GHSA-246p-cpwv-v3jq - Echo has Rate Limiter Bypass via Spoofed X-Forwarded-For Header in Default Configuration

## Summary When `echo.New()` is used without explicitly setting `Echo#IPExtractor`, the `RealIP()` method falls back to reading the client-supplied `X-Forwarded-For` (or `X-Real-IP`) header without any proxy-origin validation. The default `RateLimiter()` middleware derives its per-client identifier from `ctx.RealIP()`, so an attacker can rotate arbitrary values in the `X-Forwarded-For` header on each request to obtain a fresh rate-limit bucket and bypass IP-based throttling entirely. ## Affected Component - `context.go` — `(*Context).RealIP()` (lines 186–208), legacy fallback branch (lines 190–200) - `middleware/rate_limiter.go` — `DefaultRateLimiterConfig.IdentifierExtractor` (lines 46–58); `RateLimiter()` (lines 71–76) - `echo.go` — `New()` (lines 326–343), `IPExtractor` left `nil` by default ## Description ### `RealIP()` unconditionally trusts `X-Forwarded-For` when `IPExtractor` is unset `Context.RealIP()` (`context.go:186-208`) first checks `c.echo.IPExtractor`. When this field is `nil` (the default for `echo.New()`), it falls through to legacy behavior and returns the **first IP** from the `X-Forwarded-For` header — the position an attacker has full control over: ```go // context.go:186-200 func (c *Context) RealIP() string { if c.echo != nil && c.echo.IPExtractor != nil { return c.echo.IPExtractor(c.request) } // Fall back to legacy behavior if ip := c.request.Header.Get(HeaderXForwardedFor); ip != "" { i := strings.IndexAny(ip, ",") if i > 0 { xffip := strings.TrimSpace(ip[:i]) xffip = strings.TrimPrefix(xffip, "[") xffip = strings.TrimSuffix(xffip, "]") return xffip // ← attacker-controlled first IP } return ip } if ip := c.request.Header.Get(HeaderXRealIP); ip != "" { ... return ip // ← also attacker-controlled } ra, _, _ := net.SplitHostPort(c.request.RemoteAddr) return ra } ``` ### Rate limiter def