CVE-2026-52816

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

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>

<a href="data:text/html,<script>alert(document.cookie)</script>">click</a>

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
}

m.Group("/-", func() {
	m.Get("/metrics", app.MetricsFilter(), promhttp.Handler())
	m.Group("/api", func() {
		m.Post("/sanitize_ipynb", app.SanitizeIpynb())  // <-- No auth middleware
	})
})

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

Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.

Kodem intelligence

Already deployed Kodem?

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")

1. What is CVE-2026-52816? CVE-2026-52816 is a medium-severity cross-site scripting (XSS) vulnerability in gogs.io/gogs (go), affecting versions < 0.14.3. It is fixed in 0.14.3. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
2. Which versions of gogs.io/gogs are affected by CVE-2026-52816? gogs.io/gogs (go) versions < 0.14.3 is affected.
3. Is there a fix for CVE-2026-52816? Yes. CVE-2026-52816 is fixed in 0.14.3. Upgrade to this version or later.
4. Is CVE-2026-52816 exploitable, and should I be worried? Whether CVE-2026-52816 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-52816 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-52816? Upgrade gogs.io/gogs to 0.14.3 or later.

Other vulnerabilities in gogs.io/gogs