Passthrough request headers (#4172)

* Add passthrough request headers for secret/auth mounts

* Update comments

* Fix SyncCache deletion of passthrough_request_headers

* Remove debug line

* Case-insensitive header comparison

* Remove unnecessary allocation

* Short-circuit filteredPassthroughHeaders if there's nothing to filter

* Add whitelistedHeaders list

* Update router logic after merge

* Add whitelist test

* Add lowercase x-vault-kv-client to whitelist

* Add back const

* Refactor whitelist logic

api/sys_auth.go

@@ -98,6 +98,7 @@ type AuthConfigInput struct {
 	AuditNonHMACRequestKeys   []string `json:"audit_non_hmac_request_keys,omitempty" structs:"audit_non_hmac_request_keys" mapstructure:"audit_non_hmac_request_keys"`
 	AuditNonHMACResponseKeys  []string `json:"audit_non_hmac_response_keys,omitempty" structs:"audit_non_hmac_response_keys" mapstructure:"audit_non_hmac_response_keys"`
 	ListingVisibility         string   `json:"listing_visibility,omitempty" structs:"listing_visibility" mapstructure:"listing_visibility"`
+	PassthroughRequestHeaders []string `json:"passthrough_request_headers,omitempty" structs:"passthrough_request_headers" mapstructure:"passthrough_request_headers"`
 }
 
 type AuthMount struct {
@@ -117,4 +118,5 @@ type AuthConfigOutput struct {
 	AuditNonHMACRequestKeys   []string `json:"audit_non_hmac_request_keys,omitempty" structs:"audit_non_hmac_request_keys" mapstructure:"audit_non_hmac_request_keys"`
 	AuditNonHMACResponseKeys  []string `json:"audit_non_hmac_response_keys,omitempty" structs:"audit_non_hmac_response_keys" mapstructure:"audit_non_hmac_response_keys"`
 	ListingVisibility         string   `json:"listing_visibility,omitempty" structs:"listing_visibility" mapstructure:"listing_visibility"`
+	PassthroughRequestHeaders []string `json:"passthrough_request_headers,omitempty" structs:"passthrough_request_headers" mapstructure:"passthrough_request_headers"`
 }

api/sys_mounts.go

@@ -138,6 +138,7 @@ type MountConfigInput struct {
 	AuditNonHMACRequestKeys   []string          `json:"audit_non_hmac_request_keys,omitempty" structs:"audit_non_hmac_request_keys" mapstructure:"audit_non_hmac_request_keys"`
 	AuditNonHMACResponseKeys  []string          `json:"audit_non_hmac_response_keys,omitempty" structs:"audit_non_hmac_response_keys" mapstructure:"audit_non_hmac_response_keys"`
 	ListingVisibility         string            `json:"listing_visibility,omitempty" structs:"listing_visibility" mapstructure:"listing_visibility"`
+	PassthroughRequestHeaders []string          `json:"passthrough_request_headers,omitempty" structs:"passthrough_request_headers" mapstructure:"passthrough_request_headers"`
 }
 
 type MountOutput struct {
@@ -159,4 +160,5 @@ type MountConfigOutput struct {
 	AuditNonHMACRequestKeys   []string          `json:"audit_non_hmac_request_keys,omitempty" structs:"audit_non_hmac_request_keys" mapstructure:"audit_non_hmac_request_keys"`
 	AuditNonHMACResponseKeys  []string          `json:"audit_non_hmac_response_keys,omitempty" structs:"audit_non_hmac_response_keys" mapstructure:"audit_non_hmac_response_keys"`
 	ListingVisibility         string            `json:"listing_visibility,omitempty" structs:"listing_visibility" mapstructure:"listing_visibility"`
+	PassthroughRequestHeaders []string          `json:"passthrough_request_headers,omitempty" structs:"passthrough_request_headers" mapstructure:"passthrough_request_headers"`
 }

command/auth_enable.go

@@ -24,6 +24,7 @@ type AuthEnableCommand struct {
 	flagAuditNonHMACRequestKeys   []string
 	flagAuditNonHMACResponseKeys  []string
 	flagListingVisibility         string
+	flagPassthroughRequestHeaders []string
 	flagPluginName                string
 	flagOptions                   map[string]string
 	flagLocal                     bool
@@ -121,6 +122,13 @@ func (c *AuthEnableCommand) Flags() *FlagSets {
 		Usage:  "Determines the visibility of the mount in the UI-specific listing endpoint.",
 	})
 
+	f.StringSliceVar(&StringSliceVar{
+		Name:   flagNamePassthroughRequestHeaders,
+		Target: &c.flagPassthroughRequestHeaders,
+		Usage: "Comma-separated string or list of request header values that " +
+			"will be sent to the backend",
+	})
+
 	f.StringVar(&StringVar{
 		Name:       "plugin-name",
 		Target:     &c.flagPluginName,
@@ -229,6 +237,10 @@ func (c *AuthEnableCommand) Run(args []string) int {
 		if fl.Name == flagNameListingVisibility {
 			authOpts.Config.ListingVisibility = c.flagListingVisibility
 		}
+
+		if fl.Name == flagNamePassthroughRequestHeaders {
+			authOpts.Config.PassthroughRequestHeaders = c.flagPassthroughRequestHeaders
+		}
 	})
 
 	if err := client.Sys().EnableAuthWithOptions(authPath, authOpts); err != nil {

command/commands.go

@@ -79,6 +79,8 @@ const (
 	flagNameAuditNonHMACResponseKeys = "audit-non-hmac-response-keys"
 	// flagListingVisibility is the flag to toggle whether to show the mount in the UI-specific listing endpoint
 	flagNameListingVisibility = "listing-visibility"
+	// flagNamePassthroughRequestHeaders is the flag name used to set passthrough request headers to the backend
+	flagNamePassthroughRequestHeaders = "passthrough-request-headers"
 )
 
 var (

command/secrets_enable.go

@@ -24,6 +24,7 @@ type SecretsEnableCommand struct {
 	flagAuditNonHMACRequestKeys   []string
 	flagAuditNonHMACResponseKeys  []string
 	flagListingVisibility         string
+	flagPassthroughRequestHeaders []string
 	flagForceNoCache              bool
 	flagPluginName                string
 	flagOptions                   map[string]string
@@ -129,6 +130,13 @@ func (c *SecretsEnableCommand) Flags() *FlagSets {
 		Usage:  "Determines the visibility of the mount in the UI-specific listing endpoint.",
 	})
 
+	f.StringSliceVar(&StringSliceVar{
+		Name:   flagNamePassthroughRequestHeaders,
+		Target: &c.flagPassthroughRequestHeaders,
+		Usage: "Comma-separated string or list of request header values that " +
+			"will be sent to the backend",
+	})
+
 	f.BoolVar(&BoolVar{
 		Name:    "force-no-cache",
 		Target:  &c.flagForceNoCache,
@@ -249,6 +257,10 @@ func (c *SecretsEnableCommand) Run(args []string) int {
 		if fl.Name == flagNameListingVisibility {
 			mountInput.Config.ListingVisibility = c.flagListingVisibility
 		}
+
+		if fl.Name == flagNamePassthroughRequestHeaders {
+			mountInput.Config.PassthroughRequestHeaders = c.flagPassthroughRequestHeaders
+		}
 	})
 
 	if err := client.Sys().Mount(mountPath, mountInput); err != nil {

http/sys_mount_test.go

@@ -1287,6 +1287,7 @@ func TestSysTuneMount_nonHMACKeys(t *testing.T) {
 	resp = testHttpPost(t, token, addr+"/v1/sys/mounts/secret/tune", map[string]interface{}{
 		"audit_non_hmac_response_keys": "",
 	})
+	testResponseStatus(t, resp, 204)
 
 	// Check results
 	resp = testHttpGet(t, token, addr+"/v1/sys/mounts/secret/tune")
@@ -1318,7 +1319,7 @@ func TestSysTuneMount_nonHMACKeys(t *testing.T) {
 	}
 }
 
-func TestSysTuneMount_showUIMount(t *testing.T) {
+func TestSysTuneMount_listingVisibility(t *testing.T) {
 	core, _, token := vault.TestCoreUnsealed(t)
 	ln, addr := TestServer(t, core)
 	defer ln.Close()
@@ -1390,3 +1391,78 @@ func TestSysTuneMount_showUIMount(t *testing.T) {
 		t.Fatalf("bad:\nExpected: %#v\nActual:%#v", expected, actual)
 	}
 }
+
+func TestSysTuneMount_passthroughRequestHeaders(t *testing.T) {
+	core, _, token := vault.TestCoreUnsealed(t)
+	ln, addr := TestServer(t, core)
+	defer ln.Close()
+	TestServerAuth(t, addr, token)
+
+	// Mount-tune the audit_non_hmac_request_keys
+	resp := testHttpPost(t, token, addr+"/v1/sys/mounts/secret/tune", map[string]interface{}{
+		"passthrough_request_headers": "X-Vault-Foo",
+	})
+	testResponseStatus(t, resp, 204)
+
+	// Check results
+	resp = testHttpGet(t, token, addr+"/v1/sys/mounts/secret/tune")
+	testResponseStatus(t, resp, 200)
+
+	actual := map[string]interface{}{}
+	expected := map[string]interface{}{
+		"lease_id":       "",
+		"renewable":      false,
+		"lease_duration": json.Number("0"),
+		"wrap_info":      nil,
+		"warnings":       nil,
+		"auth":           nil,
+		"data": map[string]interface{}{
+			"default_lease_ttl":           json.Number("2764800"),
+			"max_lease_ttl":               json.Number("2764800"),
+			"force_no_cache":              false,
+			"passthrough_request_headers": []interface{}{"X-Vault-Foo"},
+		},
+		"default_lease_ttl":           json.Number("2764800"),
+		"max_lease_ttl":               json.Number("2764800"),
+		"force_no_cache":              false,
+		"passthrough_request_headers": []interface{}{"X-Vault-Foo"},
+	}
+	testResponseBody(t, resp, &actual)
+	expected["request_id"] = actual["request_id"]
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("bad:\nExpected: %#v\nActual:%#v", expected, actual)
+	}
+
+	// Unset the mount tune value
+	resp = testHttpPost(t, token, addr+"/v1/sys/mounts/secret/tune", map[string]interface{}{
+		"passthrough_request_headers": "",
+	})
+	testResponseStatus(t, resp, 204)
+
+	// Check results
+	resp = testHttpGet(t, token, addr+"/v1/sys/mounts/secret/tune")
+	testResponseStatus(t, resp, 200)
+
+	actual = map[string]interface{}{}
+	expected = map[string]interface{}{
+		"lease_id":       "",
+		"renewable":      false,
+		"lease_duration": json.Number("0"),
+		"wrap_info":      nil,
+		"warnings":       nil,
+		"auth":           nil,
+		"data": map[string]interface{}{
+			"default_lease_ttl": json.Number("2764800"),
+			"max_lease_ttl":     json.Number("2764800"),
+			"force_no_cache":    false,
+		},
+		"default_lease_ttl": json.Number("2764800"),
+		"max_lease_ttl":     json.Number("2764800"),
+		"force_no_cache":    false,
+	}
+	testResponseBody(t, resp, &actual)
+	expected["request_id"] = actual["request_id"]
+	if !reflect.DeepEqual(actual, expected) {
+		t.Fatalf("bad:\nExpected: %#v\nActual:%#v", expected, actual)
+	}
+}

vault/core_test.go

@@ -2276,3 +2276,88 @@ func TestCore_Standby_Rotate(t *testing.T) {
 		t.Fatalf("bad: %#v", resp)
 	}
 }
+
+// Ensure that InternalData is never returned
+func TestCore_HandleRequest_Headers(t *testing.T) {
+	noop := &NoopBackend{
+		Response: &logical.Response{
+			Data: map[string]interface{}{},
+		},
+	}
+
+	c, _, root := TestCoreUnsealed(t)
+	c.logicalBackends["noop"] = func(context.Context, *logical.BackendConfig) (logical.Backend, error) {
+		return noop, nil
+	}
+
+	// Enable the backend
+	req := logical.TestRequest(t, logical.UpdateOperation, "sys/mounts/foo")
+	req.Data["type"] = "noop"
+	req.ClientToken = root
+	_, err := c.HandleRequest(req)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Mount tune
+	req = logical.TestRequest(t, logical.UpdateOperation, "sys/mounts/foo/tune")
+	req.Data["passthrough_request_headers"] = []string{"Should-Passthrough", "should-passthrough-case-insensitive"}
+	req.ClientToken = root
+	_, err = c.HandleRequest(req)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Attempt to read
+	lreq := &logical.Request{
+		Operation:   logical.ReadOperation,
+		Path:        "foo/test",
+		ClientToken: root,
+		Headers: map[string][]string{
+			"X-Vault-Kv-Client":                   []string{"foo"},
+			"Should-Passthrough":                  []string{"foo"},
+			"Should-Passthrough-Case-Insensitive": []string{"baz"},
+			"Should-Not-Passthrough":              []string{"bar"},
+		},
+	}
+	_, err = c.HandleRequest(lreq)
+	if err != nil {
+		t.Fatalf("err: %v", err)
+	}
+
+	// Check the headers
+	headers := noop.Requests[0].Headers
+
+	// Test whitelisted values
+	if val, ok := headers["X-Vault-Kv-Client"]; ok {
+		expected := []string{"foo"}
+		if !reflect.DeepEqual(val, expected) {
+			t.Fatalf("expected: %v, got: %v", expected, val)
+		}
+	} else {
+		t.Fatalf("expected 'X-Vault-Kv-Client' to be present in the headers map")
+	}
+
+	// Test passthrough values
+	if val, ok := headers["Should-Passthrough"]; ok {
+		expected := []string{"foo"}
+		if !reflect.DeepEqual(val, expected) {
+			t.Fatalf("expected: %v, got: %v", expected, val)
+		}
+	} else {
+		t.Fatalf("expected 'Should-Passthrough' to be present in the headers map")
+	}
+
+	if val, ok := headers["Should-Passthrough-Case-Insensitive"]; ok {
+		expected := []string{"baz"}
+		if !reflect.DeepEqual(val, expected) {
+			t.Fatalf("expected: %v, got: %v", expected, val)
+		}
+	} else {
+		t.Fatalf("expected 'Should-Passthrough-Case-Insensitive' to be present in the headers map")
+	}
+
+	if _, ok := headers["Should-Not-Passthrough"]; ok {
+		t.Fatalf("did not expect 'Should-Not-Passthrough' to be in the headers map")
+	}
+}

vault/logical_system.go

@@ -275,6 +275,10 @@ func NewSystemBackend(core *Core) *SystemBackend {
 						Type:        framework.TypeString,
 						Description: strings.TrimSpace(sysHelp["listing_visibility"][0]),
 					},
+					"passthrough_request_headers": &framework.FieldSchema{
+						Type:        framework.TypeCommaStringSlice,
+						Description: strings.TrimSpace(sysHelp["passthrough_request_headers"][0]),
+					},
 				},
 				Callbacks: map[logical.Operation]framework.OperationFunc{
 					logical.ReadOperation:   b.handleAuthTuneRead,
@@ -320,6 +324,10 @@ func NewSystemBackend(core *Core) *SystemBackend {
 						Type:        framework.TypeString,
 						Description: strings.TrimSpace(sysHelp["listing_visibility"][0]),
 					},
+					"passthrough_request_headers": &framework.FieldSchema{
+						Type:        framework.TypeCommaStringSlice,
+						Description: strings.TrimSpace(sysHelp["passthrough_request_headers"][0]),
+					},
 				},
 
 				Callbacks: map[logical.Operation]framework.OperationFunc{
@@ -1498,6 +1506,10 @@ func (b *SystemBackend) handleMountTable(ctx context.Context, req *logical.Reque
 		if len(entry.Config.ListingVisibility) > 0 {
 			entryConfig["listing_visibility"] = entry.Config.ListingVisibility
 		}
+		if rawVal, ok := entry.synthesizedConfigCache.Load("passthrough_request_headers"); ok {
+			entryConfig["passthrough_request_headers"] = rawVal.([]string)
+		}
+
 		info["config"] = entryConfig
 		resp.Data[entry.Path] = info
 	}
@@ -1612,6 +1624,9 @@ func (b *SystemBackend) handleMount(ctx context.Context, req *logical.Request, d
 	if len(apiConfig.AuditNonHMACResponseKeys) > 0 {
 		config.AuditNonHMACResponseKeys = apiConfig.AuditNonHMACResponseKeys
 	}
+	if len(apiConfig.PassthroughRequestHeaders) > 0 {
+		config.PassthroughRequestHeaders = apiConfig.PassthroughRequestHeaders
+	}
 
 	// Create the mount entry
 	me := &MountEntry{
@@ -1782,6 +1797,10 @@ func (b *SystemBackend) handleTuneReadCommon(path string) (*logical.Response, er
 		resp.Data["listing_visibility"] = mountEntry.Config.ListingVisibility
 	}
 
+	if rawVal, ok := mountEntry.synthesizedConfigCache.Load("passthrough_request_headers"); ok {
+		resp.Data["passthrough_request_headers"] = rawVal.([]string)
+	}
+
 	if len(mountEntry.Options) > 0 {
 		resp.Data["options"] = mountEntry.Options
 	}
@@ -2000,6 +2019,32 @@ func (b *SystemBackend) handleTuneWriteCommon(ctx context.Context, path string,
 		}
 	}
 
+	if rawVal, ok := data.GetOk("passthrough_request_headers"); ok {
+		headers := rawVal.([]string)
+
+		oldVal := mountEntry.Config.PassthroughRequestHeaders
+		mountEntry.Config.PassthroughRequestHeaders = headers
+
+		// Update the mount table
+		var err error
+		switch {
+		case strings.HasPrefix(path, "auth/"):
+			err = b.Core.persistAuth(ctx, b.Core.auth, mountEntry.Local)
+		default:
+			err = b.Core.persistMounts(ctx, b.Core.mounts, mountEntry.Local)
+		}
+		if err != nil {
+			mountEntry.Config.PassthroughRequestHeaders = oldVal
+			return handleError(err)
+		}
+
+		mountEntry.SyncCache()
+
+		if b.Core.logger.IsInfo() {
+			b.Core.logger.Info("core: mount tuning of passthrough_request_headers successful", "path", path)
+		}
+	}
+
 	var resp *logical.Response
 	if optionsRaw, ok := data.GetOk("options"); ok {
 		b.Core.logger.Info("core: mount tuning of options", "path", path)
@@ -2050,6 +2095,7 @@ func (b *SystemBackend) handleTuneWriteCommon(ctx context.Context, path string,
 
 		delete(mountEntry.Options, "upgrade")
 	}
+
 	return resp, nil
 }
 
@@ -2214,6 +2260,10 @@ func (b *SystemBackend) handleAuthTable(ctx context.Context, req *logical.Reques
 		if len(entry.Config.ListingVisibility) > 0 {
 			entryConfig["listing_visibility"] = entry.Config.ListingVisibility
 		}
+		if rawVal, ok := entry.synthesizedConfigCache.Load("passthrough_request_headers"); ok {
+			entryConfig["passthrough_request_headers"] = rawVal.([]string)
+		}
+
 		info["config"] = entryConfig
 		resp.Data[entry.Path] = info
 	}
@@ -2320,6 +2370,9 @@ func (b *SystemBackend) handleEnableAuth(ctx context.Context, req *logical.Reque
 	if len(apiConfig.AuditNonHMACResponseKeys) > 0 {
 		config.AuditNonHMACResponseKeys = apiConfig.AuditNonHMACResponseKeys
 	}
+	if len(apiConfig.PassthroughRequestHeaders) > 0 {
+		config.PassthroughRequestHeaders = apiConfig.PassthroughRequestHeaders
+	}
 
 	// Create the mount entry
 	me := &MountEntry{
@@ -3820,4 +3873,7 @@ This path responds to the following HTTP methods.
 	"listing_visibility": {
 		"Determines the visibility of the mount in the UI-specific listing endpoint.",
 	},
+	"passthrough_request_headers": {
+		"A list of headers to whitelist and pass from the request to the backend.",
+	},
 }

vault/mount.go

@@ -184,7 +184,10 @@ type MountEntry struct {
 	SealWrap         bool              `json:"seal_wrap"`          // Whether to wrap CSPs
 	Tainted          bool              `json:"tainted,omitempty"`  // Set as a Write-Ahead flag for unmount/remount
 
-	// synthesizedConfigCache is used to cache configuration values
+	// synthesizedConfigCache is used to cache configuration values. These
+	// particular values are cached since we want to get them at a point-in-time
+	// without separately managing their locks individually. See SyncCache() for
+	// the specific values that are being cached.
 	synthesizedConfigCache sync.Map
 }
 
@@ -197,6 +200,7 @@ type MountConfig struct {
 	AuditNonHMACRequestKeys   []string             `json:"audit_non_hmac_request_keys,omitempty" structs:"audit_non_hmac_request_keys" mapstructure:"audit_non_hmac_request_keys"`
 	AuditNonHMACResponseKeys  []string             `json:"audit_non_hmac_response_keys,omitempty" structs:"audit_non_hmac_response_keys" mapstructure:"audit_non_hmac_response_keys"`
 	ListingVisibility         ListingVisiblityType `json:"listing_visibility,omitempty" structs:"listing_visibility" mapstructure:"listing_visibility"`
+	PassthroughRequestHeaders []string             `json:"passthrough_request_headers,omitempty" structs:"passthrough_request_headers" mapstructure:"passthrough_request_headers"`
 }
 
 // APIMountConfig is an embedded struct of api.MountConfigInput
@@ -208,6 +212,7 @@ type APIMountConfig struct {
 	AuditNonHMACRequestKeys   []string             `json:"audit_non_hmac_request_keys,omitempty" structs:"audit_non_hmac_request_keys" mapstructure:"audit_non_hmac_request_keys"`
 	AuditNonHMACResponseKeys  []string             `json:"audit_non_hmac_response_keys,omitempty" structs:"audit_non_hmac_response_keys" mapstructure:"audit_non_hmac_response_keys"`
 	ListingVisibility         ListingVisiblityType `json:"listing_visibility,omitempty" structs:"listing_visibility" mapstructure:"listing_visibility"`
+	PassthroughRequestHeaders []string             `json:"passthrough_request_headers,omitempty" structs:"passthrough_request_headers" mapstructure:"passthrough_request_headers"`
 }
 
 // Clone returns a deep copy of the mount entry
@@ -219,7 +224,9 @@ func (e *MountEntry) Clone() (*MountEntry, error) {
 	return cp.(*MountEntry), nil
 }
 
-// SyncCache syncs tunable configuration values to the cache
+// SyncCache syncs tunable configuration values to the cache. In the case of
+// cached values, they should be retrieved via synthesizedConfigCache.Load()
+// instead of accessing them directly through MountConfig.
 func (e *MountEntry) SyncCache() {
 	if len(e.Config.AuditNonHMACRequestKeys) == 0 {
 		e.synthesizedConfigCache.Delete("audit_non_hmac_request_keys")
@@ -232,6 +239,12 @@ func (e *MountEntry) SyncCache() {
 	} else {
 		e.synthesizedConfigCache.Store("audit_non_hmac_response_keys", e.Config.AuditNonHMACResponseKeys)
 	}
+
+	if len(e.Config.PassthroughRequestHeaders) == 0 {
+		e.synthesizedConfigCache.Delete("passthrough_request_headers")
+	} else {
+		e.synthesizedConfigCache.Store("passthrough_request_headers", e.Config.PassthroughRequestHeaders)
+	}
 }
 
 // Mount is used to mount a new backend to the mount table.

vault/router.go

@@ -15,6 +15,12 @@ import (
 	"github.com/hashicorp/vault/logical"
 )
 
+var (
+	whitelistedHeaders = []string{
+		consts.VaultKVCLIClientHeader,
+	}
+)
+
 // Router is used to do prefix based routing of a request to a logical backend
 type Router struct {
 	l                  sync.RWMutex
@@ -473,16 +479,15 @@ func (r *Router) routeCommon(ctx context.Context, req *logical.Request, existenc
 	originalClientTokenRemainingUses := req.ClientTokenRemainingUses
 	req.ClientTokenRemainingUses = 0
 
-	// Cache the headers and hide them from backends
+	// Cache the headers
 	headers := req.Headers
-	req.Headers = nil
 
-	// Whitelist the X-Vault-Kv-Client header for use in the kv backend.
-	if val, ok := headers[consts.VaultKVCLIClientHeader]; ok {
-		req.Headers = map[string][]string{
-			consts.VaultKVCLIClientHeader: val,
-		}
+	// Filter and add passthrough headers to the backend
+	var passthroughRequestHeaders []string
+	if rawVal, ok := re.mountEntry.synthesizedConfigCache.Load("passthrough_request_headers"); ok {
+		passthroughRequestHeaders = rawVal.([]string)
 	}
+	req.Headers = filteredPassthroughHeaders(headers, passthroughRequestHeaders)
 
 	// Cache the wrap info of the request
 	var wrapInfo *logical.RequestWrapInfo
@@ -624,3 +629,48 @@ func pathsToRadix(paths []string) *radix.Tree {
 
 	return tree
 }
+
+// filteredPassthroughHeaders returns a headers map[string][]string that
+// contains the filtered values contained in passthroughHeaders, as well as the
+// values in whitelistedHeaders. Filtering of passthroughHeaders from the
+// origHeaders is done is a case-insensitive manner.
+func filteredPassthroughHeaders(origHeaders map[string][]string, passthroughHeaders []string) map[string][]string {
+	retHeaders := make(map[string][]string)
+
+	// Handle whitelisted values
+	for _, header := range whitelistedHeaders {
+		if val, ok := origHeaders[header]; ok {
+			retHeaders[header] = val
+		} else {
+			// Try to check if a lowercased version of the header exists in the
+			// originating request. The header key that gets used is the one from the
+			// whitelist.
+			if val, ok := origHeaders[strings.ToLower(header)]; ok {
+				retHeaders[header] = val
+			}
+		}
+	}
+
+	// Short-circuit if there's nothing to filter
+	if len(passthroughHeaders) == 0 {
+		return retHeaders
+	}
+
+	// Create a map that uses lowercased header values as the key and the original
+	// header naming as the value for comparison down below.
+	lowerHeadersRef := make(map[string]string, len(origHeaders))
+	for key := range origHeaders {
+		lowerHeadersRef[strings.ToLower(key)] = key
+	}
+
+	// Case-insensitive compare of passthrough headers against originating
+	// headers. The returned headers will be the same casing as the originating
+	// header name.
+	for _, ph := range passthroughHeaders {
+		if header, ok := lowerHeadersRef[strings.ToLower(ph)]; ok {
+			retHeaders[header] = origHeaders[header]
+		}
+	}
+
+	return retHeaders
+}

website/source/api/system/auth.html.md

@@ -92,6 +92,12 @@ For example, enable the "foo" auth method will make it accessible at
   - `audit_non_hmac_response_keys` `(array: [])` - Comma-separated list of keys
      that will not be HMAC'd by audit devices in the response data object.
 
+  - `listing_visibility` `(string: "")` - Speficies whether to show this mount
+     in the UI-specific listing endpoint.
+
+  - `passthrough_request_headers` `(array: [])` - Comma-separated list of headers
+     to whitelist and pass from the request to the backend.
+
     The plugin_name can be provided in the config map or as a top-level option,
     with the former taking precedence.
 
@@ -215,6 +221,12 @@ can be achieved without `sudo` via `sys/mounts/auth/[auth-path]/tune`._
   list of keys that will not be HMAC'd by audit devices in the response data
   object.
 
+- `listing_visibility` `(string: "")` - Speficies whether to show this mount
+    in the UI-specific listing endpoint.
+
+- `passthrough_request_headers` `(array: [])` - Comma-separated list of headers
+    to whitelist and pass from the request to the backend.
+
 ### Sample Payload
 
 ```json

website/source/api/system/mounts.html.md

@@ -97,6 +97,12 @@ This endpoint enables a new secrets engine at the given path.
   - `audit_non_hmac_response_keys` `(array: [])` - Comma-separated list of keys
      that will not be HMAC'd by audit devices in the response data object.
 
+  - `listing_visibility` `(string: "")` - Speficies whether to show this mount
+     in the UI-specific listing endpoint.
+
+  - `passthrough_request_headers` `(array: [])` - Comma-separated list of headers
+     to whitelist and pass from the request to the backend.
+
     These control the default and maximum lease time-to-live, force
     disabling backend caching, and option plugin name for plugin backends
     respectively. The first three options override the global defaults if
@@ -216,6 +222,12 @@ This endpoint tunes configuration parameters for a given mount point.
   list of keys that will not be HMAC'd by audit devices in the response data
   object.
 
+- `listing_visibility` `(string: "")` - Speficies whether to show this mount
+    in the UI-specific listing endpoint.
+
+- `passthrough_request_headers` `(array: [])` - Comma-separated list of headers
+    to whitelist and pass from the request to the backend.
+
 ### Sample Payload
 
 ```json