# GHSA-pqg7-v6wh-3pfp: TsDProxy: X-Forwarded-For header injection allows IP spoofing in proxied requests to backend services

> Authenticated users can spoof IP addresses via X-Forwarded-For headers, bypassing backend IP-based access controls and rate limiting.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Jul 2026
- **Also known as:** GO-2026-5982

## What is GHSA-pqg7-v6wh-3pfp?

GHSA-pqg7-v6wh-3pfp is a vulnerability in github.com/almeidapaulopt/tsdproxy, published on 14 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

### Official description

## 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

### Support

If this disclosure work has been useful, [sponsoring](https://github.com/sponsors/therawdev) helps fund continued open-source security audits -- appreciated either way.

## Which versions are affected by GHSA-pqg7-v6wh-3pfp?

3 open source packages ship an affected version, including github.com/almeidapaulopt/tsdproxy.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `github.com/almeidapaulopt/tsdproxy` | Go | All versions | None |
| `github.com/almeidapaulopt/tsdproxy` | Go | All versions | None |
| `github.com/almeidapaulopt/tsdproxy` | Go | All versions | None |

## How do I fix GHSA-pqg7-v6wh-3pfp?

No fix has been published for GHSA-pqg7-v6wh-3pfp yet. Limit exposure of the affected software until one is available.

## Is GHSA-pqg7-v6wh-3pfp being exploited?

There is no known exploitation of GHSA-pqg7-v6wh-3pfp in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-pqg7-v6wh-3pfp and fix it if it is affected.

What it is: GHSA-pqg7-v6wh-3pfp is a vulnerability in github.com/almeidapaulopt/tsdproxy, published on 14 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-pqg7-v6wh-3pfp in the wild and no public exploit code.

Affected packages and fixed versions:
- github.com/almeidapaulopt/tsdproxy (Go): affected all versions; no fix listed
- github.com/almeidapaulopt/tsdproxy (Go): affected all versions; no fix listed
- github.com/almeidapaulopt/tsdproxy (Go): affected all versions; no fix listed

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-pqg7-v6wh-3pfp/ (Markdown: https://insights.sbomify.com/GHSA-pqg7-v6wh-3pfp/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-pqg7-v6wh-3pfp

---
From sbomify Insights: https://insights.sbomify.com/GHSA-pqg7-v6wh-3pfp/
Summaries are generated and may contain mistakes; the official description above is the record.