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hgweb: support Content Security Policy...
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.
Gregory Szorc - - Load All Authors

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test-hgweb-csp.t
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#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)