Last updated 6 June 2026

Summary The "frames.html" file within the Yard Doc's generated documentation is vulnerable to Cross-Site Scripting (XSS) attacks due to inadequate sanitization of user input within the JavaScript segment of the "frames.erb" template file.

Details The vulnerability stems from mishandling user-controlled data retrieved from the URL hash in the embedded JavaScript code within the "frames.erb" template file. Specifically, the script lacks proper sanitization of the hash data before utilizing it to establish the top-level window's location. This oversight permits an attacker to inject malicious JavaScript payloads through carefully crafted URLs.

Snippet from "frames.erb": (v0.9.34) erb <script type="text/javascript"> var match = unescape(window.location.hash).match(/^#!(.+)/); var name = match ? match[1] : '<%= urlformain %>'; name = name.replace(/^(\w+):\/\//, '').replace(/^\/\//, ''); window.top.location = name; </script>

(v0.9.35) erb <script type="text/javascript"> var match = decodeURIComponent(window.location.hash).match(/^#!(.+)/); var name = match ? match[1] : '<%= urlformain %>'; name = name.replace(/^((\w):)?[\/\\]/gm, '').trim(); window.top.location.replace(name) </script>

PoC (Proof of Concept) To exploit this vulnerability: 1. Gain access to the generated Yard Doc. 2. Locate and access the "frames.html" file. 3. Construct a URL containing the malicious payload in the hash segment, for instance: #!javascript:xss for v0.9.34, and #:javascript:xss for v0.9.35

Impact This XSS vulnerability presents a substantial threat by enabling an attacker to execute arbitrary JavaScript code within the user's session context. Potential ramifications include session hijacking, theft of sensitive data, unauthorized access to user accounts, and defacement of websites. Any user visiting the compromised page is susceptible to exploitation. It is critical to promptly address this vulnerability to mitigate potential harm to users and preserve the application's integrity.

Possible arbitrary path traversal and file access via yard server

Impact

A path traversal vulnerability was discovered in YARD <= 0.9.19 when using yard server to serve documentation. This bug would allow unsanitized HTTP requests to access arbitrary files on the machine of a yard server host under certain conditions.

Thanks to CuongMX from Viettel Cyber Security for discovering this vulnerability.

Patches

Please upgrade to YARD v0.9.20 immediately if you are relying on yard server to host documentation in any untrusted environments.

Workarounds

For users who cannot upgrade, it is possible to perform path sanitization of HTTP requests at your webserver level. WEBrick, for example, can perform such sanitization by default (which you can use via yard server -s webrick), as can certain rules in your webserver configuration.

lib/yard/coreext/file.rb in the server in YARD before 0.9.11 does not block relative paths with an initial ../ sequence, which allows attackers to conduct directory traversal attacks and read arbitrary files.