Google Cloud Shell is a product of the Google Cloud Platform (GCP) that provides a number of interesting features, such as an interactive shell environment and an integrated code editor, with the intent of making experimenting and administration of GCP products easier while requiring very little setup.
As part of the Google Vulnerability Reward Program (VRP) I discovered that the code editor of the Cloud Shell was vulnerable to Cross-Site Scripting (XSS) embedded into Scalable Vector Graphics (SVG) files.
While experimenting with the code editor I was curious to see how it would handle SVG files, which are XML-based vector image files. I decided to download a simple SVG file to the Cloud Shell environment and then clicked on the file in the editor's file navigator. Interestingly the file was not opened in edit mode but rather in preview mode, that is the file was rendered by my browser as an image.
Figure 1: A SVG file previewed inside the editor.
A little-known detail of the SVG specification is its support for scripting. While the HTML standard disallows the
execution of such scripts in certain contexts such as the
img element it allows script execution in other contexts
such as the
iframe element unless explicitly disallowed a mechanism known as sandboxing which can be activated by the
sandbox attribute of an iframe or by the
sandbox directive of a Content Security Policy (CSP).
Being aware of this I decided to check how the image was embedded into the editor. It turned out that the image was
included using an
iframe element with a
sandbox attribute containing, amongst others, the
allow-scripts keywords, which lifts certain sandbox restrictions and thus should effectively allow the execution
of arbitrary scripts embedded into the SVG file. As a matter of fact, the listed attributes were the default sandbox
options of the mini-browser extension of the Theia IDE, upon which Google Cloud Shell's code editor is based.
Figure 2: The SVG file being embedded using an iframe.
To test my assumptions I modified the SVG file so that when loaded it would simply show the file's location in case the script gets executed. I then opened the file by clicking on it, which confirmed that the code editor was vulnerable to XSS embedded into SVG files.
Figure 3: A SVG file whose embedded script gets executed when previewed inside the editor.
For the described vulnerability to be not only of theoretical interest the impact of a potential attack must be non-negligible and the potential attack must not be merely self-XSS.
The impact of a potential attack is determined by the domain in whose context the XSS payload would be executed. As can
be seen the domain would be of the form
970-dot-123456789-dot-devshell.appspot.com. Further analysis revealed that
the domain name varies for every Cloud Shell user. Apparently a unique ID is assigned to each Cloud Shell user and the
970-dot-$ID-dot-devshell.appspot.com, which is only accessible by the associated user, is used to address the
backend of that user's Cloud Shell code editor. Each Cloud Shell user is assigned an isolated virtual machine where
this and other Cloud Shell backend services are hosted.
Consequently, the code of the XSS payload would be able to unrestrictedly communicate with the editor's backend, which in particular allows executing arbitrary commands on the assigned user's virtual machine. A potential attacker thus could obtain access to a Cloud Shell user's GCP resources as the virtual machine is automatically authorized to access these resources.
For a potential attacker to exploit the described vulnerability they would have to trick a Cloud Shell user to put a malicious SVG file in their Cloud Shell environment and subsequently click on the file in Cloud Shell's editor. Google Cloud Shell has a feature to create a link that, when clicked, opens Cloud Shell and clones a specified git repository from GitHub or Bitbucket. Additionally, a tutorial file to be displayed to the user after cloning the repository can be specified in the link.
Using this feature, an attacker could put a malicious SVG file inside an interesting looking repository and also include a tutorial in which the Cloud Shell user would be instructed to take a look at the new logo by clicking on the SVG file. The "Open in Cloud Shell" link for cloning the repository and opening the tutorial could then be added to the repository's description.
After creating a proof of concept and reporting the vulnerability to Google it was fixed. This was done by overriding the editor's mini-browser's default sandbox options as follows, thus effectively serving files in iframes with completely restrictive iframes:
(MiniBrowserProps.SandboxOptions.DEFAULT as any) = 
After the fix was applied, when trying to open a SVG file with an embedded script inside Cloud Shell's editor, as expected the code would be blocked instead of executed.
Figure 4: A SVG file whose embedded script gets blocked by the iframe's sandbox when previewed inside the editor.
While this fix protects from the attack scenario described above, it would not be sufficient when a malicious SVG file,
or even HTML file, were to be opened inside its own browser window instead of inside the the sandboxed iframe. To
protect against such a scenario the files could be protected by the
sandbox directive of a CSP. More details about a
related vulnerability, which was fixed by applying this CSP directive to all endpoints of the editor's mini-browser, can
be found here.
Take away messages
- Even innocent-looking file formats such as image formats may contain potentially dangerous content. Google Cloud Shell editor had automatic preview mode only enabled for certain file formats like SVG, so the described attack scenario would not have worked with HTML files as these would be opened in edit mode instead.
- When integrating components, such as the Theia IDE, into larger projects, such as Google Cloud Shell, it is necessary to check whether the component's security related default settings, such as the default sandbox options, are also adequate within the context of the larger project or customization is required.
XSS in SVG
- a presentation about code execution in SVG files by Mario Heiderich
- an overview of options for protecting against XSS in SVG by @digininja
Google Cloud Shell architecture and further vulnerabilities in Google Cloud Shell
- a video by @LiveOverflow about the architecture and a vulnerability found by @wtm_offensi
- architecture overview and four more vulnerabilities by @wtm_offensi
- a XSS vulnerability in Google Cloud Shell's code editor through mini-browser endpoint
- unrelated vulnerabilites in Google Cloud Shell