GHSA-pqg7-v6wh-3pfp

## Description

The HTTP reverse proxy handler in tsdproxy does not strip the X-Forwarded-For (or X-Real-IP) header from incoming requests before calling r.SetXForwarded(). This allows an authenticated Tailscale user to inject arbitrary X-Forwarded-For values that are forwarded verbatim to backend services.

```go // internal/proxymanager/port.go -- Rewrite function Rewrite: func(r *httputil.ProxyRequest) { r.SetURL(pconfig.GetFirstTarget()) r.Out.Host = r.In.Host

// Strips tsdproxy identity headers (correct) r.Out.Header.Del(consts.HeaderID) r.Out.Header.Del(consts.HeaderRemoteUser) r.Out.Header.Del(consts.HeaderXForwardedUser) // ... other identity headers deleted ...

// X-Forwarded-For is NOT deleted before SetXForwarded! // X-Real-IP is NOT deleted at all! r.SetXForwarded() // APPENDS client IP to attacker-controlled XFF list }, ```

Per Go's httputil.ProxyRequest.SetXForwarded() documentation: > If the inbound request has an existing X-Forwarded-For header, SetXForwarded appends the inbound request's remote address to the list.

Result when attacker sends X-Forwarded-For: 127.0.0.1: - Backend receives: X-Forwarded-For: 127.0.0.1, <real-tailscale-client-ip> - If backend reads first element as "original client", attacker appears as 127.0.0.1

X-Real-IP is not handled at all -- if the attacker sets X-Real-IP: 127.0.0.1, it is forwarded to the backend verbatim without any overriding or stripping.

Many backend applications trust the first element of X-Forwarded-For (or X-Real-IP) for: - IP-based access control (admin panels restricted to 127.0.0.1) - Rate limiting tied to source IP - Audit logging - Geo-blocking or network-segment restrictions

This is particularly impactful in tsdproxy's intended use case where the backend service is only accessible through tsdproxy -- making the proxy's header handling the sole enforcement point.

## CVSS CVSS v3.1: AV:N/AC:L/PR:L/UI:N/S:C/C:L/I:H/A:N = 7.7

## Severity High

## Affected Code / Files - `internal/proxymanager/port.go` -- newPortProxy Rewrite closure - Missing: r.Out.Header.Del("X-Forwarded-For") before r.SetXForwarded() - Missing: r.Out.Header.Del("X-Real-IP") unconditional strip

## Steps to Reproduce 1. Deploy tsdproxy with a backend service that restricts /admin to 127.0.0.1 via X-Forwarded-For (e.g., Nginx with real_ip_header X-Forwarded-For and real_ip_recursive on) 2. As an authenticated Tailscale user (non-admin), make a request through the proxy:

```bash curl -H "X-Forwarded-For: 127.0.0.1" \ https://<proxy-hostname>.ts.net/admin ```

3. Backend receives: X-Forwarded-For: 127.0.0.1, <your-tailscale-ip> 4. Nginx real_ip_recursive resolves left-most non-trusted IP; if tailscale range is the only trusted range, 127.0.0.1 becomes the "real" IP, bypassing admin restriction.

For the X-Real-IP vector: ```bash curl -H "X-Real-IP: 127.0.0.1" \ https://<proxy-hostname>.ts.net/admin # Backend receives X-Real-IP: 127.0.0.1 verbatim ```

## PoC Script (if used) ```bash #!/bin/bash # Demonstrate XFF injection through tsdproxy PROXY_HOST="${1}" # e.g. myapp.my-tailnet.ts.net curl -v \ -H "X-Forwarded-For: 127.0.0.1" \ -H "X-Real-IP: 127.0.0.1" \ "https://${PROXY_HOST}/" # Expected: backend sees XFF: 127.0.0.1, <tailscale-ip> # backend sees X-Real-IP: 127.0.0.1 (unmodified) ```

## Impact

An authenticated Tailscale user who should only have regular user access can: 1. Bypass IP-based admin restrictions on the backend application by spoofing X-Forwarded-For to 127.0.0.1 2. Appear as any arbitrary IP address in audit logs 3. Bypass rate limiting tied to source IP 4. Bypass geo-blocking or network-segment restrictions enforced by the backend

This is especially impactful because tsdproxy is designed as the sole access point for backend services that are otherwise network-isolated -- making the proxy the only enforcement boundary.

Fix: Add r.Out.Header.Del("X-Forwarded-For") and r.Out.Header.Del("X-Real-IP") in the Rewrite closure before calling r.SetXForwarded(). This ensures only the real Tailscale client IP appears in the XFF chain.

### Credits

Reported by Vishal Shukla (@shukla304) using sechub.dev AI Agent

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