Summary
Rack::Directory generates an HTML directory index where each file entry is rendered as a clickable link. If a file exists on disk whose basename begins with the javascript: scheme (e.g. javascript:alert(1)), the generated index includes an anchor whose href attribute is exactly javascript:alert(1). Clicking this entry executes arbitrary JavaScript in the context of the hosting application.
This results in a client-side XSS condition in directory listings generated by Rack::Directory.
Details
Rack::Directory renders directory entries using an HTML row template similar to:
The %s placeholder is populated directly with the file’s basename. If the basename begins with javascript:, the resulting HTML contains an executable JavaScript URL:
<a href='javascript:alert(1)'>javascript:alert(1)</a>
Because the value is inserted directly into the href attribute without scheme validation or normalization, browsers interpret it as a JavaScript URI. When a user clicks the link, the JavaScript executes in the origin of the Rack application.
Impact
If Rack::Directory is used to expose filesystem contents over HTTP, an attacker who can create or upload files within that directory may introduce a malicious filename beginning with javascript:.
When a user visits the directory listing and clicks the entry, arbitrary JavaScript executes in the application's origin. Exploitation requires user interaction (clicking the malicious entry).
Mitigation
- Update to a patched version of Rack in which
Rack::Directory prefixes generated anchors with a relative path indicator (e.g. ./filename).
- Avoid exposing user-controlled directories via
Rack::Directory.
- Apply a strict Content Security Policy (CSP) to reduce impact of potential client-side execution issues.
- Where feasible, restrict or sanitize uploaded filenames to disallow dangerous URI scheme prefixes.
HackerOne profile:
https://hackerone.com/thesmartshadow
GitHub account owner:
Ali Firas (@thesmartshadow)
References
thesmartshadow Reporter
jeremyevans Remediation developer
ioquatix Coordinator
Summary
Rack::Directorygenerates an HTML directory index where each file entry is rendered as a clickable link. If a file exists on disk whose basename begins with thejavascript:scheme (e.g.javascript:alert(1)), the generated index includes an anchor whosehrefattribute is exactlyjavascript:alert(1). Clicking this entry executes arbitrary JavaScript in the context of the hosting application.This results in a client-side XSS condition in directory listings generated by
Rack::Directory.Details
Rack::Directoryrenders directory entries using an HTML row template similar to:The
%splaceholder is populated directly with the file’s basename. If the basename begins withjavascript:, the resulting HTML contains an executable JavaScript URL:Because the value is inserted directly into the
hrefattribute without scheme validation or normalization, browsers interpret it as a JavaScript URI. When a user clicks the link, the JavaScript executes in the origin of the Rack application.Impact
If
Rack::Directoryis used to expose filesystem contents over HTTP, an attacker who can create or upload files within that directory may introduce a malicious filename beginning withjavascript:.When a user visits the directory listing and clicks the entry, arbitrary JavaScript executes in the application's origin. Exploitation requires user interaction (clicking the malicious entry).
Mitigation
Rack::Directoryprefixes generated anchors with a relative path indicator (e.g../filename).Rack::Directory.HackerOne profile:
https://hackerone.com/thesmartshadow
GitHub account owner:
Ali Firas (@thesmartshadow)
References