CVE-2025-25296: Label Studio: reflected XSS via label_config param

## Description Label Studio's `/projects/upload-example` endpoint allows injection of arbitrary HTML through a `GET` request with an appropriately crafted `label_config` query parameter. By crafting a specially formatted XML label config with inline task data containing malicious HTML/JavaScript, an attacker can achieve Cross-Site Scripting (XSS). While the application has a Content Security Policy (CSP), it is only set in report-only mode, making it ineffective at preventing script execution. The vulnerability exists because the upload-example endpoint renders user-provided HTML content without proper sanitization on a GET request. This allows attackers to inject and execute arbitrary JavaScript in victims' browsers by getting them to visit a maliciously crafted URL. This is considered vulnerable because it enables attackers to execute JavaScript in victims' contexts, potentially allowing theft of sensitive data, session hijacking, or other malicious actions. ## Steps to reproduce 1. Create a malicious label config that includes an XSS payload in embedded task data: ```xml <View><!-- {"data": {"text": "<div><img src=x onerror=eval(atob(`YWxlcnQoIlhTUyIp`))></div>"}} --><HyperText name="text" value="$text"/></View> ``` 2. URL encode the payload and access the following URL: - http://app/projects/upload-example/?label_config=%3CView%3E%3C!--%20{%22data%22:%20{%22text%22:%20%22%3Cdiv%3E%3Cimg%20src=x%20onerror=eval(atob(`YWxlcnQoIlhTUyIp`))%3E%3C/div%3E%22}}%20--%3E%3CHyperText%20name=%22text%22%20value=%22$text%22/%3E%3C/View%3E When executed, the payload causes the application to render an HTML page containing an img tag that fails to load, triggering the onerror event handler which executes base64-decoded JavaScript, demonstrating successful XSS execution in the victim's browser. ## Mitigations - Enable the Content Security Policy in enforcement mode instead of report-only mode to actively block unauthorized script execution - Deprecate the `GET` behavior at the `example-config` endpoint since it's not used ## Impact The vulnerability requires no special privileges and can be exploited by getting a victim to visit a crafted URL. The impact is high as it allows arbitrary JavaScript execution in victims' browsers, potentially exposing sensitive data or enabling account takeover through session theft.

An adversary targeting an organization's ML training pipeline uses Shodan or Censys to identify exposed Label Studio instances. They craft a GET URL to /projects/upload-example with a label_config parameter containing a base64-encoded JavaScript payload embedded in an XML HyperText config. The URL is delivered via spearphishing email to a data annotator or ML engineer. When the target clicks the link, their browser renders the malicious HTML, executes the JavaScript payload, and exfiltrates the session cookie to an attacker-controlled server. The attacker uses the stolen session to access annotation projects, download training datasets, or inject poisoned labels—silently corrupting the ML model being trained without triggering any authentication alerts.