Description

GoFiber Vulnerable to 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

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") {...
Show more

Data Flow

Attacker sends request with X-Real-IP: 10.0.0.1 (spoofed internal IP)

BalancerForward handler executes at line 282

c.Request().Header.Add("X-Real-IP", c.IP()) APPENDS the real IP as a second header

Upstream server receives: X-Real-IP: 10.0.0.1 AND X-Real-IP: <real-attacker-ip>

Most HTTP servers (nginx, Node.js, Apache) read the FIRST value

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:

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

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

Mitigation

Aliases

References