PYSEC-2026-1868 
### 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**
 

<img width="623" height="497" alt="image" src="https://github.com/user-attachments/assets/55ef4828-09fa-451b-a7cc-8fcaad6a2a21" />


- 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

<img width="1917" height="949" alt="image" src="https://github.com/user-attachments/assets/e565dc4e-a59b-44d4-a92a-ebf128489e88" />

<img width="1913" height="977" alt="image" src="https://github.com/user-attachments/assets/496fa585-76ea-4d2d-80f8-0ab79f51229e" />




### 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.
CVSS v3.1 score: 3.1