httphandlers.go

// Copyright 2015 Matthew Holt
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package certmagic

import (
	"fmt"
	"net/http"
	"net/url"
	"strings"

	"github.com/mholt/acmez/v3/acme"
	"go.uber.org/zap"
)

// HTTPChallengeHandler wraps h in a handler that can solve the ACME
// HTTP challenge. cfg is required, and it must have a certificate
// cache backed by a functional storage facility, since that is where
// the challenge state is stored between initiation and solution.
//
// If a request is not an ACME HTTP challenge, h will be invoked.
func (am *ACMEIssuer) HTTPChallengeHandler(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if am.HandleHTTPChallenge(w, r) {
			return
		}
		h.ServeHTTP(w, r)
	})
}

// HandleHTTPChallenge uses am to solve challenge requests from an ACME
// server that were initiated by this instance or any other instance in
// this cluster (being, any instances using the same storage am does).
//
// If the HTTP challenge is disabled, this function is a no-op.
//
// If am is nil or if am does not have a certificate cache backed by
// usable storage, solving the HTTP challenge will fail.
//
// It returns true if it handled the request; if so, the response has
// already been written. If false is returned, this call was a no-op and
// the request has not been handled.
func (am *ACMEIssuer) HandleHTTPChallenge(w http.ResponseWriter, r *http.Request) bool {
	if am == nil {
		return false
	}
	if am.DisableHTTPChallenge {
		return false
	}
	if !LooksLikeHTTPChallenge(r) {
		return false
	}
	return am.distributedHTTPChallengeSolver(w, r)
}

// distributedHTTPChallengeSolver checks to see if this challenge
// request was initiated by this or another instance which uses the
// same storage as am does, and attempts to complete the challenge for
// it. It returns true if the request was handled; false otherwise.
func (am *ACMEIssuer) distributedHTTPChallengeSolver(w http.ResponseWriter, r *http.Request) bool {
	if am == nil {
		return false
	}
	host := hostOnly(r.Host)
	chalInfo, distributed, err := am.config.getACMEChallengeInfo(r.Context(), host, !am.DisableDistributedSolvers)
	if err != nil {
		if am.DisableDistributedSolvers {
			// Distributed solvers are disabled, so the only way an error can be returned is if
			// this instance didn't initiate the challenge (or if the process exited after, but
			// either way, we don't have the challenge info). Assuming this is a legitimate
			// challenge request, we may still be able to solve it if we can present the correct
			// account thumbprint with the token, since the token is given to us in the URL path.
			//
			// NOTE: About doing this, RFC 8555 section 8.3 says:
			//
			// 	Note that because the token appears both in the request sent by the
			// 	ACME server and in the key authorization in the response, it is
			// 	possible to build clients that copy the token from request to
			// 	response.  Clients should avoid this behavior because it can lead to
			// 	cross-site scripting vulnerabilities; instead, clients should be
			// 	explicitly configured on a per-challenge basis.  A client that does
			// 	copy tokens from requests to responses MUST validate that the token
			// 	in the request matches the token syntax above (e.g., that it includes
			// 	only characters from the base64url alphabet).
			//
			// Also, since we're just blindly solving a challenge, we're unable to mitigate DNS
			// rebinding attacks, because we don't know what host to expect in the URL. So this
			// is not ideal, but we do at least validate the copied token is in the base64url set.
			if strings.HasPrefix(r.URL.Path, acmeHTTPChallengeBasePath) &&
				strings.Count(r.URL.Path, "/") == 3 &&
				r.Method == http.MethodGet {
				tokenStart := strings.LastIndex(r.URL.Path, "/") + 1
				token := r.URL.Path[tokenStart:]
				if allBase64URL(token) {
					if err := am.solveHTTPChallengeBlindly(w, r); err != nil {
						am.Logger.Error("solving http-01 challenge blindly",
							zap.String("identifier", host),
							zap.Error(err))
					}
					return true
				}
			}
		}

		// couldn't get challenge info even with distributed solver
		am.Logger.Warn("looking up info for HTTP challenge",
			zap.String("uri", r.RequestURI),
			zap.String("identifier", host),
			zap.String("remote_addr", r.RemoteAddr),
			zap.String("user_agent", r.Header.Get("User-Agent")),
			zap.Error(err))
		return false
	}

	return solveHTTPChallenge(am.Logger, w, r, chalInfo.Challenge, distributed)
}

// solveHTTPChallengeBlindly will try to respond correctly with an http-01 challenge response.
// The request must be an http-01 challenge request. We cannot know for sure the ACME CA that
// is requesting this, so we have to guess as we load the account to use for a thumbprint as
// part of the response body. It is a no-op if the last component of the URL path contains
// characters outside of the base64url alphabet.
func (am *ACMEIssuer) solveHTTPChallengeBlindly(w http.ResponseWriter, r *http.Request) error {
	tokenStart := strings.LastIndex(r.URL.Path, "/") + 1
	token := r.URL.Path[tokenStart:]
	if allBase64URL(token) {
		acct, err := am.getAccountToUse(r.Context(), am.CA) // assume production CA, I guess
		if err != nil {
			return fmt.Errorf("getting an account to use: %v", err)
		}
		thumbprint, err := acct.Thumbprint()
		if err != nil {
			return fmt.Errorf("could not encode account thumbprint: %v", err)
		}
		w.Header().Add("Content-Type", "text/plain")
		_, _ = w.Write([]byte(token + "." + thumbprint))
		r.Close = true
		am.Logger.Info("served key authentication",
			zap.String("identifier", hostOnly(r.Host)),
			zap.String("challenge", "http-01"),
			zap.String("remote", r.RemoteAddr),
			zap.Bool("distributed", false),
			zap.Bool("blind", true),
			zap.String("ca", am.CA))
	}
	return nil
}

// allBase64URL returns true if all characters of s are in the base64url alphabet.
func allBase64URL(s string) bool {
	for _, c := range s {
		if (c >= 'A' && c <= 'Z') ||
			(c >= 'a' && c <= 'z') ||
			(c >= '0' && c <= '9') ||
			c == '-' || c == '_' {
			continue
		}
		return false
	}
	return true
}

// solveHTTPChallenge solves the HTTP challenge using the given challenge information.
// If the challenge is being solved in a distributed fahsion, set distributed to true for logging purposes.
// It returns true the properties of the request check out in relation to the HTTP challenge.
// Most of this code borrowed from xenolf's built-in HTTP-01 challenge solver in March 2018.
func solveHTTPChallenge(logger *zap.Logger, w http.ResponseWriter, r *http.Request, challenge acme.Challenge, distributed bool) bool {
	challengeReqPath := challenge.HTTP01ResourcePath()
	if r.URL.Path == challengeReqPath &&
		strings.EqualFold(hostOnly(r.Host), challenge.Identifier.Value) && // mitigate DNS rebinding attacks
		r.Method == http.MethodGet {
		w.Header().Add("Content-Type", "text/plain")
		w.Write([]byte(challenge.KeyAuthorization))
		r.Close = true
		logger.Info("served key authentication",
			zap.String("identifier", challenge.Identifier.Value),
			zap.String("challenge", "http-01"),
			zap.String("remote", r.RemoteAddr),
			zap.Bool("distributed", distributed))
		return true
	}
	return false
}

// SolveHTTPChallenge solves the HTTP challenge. It should be used only on HTTP requests that are
// from ACME servers trying to validate an identifier (i.e. LooksLikeHTTPChallenge() == true). It
// returns true if the request criteria check out and it answered with key authentication, in which
// case no further handling of the request is necessary.
func SolveHTTPChallenge(logger *zap.Logger, w http.ResponseWriter, r *http.Request, challenge acme.Challenge) bool {
	return solveHTTPChallenge(logger, w, r, challenge, false)
}

// LooksLikeHTTPChallenge returns true if r looks like an ACME
// HTTP challenge request from an ACME server.
func LooksLikeHTTPChallenge(r *http.Request) bool {
	return r.Method == http.MethodGet &&
		strings.HasPrefix(r.URL.Path, acmeHTTPChallengeBasePath)
}

// LooksLikeZeroSSLHTTPValidation returns true if the request appears to be
// domain validation from a ZeroSSL/Sectigo CA. NOTE: This API is
// non-standard and is subject to change.
func LooksLikeZeroSSLHTTPValidation(r *http.Request) bool {
	return r.Method == http.MethodGet &&
		strings.HasPrefix(r.URL.Path, zerosslHTTPValidationBasePath)
}

// HTTPValidationHandler wraps the ZeroSSL HTTP validation handler such that
// it can pass verification checks from ZeroSSL's API.
//
// If a request is not a ZeroSSL HTTP validation request, h will be invoked.
func (iss *ZeroSSLIssuer) HTTPValidationHandler(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if iss.HandleZeroSSLHTTPValidation(w, r) {
			return
		}
		h.ServeHTTP(w, r)
	})
}

// HandleZeroSSLHTTPValidation is to ZeroSSL API HTTP validation requests like HandleHTTPChallenge
// is to ACME HTTP challenge requests.
func (iss *ZeroSSLIssuer) HandleZeroSSLHTTPValidation(w http.ResponseWriter, r *http.Request) bool {
	if iss == nil {
		return false
	}
	if !LooksLikeZeroSSLHTTPValidation(r) {
		return false
	}
	return iss.distributedHTTPValidationAnswer(w, r)
}

func (iss *ZeroSSLIssuer) distributedHTTPValidationAnswer(w http.ResponseWriter, r *http.Request) bool {
	if iss == nil {
		return false
	}
	logger := iss.Logger
	if logger == nil {
		logger = zap.NewNop()
	}
	host := hostOnly(r.Host)
	valInfo, distributed, err := iss.getDistributedValidationInfo(r.Context(), host)
	if err != nil {
		logger.Warn("looking up info for HTTP validation",
			zap.String("host", host),
			zap.String("remote_addr", r.RemoteAddr),
			zap.String("user_agent", r.Header.Get("User-Agent")),
			zap.Error(err))
		return false
	}
	return answerHTTPValidation(logger, w, r, valInfo, distributed)
}

func answerHTTPValidation(logger *zap.Logger, rw http.ResponseWriter, req *http.Request, valInfo acme.Challenge, distributed bool) bool {
	// ensure URL matches
	validationURL, err := url.Parse(valInfo.URL)
	if err != nil {
		logger.Error("got invalid URL from CA",
			zap.String("file_validation_url", valInfo.URL),
			zap.Error(err))
		rw.WriteHeader(http.StatusInternalServerError)
		return true
	}
	if req.URL.Path != validationURL.Path {
		rw.WriteHeader(http.StatusNotFound)
		return true
	}

	rw.Header().Add("Content-Type", "text/plain")
	req.Close = true

	rw.Write([]byte(valInfo.Token))

	logger.Info("served HTTP validation credential",
		zap.String("validation_path", valInfo.URL),
		zap.String("challenge", "http-01"),
		zap.String("remote", req.RemoteAddr),
		zap.Bool("distributed", distributed))

	return true
}

const (
	acmeHTTPChallengeBasePath     = "/.well-known/acme-challenge"
	zerosslHTTPValidationBasePath = "/.well-known/pki-validation/"
)