Add ACME HTTP-01 Challenge (hashicorp#20141)

* Add HTTP challenge validator

This will attempt to safely validate HTTP challenges, following a
limited number of redirects and timing out after too much time has
passed.

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Add test for ValidateKeyAuthorization

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Add test cases for ValidateHTTP01Challenge

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

* Add token to HTTP challenge

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

---------

Signed-off-by: Alexander Scheel <alex.scheel@hashicorp.com>

builtin/logical/pki/acme_challenges.go

@@ -0,0 +1,111 @@
+package pki
+
+import (
+	"fmt"
+	"io"
+	"net"
+	"net/http"
+	"strings"
+	"time"
+)
+
+// ValidateKeyAuthorization validates that the given keyAuthz from a challenge
+// matches our expectation, returning (true, nil) if so, or (false, err) if
+// not.
+func ValidateKeyAuthorization(keyAuthz string, token string, thumbprint string) (bool, error) {
+	parts := strings.Split(keyAuthz, ".")
+	if len(parts) != 2 {
+		return false, fmt.Errorf("invalid authorization: got %v parts, expected 2", len(parts))
+	}
+
+	tokenPart := parts[0]
+	thumbprintPart := parts[1]
+
+	if token != tokenPart || thumbprint != thumbprintPart {
+		return false, fmt.Errorf("key authorization was invalid")
+	}
+
+	return true, nil
+}
+
+// Validates a given ACME http-01 challenge against the specified domain,
+// per RFC 8555.
+//
+// We attempt to be defensive here against timeouts, extra redirects, &c.
+func ValidateHTTP01Challenge(domain string, token string, thumbprint string) (bool, error) {
+	path := "http://" + domain + "/.well-known/acme-challenge/" + token
+
+	transport := &http.Transport{
+		// Only a single request is sent to this server as we do not do any
+		// batching of validation attempts. There is no need to do an HTTP
+		// KeepAlive as a result.
+		DisableKeepAlives:   true,
+		MaxIdleConns:        1,
+		MaxIdleConnsPerHost: 1,
+		MaxConnsPerHost:     1,
+		IdleConnTimeout:     1 * time.Second,
+
+		// We'd rather timeout and re-attempt validation later than hang
+		// too many validators waiting for slow hosts.
+		DialContext: (&net.Dialer{
+			Timeout:   10 * time.Second,
+			KeepAlive: -1 * time.Second,
+		}).DialContext,
+		ResponseHeaderTimeout: 10 * time.Second,
+	}
+
+	maxRedirects := 10
+	urlLength := 2000
+
+	client := &http.Client{
+		Transport: transport,
+		CheckRedirect: func(req *http.Request, via []*http.Request) error {
+			if len(via)+1 >= maxRedirects {
+				return fmt.Errorf("http-01: too many redirects: %v", len(via)+1)
+			}
+
+			reqUrlLen := len(req.URL.String())
+			if reqUrlLen > urlLength {
+				return fmt.Errorf("http-01: redirect url length too long: %v", reqUrlLen)
+			}
+
+			return nil
+		},
+	}
+
+	resp, err := client.Get(path)
+	if err != nil {
+		return false, fmt.Errorf("http-01: failed to fetch path %v: %w", path, err)
+	}
+
+	// We provision a buffer which allows for a variable size challenge, some
+	// whitespace, and a detection gap for too long of a message.
+	minExpected := len(token) + 1 + len(thumbprint)
+	maxExpected := 512
+
+	defer resp.Body.Close()
+
+	// Attempt to read the body, but don't do so infinitely.
+	body, err := io.ReadAll(io.LimitReader(resp.Body, int64(maxExpected+1)))
+	if err != nil {
+		return false, fmt.Errorf("http-01: unexpected error while reading body: %w", err)
+	}
+
+	if len(body) > maxExpected {
+		return false, fmt.Errorf("http-01: response too large: received %v > %v bytes", len(body), maxExpected)
+	}
+
+	if len(body) < minExpected {
+		return false, fmt.Errorf("http-01: response too small: received %v < %v bytes", len(body), minExpected)
+	}
+
+	// Per RFC 8555 Section 8.3. HTTP Challenge:
+	//
+	// > The server SHOULD ignore whitespace characters at the end of the body.
+	keyAuthz := string(body)
+	keyAuthz = strings.TrimSpace(keyAuthz)
+
+	// If we got here, we got no non-EOF error while reading. Try to validate
+	// the token because we're bounded by a reasonable amount of length.
+	return ValidateKeyAuthorization(keyAuthz, token, thumbprint)
+}

builtin/logical/pki/acme_challenges_test.go

@@ -0,0 +1,184 @@
+package pki
+
+import (
+	"net/http"
+	"net/http/httptest"
+	"strings"
+	"testing"
+	"time"
+)
+
+type keyAuthorizationTestCase struct {
+	keyAuthz   string
+	token      string
+	thumbprint string
+	shouldFail bool
+}
+
+var keyAuthorizationTestCases = []keyAuthorizationTestCase{
+	{
+		// Entirely empty
+		"",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		true,
+	},
+	{
+		// Both empty
+		".",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		true,
+	},
+	{
+		// Not equal
+		"non-.non-",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		true,
+	},
+	{
+		// Empty thumbprint
+		"non-.",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		true,
+	},
+	{
+		// Empty token
+		".non-",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		true,
+	},
+	{
+		// Wrong order
+		"non-empty-thumbprint.non-empty-token",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		true,
+	},
+	{
+		// Too many pieces
+		"one.two.three",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		true,
+	},
+	{
+		// Valid
+		"non-empty-token.non-empty-thumbprint",
+		"non-empty-token",
+		"non-empty-thumbprint",
+		false,
+	},
+}
+
+func TestAcmeValidateKeyAuthorization(t *testing.T) {
+	t.Parallel()
+
+	for index, tc := range keyAuthorizationTestCases {
+		isValid, err := ValidateKeyAuthorization(tc.keyAuthz, tc.token, tc.thumbprint)
+		if !isValid && err == nil {
+			t.Fatalf("[%d] expected failure to give reason via err (%v / %v)", index, isValid, err)
+		}
+
+		expectedValid := !tc.shouldFail
+		if expectedValid != isValid {
+			t.Fatalf("[%d] got ret=%v, expected ret=%v (shouldFail=%v)", index, isValid, expectedValid, tc.shouldFail)
+		}
+	}
+}
+
+func TestAcmeValidateHTTP01Challenge(t *testing.T) {
+	t.Parallel()
+
+	for index, tc := range keyAuthorizationTestCases {
+		validFunc := func(w http.ResponseWriter, r *http.Request) {
+			w.Write([]byte(tc.keyAuthz))
+		}
+		withPadding := func(w http.ResponseWriter, r *http.Request) {
+			w.Write([]byte("  " + tc.keyAuthz + "     "))
+		}
+		withRedirect := func(w http.ResponseWriter, r *http.Request) {
+			if strings.Contains(r.URL.Path, "/.well-known/") {
+				http.Redirect(w, r, "/my-http-01-challenge-response", 301)
+				return
+			}
+
+			w.Write([]byte(tc.keyAuthz))
+		}
+		withSleep := func(w http.ResponseWriter, r *http.Request) {
+			// Long enough to ensure any excessively short timeouts are hit,
+			// not long enough to trigger a failure (hopefully).
+			time.Sleep(5 * time.Second)
+			w.Write([]byte(tc.keyAuthz))
+		}
+
+		validHandlers := []http.HandlerFunc{
+			http.HandlerFunc(validFunc), http.HandlerFunc(withPadding),
+			http.HandlerFunc(withRedirect), http.HandlerFunc(withSleep),
+		}
+
+		for handlerIndex, handler := range validHandlers {
+			func() {
+				ts := httptest.NewServer(handler)
+				defer ts.Close()
+
+				host := ts.URL[7:]
+				isValid, err := ValidateHTTP01Challenge(host, tc.token, tc.thumbprint)
+				if !isValid && err == nil {
+					t.Fatalf("[tc=%d/handler=%d] expected failure to give reason via err (%v / %v)", handlerIndex, index, isValid, err)
+				}
+
+				expectedValid := !tc.shouldFail
+				if expectedValid != isValid {
+					t.Fatalf("[tc=%d/handler=%d] got ret=%v (err=%v), expected ret=%v (shouldFail=%v)", index, handlerIndex, isValid, err, expectedValid, tc.shouldFail)
+				}
+			}()
+		}
+	}
+
+	// Negative test cases for various HTTP-specific scenarios.
+	redirectLoop := func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "/my-http-01-challenge-response", 301)
+	}
+	publicRedirect := func(w http.ResponseWriter, r *http.Request) {
+		http.Redirect(w, r, "http://hashicorp.com/", 301)
+	}
+	noData := func(w http.ResponseWriter, r *http.Request) {}
+	noContent := func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNoContent)
+	}
+	notFound := func(w http.ResponseWriter, r *http.Request) {
+		w.WriteHeader(http.StatusNotFound)
+	}
+	simulateHang := func(w http.ResponseWriter, r *http.Request) {
+		time.Sleep(30 * time.Second)
+		w.Write([]byte("my-token.my-thumbprint"))
+	}
+	tooLarge := func(w http.ResponseWriter, r *http.Request) {
+		for i := 0; i < 512; i++ {
+			w.Write([]byte("my-token.my-thumbprint"))
+		}
+	}
+
+	validHandlers := []http.HandlerFunc{
+		http.HandlerFunc(redirectLoop), http.HandlerFunc(publicRedirect),
+		http.HandlerFunc(noData), http.HandlerFunc(noContent),
+		http.HandlerFunc(notFound), http.HandlerFunc(simulateHang),
+		http.HandlerFunc(tooLarge),
+	}
+	for handlerIndex, handler := range validHandlers {
+		func() {
+			ts := httptest.NewServer(handler)
+			defer ts.Close()
+
+			host := ts.URL[7:]
+			isValid, err := ValidateHTTP01Challenge(host, "my-token", "my-thumbprint")
+			if isValid || err == nil {
+				t.Fatalf("[handler=%d] expected failure validating challenge (%v / %v)", handlerIndex, isValid, err)
+			}
+		}()
+	}
+}

builtin/logical/pki/acme_state.go

@@ -22,6 +22,13 @@ const (
 	// How long nonces are considered valid.
 	nonceExpiry = 15 * time.Minute
 
+	// How many bytes are in a token. Per RFC 8555 Section
+	// 8.3. HTTP Challenge and Section 11.3 Token Entropy:
+	//
+	// > token (required, string):  A random value that uniquely identifies
+	// >   the challenge.  This value MUST have at least 128 bits of entropy.
+	tokenBytes = 128 / 8
+
 	// Path Prefixes
 	acmePathPrefix       = "acme/"
 	acmeAccountPrefix    = acmePathPrefix + "accounts/"
@@ -46,6 +53,10 @@ func NewACMEState() *acmeState {
 }
 
 func generateNonce() (string, error) {
+	return generateRandomBase64(21)
+}
+
+func generateRandomBase64(srcBytes int) (string, error) {
 	data := make([]byte, 21)
 	if _, err := io.ReadFull(rand.Reader, data); err != nil {
 		return "", err
@@ -447,3 +458,7 @@ func getAuthorizationPath(accountId string, authId string) string {
 func getOrderPath(accountId string, orderId string) string {
 	return acmeAccountPrefix + accountId + "/orders/" + orderId
 }
+
+func getACMEToken() (string, error) {
+	return generateRandomBase64(tokenBytes)
+}

builtin/logical/pki/path_acme_order.go

@@ -195,7 +195,10 @@ func (b *backend) acmeNewOrderHandler(ac *acmeContext, r *logical.Request, _ *fr
 	var authorizations []*ACMEAuthorization
 	var authorizationIds []string
 	for _, identifier := range identifiers {
-		authz := generateAuthorization(ac, account, identifier)
+		authz, err := generateAuthorization(ac, account, identifier)
+		if err != nil {
+			return nil, fmt.Errorf("error generating authorizations: %w", err)
+		}
 		authorizations = append(authorizations, authz)
 
 		err = b.acmeState.SaveAuthorization(ac, authz)
@@ -295,15 +298,24 @@ func buildOrderUrl(acmeCtx *acmeContext, orderId string) string {
 	return acmeCtx.baseUrl.JoinPath("order", orderId).String()
 }
 
-func generateAuthorization(acmeCtx *acmeContext, acct *acmeAccount, identifier *ACMEIdentifier) *ACMEAuthorization {
+func generateAuthorization(acmeCtx *acmeContext, acct *acmeAccount, identifier *ACMEIdentifier) (*ACMEAuthorization, error) {
 	authId := genUuid()
+	var challenges []*ACMEChallenge
+	for _, challengeType := range []ACMEChallengeType{ACMEHTTPChallenge} {
+		token, err := getACMEToken()
+		if err != nil {
+			return nil, err
+		}
 
-	challenges := []*ACMEChallenge{
-		{
-			Type:            ACMEHTTPChallenge,
-			Status:          ACMEChallengePending,
-			ChallengeFields: map[string]interface{}{}, // TODO fill this in properly
-		},
+		challenge := &ACMEChallenge{
+			Type:   challengeType,
+			Status: ACMEChallengePending,
+			ChallengeFields: map[string]interface{}{
+				"token": token,
+			},
+		}
+
+		challenges = append(challenges, challenge)
 	}
 
 	return &ACMEAuthorization{
@@ -314,7 +326,7 @@ func generateAuthorization(acmeCtx *acmeContext, acct *acmeAccount, identifier *
 		Expires:    "", // only populated when it switches to valid.
 		Challenges: challenges,
 		Wildcard:   strings.HasPrefix(identifier.Value, "*."),
-	}
+	}, nil
 }
 
 func parseOptRFC3339Field(data map[string]interface{}, keyName string) (time.Time, error) {