csrf.go

     1  // Copyright 2025 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  package http
     6  
     7  import (
     8  	"errors"
     9  	"fmt"
    10  	"net/url"
    11  	"sync"
    12  	"sync/atomic"
    13  )
    14  
    15  // CrossOriginProtection implements protections against [Cross-Site Request
    16  // Forgery (CSRF)] by rejecting non-safe cross-origin browser requests.
    17  //
    18  // Cross-origin requests are currently detected with the [Sec-Fetch-Site]
    19  // header, available in all browsers since 2023, or by comparing the hostname of
    20  // the [Origin] header with the Host header.
    21  //
    22  // The GET, HEAD, and OPTIONS methods are [safe methods] and are always allowed.
    23  // It's important that applications do not perform any state changing actions
    24  // due to requests with safe methods.
    25  //
    26  // Requests without Sec-Fetch-Site or Origin headers are currently assumed to be
    27  // either same-origin or non-browser requests, and are allowed.
    28  //
    29  // The zero value of CrossOriginProtection is valid and has no trusted origins
    30  // or bypass patterns.
    31  //
    32  // [Sec-Fetch-Site]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Sec-Fetch-Site
    33  // [Origin]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Origin
    34  // [Cross-Site Request Forgery (CSRF)]: https://developer.mozilla.org/en-US/docs/Web/Security/Attacks/CSRF
    35  // [safe methods]: https://developer.mozilla.org/en-US/docs/Glossary/Safe/HTTP
    36  type CrossOriginProtection struct {
    37  	bypass    atomic.Pointer[ServeMux]
    38  	trustedMu sync.RWMutex
    39  	trusted   map[string]bool
    40  	deny      atomic.Pointer[Handler]
    41  }
    42  
    43  // NewCrossOriginProtection returns a new [CrossOriginProtection] value.
    44  func NewCrossOriginProtection() *CrossOriginProtection {
    45  	return &CrossOriginProtection{}
    46  }
    47  
    48  // AddTrustedOrigin allows all requests with an [Origin] header
    49  // which exactly matches the given value.
    50  //
    51  // Origin header values are of the form "scheme://host[:port]".
    52  //
    53  // AddTrustedOrigin can be called concurrently with other methods
    54  // or request handling, and applies to future requests.
    55  //
    56  // [Origin]: https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Origin
    57  func (c *CrossOriginProtection) AddTrustedOrigin(origin string) error {
    58  	u, err := url.Parse(origin)
    59  	if err != nil {
    60  		return fmt.Errorf("invalid origin %q: %w", origin, err)
    61  	}
    62  	if u.Scheme == "" {
    63  		return fmt.Errorf("invalid origin %q: scheme is required", origin)
    64  	}
    65  	if u.Host == "" {
    66  		return fmt.Errorf("invalid origin %q: host is required", origin)
    67  	}
    68  	if u.Path != "" || u.RawQuery != "" || u.Fragment != "" {
    69  		return fmt.Errorf("invalid origin %q: path, query, and fragment are not allowed", origin)
    70  	}
    71  	c.trustedMu.Lock()
    72  	defer c.trustedMu.Unlock()
    73  	if c.trusted == nil {
    74  		c.trusted = make(map[string]bool)
    75  	}
    76  	c.trusted[origin] = true
    77  	return nil
    78  }
    79  
    80  type noopHandler struct{}
    81  
    82  func (noopHandler) ServeHTTP(ResponseWriter, *Request) {}
    83  
    84  var sentinelHandler Handler = &noopHandler{}
    85  
    86  // AddInsecureBypassPattern permits all requests that match the given pattern.
    87  //
    88  // The pattern syntax and precedence rules are the same as [ServeMux]. Only
    89  // requests that match the pattern directly are permitted. Those that ServeMux
    90  // would redirect to a pattern (e.g. after cleaning the path or adding a
    91  // trailing slash) are not.
    92  //
    93  // AddInsecureBypassPattern can be called concurrently with other methods or
    94  // request handling, and applies to future requests.
    95  func (c *CrossOriginProtection) AddInsecureBypassPattern(pattern string) {
    96  	var bypass *ServeMux
    97  
    98  	// Lazily initialize c.bypass
    99  	for {
   100  		bypass = c.bypass.Load()
   101  		if bypass != nil {
   102  			break
   103  		}
   104  		bypass = NewServeMux()
   105  		if c.bypass.CompareAndSwap(nil, bypass) {
   106  			break
   107  		}
   108  	}
   109  
   110  	bypass.Handle(pattern, sentinelHandler)
   111  }
   112  
   113  // SetDenyHandler sets a handler to invoke when a request is rejected.
   114  // The default error handler responds with a 403 Forbidden status.
   115  //
   116  // SetDenyHandler can be called concurrently with other methods
   117  // or request handling, and applies to future requests.
   118  //
   119  // Check does not call the error handler.
   120  func (c *CrossOriginProtection) SetDenyHandler(h Handler) {
   121  	if h == nil {
   122  		c.deny.Store(nil)
   123  		return
   124  	}
   125  	c.deny.Store(&h)
   126  }
   127  
   128  // Check applies cross-origin checks to a request.
   129  // It returns an error if the request should be rejected.
   130  func (c *CrossOriginProtection) Check(req *Request) error {
   131  	switch req.Method {
   132  	case "GET", "HEAD", "OPTIONS":
   133  		// Safe methods are always allowed.
   134  		return nil
   135  	}
   136  
   137  	switch req.Header.Get("Sec-Fetch-Site") {
   138  	case "":
   139  		// No Sec-Fetch-Site header is present.
   140  		// Fallthrough to check the Origin header.
   141  	case "same-origin", "none":
   142  		return nil
   143  	default:
   144  		if c.isRequestExempt(req) {
   145  			return nil
   146  		}
   147  		return errCrossOriginRequest
   148  	}
   149  
   150  	origin := req.Header.Get("Origin")
   151  	if origin == "" {
   152  		// Neither Sec-Fetch-Site nor Origin headers are present.
   153  		// Either the request is same-origin or not a browser request.
   154  		return nil
   155  	}
   156  
   157  	if o, err := url.Parse(origin); err == nil && o.Host == req.Host {
   158  		// The Origin header matches the Host header. Note that the Host header
   159  		// doesn't include the scheme, so we don't know if this might be an
   160  		// HTTP→HTTPS cross-origin request. We fail open, since all modern
   161  		// browsers support Sec-Fetch-Site since 2023, and running an older
   162  		// browser makes a clear security trade-off already. Sites can mitigate
   163  		// this with HTTP Strict Transport Security (HSTS).
   164  		return nil
   165  	}
   166  
   167  	if c.isRequestExempt(req) {
   168  		return nil
   169  	}
   170  	return errCrossOriginRequestFromOldBrowser
   171  }
   172  
   173  var (
   174  	errCrossOriginRequest               = errors.New("cross-origin request detected from Sec-Fetch-Site header")
   175  	errCrossOriginRequestFromOldBrowser = errors.New("cross-origin request detected, and/or browser is out of date: " +
   176  		"Sec-Fetch-Site is missing, and Origin does not match Host")
   177  )
   178  
   179  // isRequestExempt checks the bypasses which require taking a lock, and should
   180  // be deferred until the last moment.
   181  func (c *CrossOriginProtection) isRequestExempt(req *Request) bool {
   182  	if bypass := c.bypass.Load(); bypass != nil {
   183  		if h, _ := bypass.Handler(req); h == sentinelHandler {
   184  			// The request matches a bypass pattern.
   185  			return true
   186  		}
   187  	}
   188  
   189  	c.trustedMu.RLock()
   190  	defer c.trustedMu.RUnlock()
   191  	origin := req.Header.Get("Origin")
   192  	// The request matches a trusted origin.
   193  	return origin != "" && c.trusted[origin]
   194  }
   195  
   196  // Handler returns a handler that applies cross-origin checks
   197  // before invoking the handler h.
   198  //
   199  // If a request fails cross-origin checks, the request is rejected
   200  // with a 403 Forbidden status or handled with the handler passed
   201  // to [CrossOriginProtection.SetDenyHandler].
   202  func (c *CrossOriginProtection) Handler(h Handler) Handler {
   203  	return HandlerFunc(func(w ResponseWriter, r *Request) {
   204  		if err := c.Check(r); err != nil {
   205  			if deny := c.deny.Load(); deny != nil {
   206  				(*deny).ServeHTTP(w, r)
   207  				return
   208  			}
   209  			Error(w, err.Error(), StatusForbidden)
   210  			return
   211  		}
   212  		h.ServeHTTP(w, r)
   213  	})
   214  }
   215
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