortlog.tmpl
@@ -41,7 +41,7 @@ 
     {changenav%navshort}
     </div>
 
-    <script type="text/javascript">
+    <script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
     ajaxScrollInit(
             '{url|urlescape}shortlog/%next%{sessionvars%urlparameter}',
             '{nextentry%"{node}"}', <!-- NEXTHASH
diff --git a/mercurial/templates/paper/graph.tmpl b/mercurial/templates/paper/graph.tmpl
--- a/mercurial/templates/paper/graph.tmpl
+++ b/mercurial/templates/paper/graph.tmpl
@@ -59,7 +59,7 @@ 
 <ul id="graphnodes"></ul>
 </div>
 
-<script type="text/javascript">
+<script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
 <!-- hide script content
 
 var data = {jsdata|json};
@@ -121,7 +121,7 @@  graph.render(data);
 | rev {rev}: {changenav%navgraph}
 </div>
 
-<script type="text/javascript">
+<script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
     ajaxScrollInit(
             '{url|urlescape}graph/{rev}?revcount=%next%&style={style}',
             {revcount}+60,
diff --git a/mercurial/templates/paper/shortlog.tmpl b/mercurial/templates/paper/shortlog.tmpl
--- a/mercurial/templates/paper/shortlog.tmpl
+++ b/mercurial/templates/paper/shortlog.tmpl
@@ -72,7 +72,7 @@ 
 | rev {rev}: {changenav%navshort}
 </div>
 
-<script type="text/javascript">
+<script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
     ajaxScrollInit(
             '{url|urlescape}shortlog/%next%{sessionvars%urlparameter}',
             '{nextentry%"{node}"}', <!-- NEXTHASH
diff --git a/mercurial/templates/spartan/graph.tmpl b/mercurial/templates/spartan/graph.tmpl
--- a/mercurial/templates/spartan/graph.tmpl
+++ b/mercurial/templates/spartan/graph.tmpl
@@ -36,7 +36,7 @@  navigate: <small class="navigate">{chang
 <ul id="graphnodes"></ul>
 </div>
 
-<script type="text/javascript">
+<script type="text/javascript"{if(nonce, ' nonce="{nonce}"')}>
 <!-- hide script content
 
 var data = {jsdata|json};
diff --git a/tests/test-hgweb-csp.t b/tests/test-hgweb-csp.t
new file mode 100644
--- /dev/null
+++ b/tests/test-hgweb-csp.t
@@ -0,0 +1,129 @@ 
+#require serve
+
+  $ cat > web.conf << EOF
+  > [paths]
+  > / = $TESTTMP/*
+  > EOF
+
+  $ hg init repo1
+  $ cd repo1
+  $ touch foo
+  $ hg -q commit -A -m initial
+  $ cd ..
+
+  $ hg serve -p $HGPORT -d --pid-file=hg.pid --web-conf web.conf
+  $ cat hg.pid >> $DAEMON_PIDS
+
+repo index should not send Content-Security-Policy header by default
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT '' content-security-policy etag
+  200 Script output follows
+
+static page should not send CSP by default
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT static/mercurial.js content-security-policy etag
+  200 Script output follows
+
+repo page should not send CSP by default, should send ETag
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT repo1 content-security-policy etag
+  200 Script output follows
+  etag: W/"*" (glob)
+
+  $ killdaemons.py
+
+Configure CSP without nonce
+
+  $ cat >> web.conf << EOF
+  > [web]
+  > csp = script-src https://example.com/ 'unsafe-inline'
+  > EOF
+
+  $ hg serve -p $HGPORT -d --pid-file=hg.pid --web-conf web.conf
+  $ cat hg.pid > $DAEMON_PIDS
+
+repo index should send Content-Security-Policy header when enabled
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT '' content-security-policy etag
+  200 Script output follows
+  content-security-policy: script-src https://example.com/ 'unsafe-inline'
+
+static page should send CSP when enabled
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT static/mercurial.js content-security-policy etag
+  200 Script output follows
+  content-security-policy: script-src https://example.com/ 'unsafe-inline'
+
+repo page should send CSP by default, include etag w/o nonce
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT repo1 content-security-policy etag
+  200 Script output follows
+  content-security-policy: script-src https://example.com/ 'unsafe-inline'
+  etag: W/"*" (glob)
+
+nonce should not be added to html if CSP doesn't use it
+
+  $ get-with-headers.py localhost:$HGPORT repo1/graph/tip | egrep 'content-security-policy|<script'
+  <script type="text/javascript" src="/repo1/static/mercurial.js"></script>
+  <!--[if IE]><script type="text/javascript" src="/repo1/static/excanvas.js"></script><![endif]-->
+  <script type="text/javascript">
+  <script type="text/javascript">
+
+Configure CSP with nonce
+
+  $ killdaemons.py
+  $ cat >> web.conf << EOF
+  > csp = image-src 'self'; script-src https://example.com/ 'nonce-%nonce%'
+  > EOF
+
+  $ hg serve -p $HGPORT -d --pid-file=hg.pid --web-conf web.conf
+  $ cat hg.pid > $DAEMON_PIDS
+
+nonce should be substituted in CSP header
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT '' content-security-policy etag
+  200 Script output follows
+  content-security-policy: image-src 'self'; script-src https://example.com/ 'nonce-*' (glob)
+
+nonce should be included in CSP for static pages
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT static/mercurial.js content-security-policy etag
+  200 Script output follows
+  content-security-policy: image-src 'self'; script-src https://example.com/ 'nonce-*' (glob)
+
+repo page should have nonce, no ETag
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT repo1 content-security-policy etag
+  200 Script output follows
+  content-security-policy: image-src 'self'; script-src https://example.com/ 'nonce-*' (glob)
+
+nonce should be added to html when used
+
+  $ get-with-headers.py localhost:$HGPORT repo1/graph/tip content-security-policy | egrep 'content-security-policy|<script'
+  content-security-policy: image-src 'self'; script-src https://example.com/ 'nonce-*' (glob)
+  <script type="text/javascript" src="/repo1/static/mercurial.js"></script>
+  <!--[if IE]><script type="text/javascript" src="/repo1/static/excanvas.js"></script><![endif]-->
+  <script type="text/javascript" nonce="*"> (glob)
+  <script type="text/javascript" nonce="*"> (glob)
+
+hgweb_mod w/o hgwebdir works as expected
+
+  $ killdaemons.py
+
+  $ hg -R repo1 serve -p $HGPORT -d --pid-file=hg.pid --config "web.csp=image-src 'self'; script-src https://example.com/ 'nonce-%nonce%'"
+  $ cat hg.pid > $DAEMON_PIDS
+
+static page sends CSP
+
+  $ get-with-headers.py --headeronly localhost:$HGPORT static/mercurial.js content-security-policy etag
+  200 Script output follows
+  content-security-policy: image-src 'self'; script-src https://example.com/ 'nonce-*' (glob)
+
+nonce included in <script> and headers
+
+  $ get-with-headers.py localhost:$HGPORT graph/tip content-security-policy  | egrep 'content-security-policy|<script'
+  content-security-policy: image-src 'self'; script-src https://example.com/ 'nonce-*' (glob)
+  <script type="text/javascript" src="/static/mercurial.js"></script>
+  <!--[if IE]><script type="text/javascript" src="/static/excanvas.js"></script><![endif]-->
+  <script type="text/javascript" nonce="*"> (glob)
+  <script type="text/javascript" nonce="*"> (glob)