google/internal/externalaccount: create executable credenitals

google/internal/externalaccount/basecredentials.go

@@ -171,6 +171,8 @@ type CredentialSource struct {
 	URL     string            `json:"url"`
 	Headers map[string]string `json:"headers"`
 
+	Executable ExecutableConfig `json:"executable"`
+
 	EnvironmentID               string `json:"environment_id"`
 	RegionURL                   string `json:"region_url"`
 	RegionalCredVerificationURL string `json:"regional_cred_verification_url"`
@@ -179,6 +181,12 @@ type CredentialSource struct {
 	Format                      format `json:"format"`
 }
 
+type ExecutableConfig struct {
+	Command       string `json:"command"`
+	TimeoutMillis int    `json:"timeout_millis"`
+	OutputFile    string `json:"output_file"`
+}
+
 // parse determines the type of CredentialSource needed
 func (c *Config) parse(ctx context.Context) (baseCredentialSource, error) {
 	if len(c.CredentialSource.EnvironmentID) > 3 && c.CredentialSource.EnvironmentID[:3] == "aws" {
@@ -205,6 +213,8 @@ func (c *Config) parse(ctx context.Context) (baseCredentialSource, error) {
 		return fileCredentialSource{File: c.CredentialSource.File, Format: c.CredentialSource.Format}, nil
 	} else if c.CredentialSource.URL != "" {
 		return urlCredentialSource{URL: c.CredentialSource.URL, Headers: c.CredentialSource.Headers, Format: c.CredentialSource.Format, ctx: ctx}, nil
+	} else if c.CredentialSource.Executable.Command != "" {
+		return CreateExecutableCredential(c.CredentialSource.Executable, c, ctx), nil
 	}
 	return nil, fmt.Errorf("oauth2/google: unable to parse credential source")
 }

google/internal/externalaccount/executablecredsource.go

@@ -0,0 +1,217 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package externalaccount
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"os"
+	"os/exec"
+	"regexp"
+	"time"
+)
+
+var (
+	EXECUTABLE_SUPPORTED_MAX_VERSION = 1
+)
+
+var baseEnv = os.Environ
+
+// runCommand is basically an alias of exec.CommandContext for testing.
+var runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+	cmd := exec.CommandContext(ctx, command)
+	cmd.Env = env
+
+	response, err := cmd.Output()
+	if ctx.Err() != nil {
+		return nil, ctx.Err()
+	}
+	return response, err
+}
+
+type executableCredentialSource struct {
+	Command    string
+	Timeout    time.Duration
+	OutputFile string
+	ctx        context.Context
+	config     *Config
+}
+
+// CreateExecutableCredential creates an executableCredentialSource given an ExecutableConfig.
+// It also performs defaulting and type conversions.
+func CreateExecutableCredential(ec ExecutableConfig, config *Config, ctx context.Context) (result executableCredentialSource) {
+	result.Command = ec.Command
+	if ec.TimeoutMillis == 0 {
+		result.Timeout = 30 * time.Second
+	} else {
+		result.Timeout = time.Duration(ec.TimeoutMillis) * time.Millisecond
+	}
+	result.OutputFile = ec.OutputFile
+	result.ctx = ctx
+	result.config = config
+	return
+}
+
+type subjectTokenResponse struct {
+	Version        *int    `json:"version"`
+	Success        *bool   `json:"success"`
+	TokenType      *string `json:"token_type"`
+	ExpirationTime *int64  `json:"expiration_time"`
+	IdToken        *string `json:"id_token"`
+	SamlResponse   *string `json:"saml_response"`
+	Code           *string `json:"code"`
+	Message        *string `json:"message"`
+}
+
+func parseSubjectToken(response []byte) (string, error) {
+	var result subjectTokenResponse
+	if err := json.Unmarshal(response, &result); err != nil {
+		return "", errors.New("oauth2/google: Unable to parse response JSON.")
+	}
+
+	if result.Version == nil {
+		return "", errors.New("oauth2/google: Response missing version field.")
+	}
+
+	if result.Success == nil {
+		return "", errors.New("oauth2/google: Response missing success field.")
+	}
+
+	if !*result.Success {
+		details := ""
+		if result.Code != nil {
+			details += fmt.Sprintf("(%v)", *result.Code)
+		}
+		if result.Message != nil {
+			if details != "" {
+				details += " "
+			}
+			details += *result.Message
+		}
+		if details == "" {
+			details = "Unknown Error"
+		}
+		return "", fmt.Errorf("oauth2/google: Executable returned unsuccessful response: %v.", details)
+	}
+
+	if *result.Version > EXECUTABLE_SUPPORTED_MAX_VERSION {
+		return "", fmt.Errorf("oauth2/google: Executable returned unsupported version: %v.", *result.Version)
+	}
+
+	if result.ExpirationTime == nil {
+		return "", errors.New("oauth2/google: Response missing expiration_time field.")
+	}
+
+	if result.TokenType == nil {
+		return "", errors.New("oauth2/google: Response missing token_type field.")
+	}
+
+	if *result.ExpirationTime < now().Unix() {
+		return "", errors.New("oauth2/google: The token returned by the executable is expired.")
+	}
+
+	if *result.TokenType == "urn:ietf:params:oauth:token-type:jwt" || *result.TokenType == "urn:ietf:params:oauth:token-type:id_token" {
+		if result.IdToken == nil {
+			return "", errors.New("oauth2/google: Response missing id_token field.")
+		}
+		return *result.IdToken, nil
+	}
+
+	if *result.TokenType == "urn:ietf:params:oauth:token-type:saml2" {
+		if result.SamlResponse == nil {
+			return "", errors.New("oauth2/google: Response missing saml_response field.")
+		}
+		return *result.SamlResponse, nil
+	}
+
+	return "", errors.New("Executable returned unsupported token type.")
+}
+
+func (cs executableCredentialSource) subjectToken() (string, error) {
+	if token, ok := cs.getTokenFromInMemoryCaching(); ok {
+		return token, nil
+	}
+
+	if token, ok := cs.getTokenFromOutputFile(); ok {
+		return token, nil
+	}
+
+	return cs.getTokenFromExecutableCommand()
+}
+
+func (cs executableCredentialSource) getTokenFromInMemoryCaching() (string, bool) {
+	// TODO
+	return "", false
+}
+
+func (cs executableCredentialSource) getTokenFromOutputFile() (string, bool) {
+	// TODO
+	return "", false
+}
+
+func (cs executableCredentialSource) getEnvironment() []string {
+	result := baseEnv()
+	for k, v := range cs.getNewEnvironmentVariables() {
+		result = append(result, fmt.Sprintf("%v=%v", k, v))
+	}
+	return result
+}
+
+var serviceAccountImpersonationCompiler = regexp.MustCompile("https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/(.*@.*):generateAccessToken")
+
+func (cs executableCredentialSource) getNewEnvironmentVariables() map[string]string {
+	result := map[string]string{
+		"GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE":   cs.config.Audience,
+		"GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE": cs.config.SubjectTokenType,
+	}
+
+	if cs.config.ServiceAccountImpersonationURL != "" {
+		matches := serviceAccountImpersonationCompiler.FindStringSubmatch(cs.config.ServiceAccountImpersonationURL)
+		if matches != nil {
+			result["GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL"] = matches[1]
+		}
+	}
+
+	if cs.isInteractive() {
+		result["GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE"] = "1"
+	} else {
+		result["GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE"] = "0"
+	}
+
+	if cs.OutputFile != "" {
+		result["GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE"] = cs.OutputFile
+	}
+
+	return result
+}
+
+func (cs executableCredentialSource) isInteractive() bool {
+	// Currently, executableCredentialSource does not yet support interactive mode.
+	return false
+}
+
+func (cs executableCredentialSource) getTokenFromExecutableCommand() (string, error) {
+	// For security reasons, we need our consumers to set this environment variable to allow executables to be run.
+	if getenv("GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES") != "1" {
+		return "", errors.New("Executables need to be explicitly allowed (set GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES to '1') to run.")
+	}
+
+	ctx, cancel := context.WithDeadline(cs.ctx, now().Add(cs.Timeout))
+	defer cancel()
+
+	if output, err := runCommand(ctx, cs.Command, cs.getEnvironment()); err != nil {
+		if err == context.DeadlineExceeded {
+			return "", fmt.Errorf("oauth2/google: executable command timed out.")
+		}
+		if exitError, ok := err.(*exec.ExitError); ok {
+			return "", fmt.Errorf("oauth2/google: executable command failed with exit code %v.", exitError.ExitCode())
+		}
+		return "", fmt.Errorf("oauth2/google: executable command failed: %v.", err.Error())
+	} else {
+		return parseSubjectToken(output)
+	}
+}

google/internal/externalaccount/executablecredsource_test.go

@@ -0,0 +1,926 @@
+// Copyright 2022 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package externalaccount
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"testing"
+	"time"
+)
+
+func Bool(b bool) *bool {
+	return &b
+}
+
+func Int(i int) *int {
+	return &i
+}
+
+func Int64(i int64) *int64 {
+	return &i
+}
+
+func String(s string) *string {
+	return &s
+}
+
+var emptyEnv = func() []string {
+	return []string{}
+}
+
+func TestCreateExecutableCredential(t *testing.T) {
+	ec := ExecutableConfig{
+		Command:       "blarg",
+		TimeoutMillis: 50000,
+	}
+
+	ecs := CreateExecutableCredential(ec, nil, context.Background())
+	if ecs.Command != "blarg" {
+		t.Errorf("ecs.Command got %v but want %v", ecs.Command, "blarg")
+	}
+	if ecs.Timeout != 50000*time.Millisecond {
+		t.Errorf("ecs.Timeout got %v but want %v", ecs.Timeout, 50000*time.Millisecond)
+	}
+}
+
+func TestCreateExecutableCredential_WithoutTimeout(t *testing.T) {
+	ec := ExecutableConfig{
+		Command: "blarg",
+	}
+
+	ecs := CreateExecutableCredential(ec, nil, context.Background())
+	if ecs.Command != "blarg" {
+		t.Errorf("ecs.Command got %v but want %v", ecs.Command, "blarg")
+	}
+	if ecs.Timeout != 30000*time.Millisecond {
+		t.Errorf("ecs.Timeout got %v but want %v", ecs.Timeout, 30000*time.Millisecond)
+	}
+}
+
+func areSlicesEquivalent(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+
+OUTER:
+	for i, aa := range a {
+		for _, bb := range b {
+			if aa == bb {
+				continue OUTER
+			}
+		}
+		return false
+	}
+	return true
+}
+
+func TestMinimalExectuableCredentialGetEnvironment(t *testing.T) {
+	config := Config{
+		Audience:         "//iam.googleapis.com/projects/123/locations/global/workloadIdentityPools/pool/providers/oidc",
+		SubjectTokenType: "urn:ietf:params:oauth:token-type:jwt",
+		CredentialSource: CredentialSource{
+			Executable: ExecutableConfig{
+				Command: "blarg",
+			},
+		},
+	}
+
+	ecs := CreateExecutableCredential(config.CredentialSource.Executable, &config, context.Background())
+
+	oldBaseEnv := baseEnv
+	defer func() { baseEnv = oldBaseEnv }()
+	baseEnv = func() []string {
+		return []string{"A=B"}
+	}
+
+	expectedEnvironment := []string{
+		"A=B",
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE=%v", config.Audience),
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE=%v", config.SubjectTokenType),
+		"GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE=0",
+	}
+
+	if got, want := ecs.getEnvironment(), expectedEnvironment; !areSlicesEquivalent(got, want) {
+		t.Errorf("Incorrect environment received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+}
+
+func TestExectuableCredentialGetEnvironmentMalformedImpersonationUrl(t *testing.T) {
+	config := Config{
+		Audience:                       "//iam.googleapis.com/projects/123/locations/global/workloadIdentityPools/pool/providers/oidc",
+		ServiceAccountImpersonationURL: "https://iamcredentials.googleapis.com/v1/projects/-/serviceAccounts/test@project.iam.gserviceaccount.com:generateAccessToken",
+		SubjectTokenType:               "urn:ietf:params:oauth:token-type:jwt",
+		CredentialSource: CredentialSource{
+			Executable: ExecutableConfig{
+				Command:    "blarg",
+				OutputFile: "/path/to/generated/cached/credentials",
+			},
+		},
+	}
+
+	ecs := CreateExecutableCredential(config.CredentialSource.Executable, &config, context.Background())
+
+	oldBaseEnv := baseEnv
+	defer func() { baseEnv = oldBaseEnv }()
+	baseEnv = func() []string {
+		return []string{"A=B"}
+	}
+
+	expectedEnvironment := []string{
+		"A=B",
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE=%v", config.Audience),
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE=%v", config.SubjectTokenType),
+		"GOOGLE_EXTERNAL_ACCOUNT_IMPERSONATED_EMAIL=test@project.iam.gserviceaccount.com",
+		"GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE=0",
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE=%v", config.CredentialSource.Executable.OutputFile),
+	}
+
+	if got, want := ecs.getEnvironment(), expectedEnvironment; !areSlicesEquivalent(got, want) {
+		t.Errorf("Incorrect environment received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+}
+func TestExectuableCredentialGetEnvironment(t *testing.T) {
+	config := Config{
+		Audience:                       "//iam.googleapis.com/projects/123/locations/global/workloadIdentityPools/pool/providers/oidc",
+		ServiceAccountImpersonationURL: "test@project.iam.gserviceaccount.com",
+		SubjectTokenType:               "urn:ietf:params:oauth:token-type:jwt",
+		CredentialSource: CredentialSource{
+			Executable: ExecutableConfig{
+				Command:    "blarg",
+				OutputFile: "/path/to/generated/cached/credentials",
+			},
+		},
+	}
+
+	ecs := CreateExecutableCredential(config.CredentialSource.Executable, &config, context.Background())
+
+	oldBaseEnv := baseEnv
+	defer func() { baseEnv = oldBaseEnv }()
+	baseEnv = func() []string {
+		return []string{"A=B"}
+	}
+
+	expectedEnvironment := []string{
+		"A=B",
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_AUDIENCE=%v", config.Audience),
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_TOKEN_TYPE=%v", config.SubjectTokenType),
+		"GOOGLE_EXTERNAL_ACCOUNT_INTERACTIVE=0",
+		fmt.Sprintf("GOOGLE_EXTERNAL_ACCOUNT_OUTPUT_FILE=%v", config.CredentialSource.Executable.OutputFile),
+	}
+
+	if got, want := ecs.getEnvironment(), expectedEnvironment; !areSlicesEquivalent(got, want) {
+		t.Errorf("Incorrect environment received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenWithoutEnvironmentVariablesSet(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv := getenv
+	defer func() { getenv = oldGetenv }()
+	getenv = setEnvironment(map[string]string{})
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "Executables need to be explicitly allowed (set GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES to '1') to run."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenTimeoutOccurs(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return nil, context.DeadlineExceeded
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: executable command timed out."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenInvalidFormat(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return []byte("tokentokentoken"), nil
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Unable to parse response JSON."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenMissingVersion(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success: Bool(true),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Response missing version field."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenMissingSuccess(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Version: Int(1),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Response missing success field."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithFields(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success: Bool(false),
+			Version: Int(1),
+			Code:    String("404"),
+			Message: String("Token Not Found"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Executable returned unsuccessful response: (404) Token Not Found."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithCode(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success: Bool(false),
+			Version: Int(1),
+			Code:    String("404"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Executable returned unsuccessful response: (404)."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithMessage(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success: Bool(false),
+			Version: Int(1),
+			Message: String("Token Not Found"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Executable returned unsuccessful response: Token Not Found."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenUnsuccessfulResponseWithoutFields(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success: Bool(false),
+			Version: Int(1),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Executable returned unsuccessful response: Unknown Error."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenNewerVersion(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success: Bool(true),
+			Version: Int(2),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Executable returned unsupported version: 2."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenExpired(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success:        Bool(true),
+			Version:        Int(1),
+			ExpirationTime: Int64(now().Unix() - 1),
+			TokenType:      String("urn:ietf:params:oauth:token-type:jwt"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: The token returned by the executable is expired."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenJwt(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success:        Bool(true),
+			Version:        Int(1),
+			ExpirationTime: Int64(now().Unix() + 3600),
+			TokenType:      String("urn:ietf:params:oauth:token-type:jwt"),
+			IdToken:        String("tokentokentoken"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	out, err := base.subjectToken()
+	if err != nil {
+		t.Fatalf("retrieveSubjectToken() failed: %v", err)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+
+	if got, want := out, "tokentokentoken"; got != want {
+		t.Errorf("Incorrect token received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenJwtMissingIdToken(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success:        Bool(true),
+			Version:        Int(1),
+			ExpirationTime: Int64(now().Unix() + 3600),
+			TokenType:      String("urn:ietf:params:oauth:token-type:jwt"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Response missing id_token field."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenIdToken(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success:        Bool(true),
+			Version:        Int(1),
+			ExpirationTime: Int64(now().Unix() + 3600),
+			TokenType:      String("urn:ietf:params:oauth:token-type:id_token"),
+			IdToken:        String("tokentokentoken"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	out, err := base.subjectToken()
+	if err != nil {
+		t.Fatalf("retrieveSubjectToken() failed: %v", err)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+
+	if got, want := out, "tokentokentoken"; got != want {
+		t.Errorf("Incorrect token received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenSaml(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success:        Bool(true),
+			Version:        Int(1),
+			ExpirationTime: Int64(now().Unix() + 3600),
+			TokenType:      String("urn:ietf:params:oauth:token-type:saml2"),
+			SamlResponse:   String("tokentokentoken"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	out, err := base.subjectToken()
+	if err != nil {
+		t.Fatalf("retrieveSubjectToken() failed: %v", err)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+
+	if got, want := out, "tokentokentoken"; got != want {
+		t.Errorf("Incorrect token received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+}
+
+func TestRetrieveExecutableSubjectTokenSamlMissingResponse(t *testing.T) {
+	cs := CredentialSource{
+		Executable: ExecutableConfig{
+			Command:       "blarg",
+			TimeoutMillis: 5000,
+		},
+	}
+
+	tfc := testFileConfig
+	tfc.CredentialSource = cs
+
+	oldGetenv, oldNow, oldRunCommand := getenv, now, runCommand
+	defer func() {
+		getenv, now, runCommand = oldGetenv, oldNow, oldRunCommand
+	}()
+
+	getenv = setEnvironment(map[string]string{"GOOGLE_EXTERNAL_ACCOUNT_ALLOW_EXECUTABLES": "1"})
+	now = setTime(defaultTime)
+	deadline, deadlineSet := now(), false
+	runCommand = func(ctx context.Context, command string, env []string) ([]byte, error) {
+		deadline, deadlineSet = ctx.Deadline()
+		return json.Marshal(subjectTokenResponse{
+			Success:        Bool(true),
+			Version:        Int(1),
+			ExpirationTime: Int64(now().Unix() + 3600),
+			TokenType:      String("urn:ietf:params:oauth:token-type:saml2"),
+		})
+	}
+
+	base, err := tfc.parse(context.Background())
+	if err != nil {
+		t.Fatalf("parse() failed %v", err)
+	}
+
+	_, err = base.subjectToken()
+	if err == nil {
+		t.Fatalf("Expected error but found none")
+	}
+	if got, want := err.Error(), "oauth2/google: Response missing saml_response field."; got != want {
+		t.Errorf("Incorrect error received.\nExpected: %s\nRecieved: %s", want, got)
+	}
+
+	if !deadlineSet {
+		t.Errorf("Command run without a deadline")
+	} else if deadline != now().Add(5*time.Second) {
+		t.Errorf("Command run with incorrect deadline")
+	}
+}