CVE-2026-25500
Stored XSS in Rack::Directory via javascript: filenames rendered into anchor href
Impact
## 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: ```html <a href='%s'>%s</a> ``` 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: ```html <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)
Affected packages
| Ecosystem | Package | Affected | Fixed in |
|---|---|---|---|
| rubygems | rack | < 2.2.22 | 2.2.22 |
Affected products
| Vendor | Product | Affected versions |
|---|---|---|
| Rack | Rack | before 2.2.22 from 3.0.0 before 3.1.20 from 3.2.0 before 3.2.5 |
Weakness classification
References
- NVD record
- CVE.org record
- GitHub advisory
- https://github.com/rack/rack/commit/f2f225f297b99fbee3d9f51255d41f601fc40aff
- https://github.com/rack/rack/security/advisories/GHSA-whrj-4476-wvmp
- https://github.com/advisories/GHSA-whrj-4476-wvmp
- https://nvd.nist.gov/vuln/detail/CVE-2026-25500
- https://github.com/rubysec/ruby-advisory-db/blob/master/gems/rack/CVE-2026-25500.yml