medium

CVE-2026-52816

Go · gogs.io/gogs

Summary

Gogs's Unauthenticated Jupyter Notebook (ipynb) Sanitizer allows arbitrary data: URIs leading to XSS

Severity
medium
EPSS
0.7% (p50)
CWE
CWE-79, CWE-80
Also known as
GHSA-3w28-36p9-w929
Published
2026-06-23
Updated
2026-06-23

Advisory details

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

Impact

An attacker with a registered user account can:

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:

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:

<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

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

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:

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:

m.Post("/sanitize_ipynb", middleware.signIn, app.SanitizeIpynb())

Option 3: Disable data URI scheme entirely

If data URIs are not required for ipynb sanitization:

// Remove this line entirely:
// p.AllowURLSchemes("data")

References

Related advisories

Is your project exposed to this? Stateward checks every dependency on every pull request and flags it only if your code actually reaches it.

Check my repo

Summarize with AI

ChatGPTClaudePerplexity

Sources: CISA KEV (public domain), OSV.dev & GitHub Advisory Database (CC-BY-4.0), FIRST EPSS, NVD/CWE (public domain). Served live from the Stateward advisory database.