reflex-dev/reflex has an Open Redirect vulnerability

Mitigation

Make sure GITHUB_CODESPACES_PORT_FORWARDING_DOMAIN is not set in a production environment. So the following is correct:

assert os.getenv("GITHUB_CODESPACES_PORT_FORWARDING_DOMAIN") is None

Vulnerability Description

Vulnerability Overview

• When the GET /auth-codespace page loads in a GitHub Codespaces environment, it automatically assigns the redirect_to query parameter value directly to client-side links without any validation and triggers automatic clicks. This allows attackers to redirect users to arbitrary external URLs.
• The route is only registered when a Codespaces environment is detected, and the detection is controlled by environment variables. This means that the same behavior can be activated in production if the corresponding environment variable is set.

Vulnerable Code Analysis

https://github.com/reflex-dev/reflex/blob/51f9f2c2f52cac4d66c07683a12bc0237311b6be/reflex/utils/codespaces.py#L18-L46

• This code assigns the redirect_to query parameter directly to a.href without any validation and immediately triggers a click (automatic navigation), allowing users to be sent to arbitrary external domains, resulting in an open redirect vulnerability.
• The execution condition is simply based on the presence of a sessionStorage flag, meaning it triggers immediately on first visits or in incognito/private browsing windows, with no server-side origin/scheme whitelist or internal path enforcement defenses in place.

PoC

PoC Description

• Used the production configuration from docker-example (docker-example/production-compose).
• Added a Codespaces detection environment variable to the app container in compose.yaml to forcibly expose the route.
• GITHUB_CODESPACES_PORT_FORWARDING_DOMAIN: dummy
• The reverse proxy (Caddy) needs to be configured to forward /auth-codespace to the backend (required depending on the environment).

curl Example

https://localhost/auth-codespace?redirect_to=http://google.com

PoC MP4 https://file.notion.so/f/f/d105d145-04bc-45c5-b46c-ed880895e9de/a86c3e3b-f67f-45d1-8fa2-4aa0ba7d0068/poc.mp4?table=block&id=26955717-5d2e-805a-b53c-e25ee03f1d4b&spaceId=d105d145-04bc-45c5-b46c-ed880895e9de&expirationTimestamp=1760508000000&signature=ZPp8PVldfGOh0gB5tVElRV6GN789R-EG0oxZgkFjjLU&downloadName=poc.mp4

Impact

Phishing/Social Engineering Attacks

Users can be exploited by immediately redirecting from a trusted domain to external malicious sites, taking advantage of user trust. This enables login page spoofing, credential harvesting, and redirection to malware distribution pages.

Authentication/Session Flow Disruption

When users with valid sessions/cookies from the same origin click the link, they are redirected to unintended external domains, which can bypass or disrupt authentication/authorization flows. When combined with redirect-based flows like OAuth/OIDC, this can escalate into security incidents.