CVE-2026-45045: Fiber: X-Real-IP Spoofing via Header.Add() in BalancerForward

Summary

The BalancerForward proxy helper in GoFiber uses Header.Add() instead of Header.Set() when injecting the X-Real-IP header. This appends the real client IP as a second header value rather than replacing any attacker-supplied value. Upstream servers that read the first X-Real-IP header (nginx, Express, most HTTP servers) use the attacker's spoofed IP for logging, rate limiting, and access control.

Vulnerable Code

File: middleware/proxy/proxy.go, lines 270-285

go func BalancerForward(servers []string, clients ...fasthttp.Client) fiber.Handler { r := &roundrobin{ current: 0, pool: servers, } return func(c fiber.Ctx) error { server := r.get() if !strings.HasPrefix(server, "http") { server = "http://" + server } c.Request().Header.Add("X-Real-IP", c.IP()) // line 282: Add, not Set return Do(c, server+c.OriginalURL(), clients...) } }

Data Flow

1. Attacker sends request with X-Real-IP: 10.0.0.1 (spoofed internal IP) 2. BalancerForward handler executes at line 282 3. c.Request().Header.Add("X-Real-IP", c.IP()) APPENDS the real IP as a second header 4. Upstream server receives: X-Real-IP: 10.0.0.1 AND X-Real-IP: <real-attacker-ip> 5. Most HTTP servers (nginx, Node.js, Apache) read the FIRST value 6. Upstream uses 10.0.0.1 for all IP-dependent logic

Impact

- Rate limit bypass: IP-based rate limiting at the upstream uses the spoofed IP, allowing unlimited requests - IP ACL bypass: Internal IP allowlists (e.g., admin panels restricted to 10.0.0.0/8) can be bypassed - Audit log poisoning: Security logs record the spoofed IP, making incident investigation unreliable - Geolocation bypass: IP-based geofencing or region restrictions are circumvented

Fix

Replace Header.Add() with Header.Set() at line 282:

go c.Request().Header.Set("X-Real-IP", c.IP())

Header.Set() replaces any existing header value, ensuring only the real client IP is forwarded.