ssh: Add logic to set accepted environment variables in SSH session (#13559)

Add logic to set environment variables that match the SSH rule's
`acceptEnv` settings in the SSH session's environment.

Updates https://github.com/tailscale/corp/issues/22775

Signed-off-by: Mario Minardi <mario@tailscale.com>

ssh/tailssh/accept_env.go

@@ -4,6 +4,7 @@
 package tailssh
 
 import (
+	"fmt"
 	"slices"
 	"strings"
 )
@@ -17,27 +18,35 @@ import (
 //
 // acceptEnv values may contain * and ? wildcard characters which match against
 // zero or one or more characters and a single character respectively.
-func filterEnv(acceptEnv []string, environ []string) []string {
+func filterEnv(acceptEnv []string, environ []string) ([]string, error) {
 	var acceptedPairs []string
 
+	// Quick return if we have an empty list.
+	if acceptEnv == nil || len(acceptEnv) == 0 {
+		return acceptedPairs, nil
+	}
+
 	for _, envPair := range environ {
-		envVar := strings.Split(envPair, "=")[0]
+		variableName, _, ok := strings.Cut(envPair, "=")
+		if !ok {
+			return nil, fmt.Errorf(`invalid environment variable: %q. Variables must be in "KEY=VALUE" format`, envPair)
+		}
 
 		// Short circuit if we have a direct match between the environment
 		// variable and an AcceptEnv value.
-		if slices.Contains(acceptEnv, envVar) {
+		if slices.Contains(acceptEnv, variableName) {
 			acceptedPairs = append(acceptedPairs, envPair)
 			continue
 		}
 
 		// Otherwise check if we have a wildcard pattern that matches.
-		if matchAcceptEnv(acceptEnv, envVar) {
+		if matchAcceptEnv(acceptEnv, variableName) {
 			acceptedPairs = append(acceptedPairs, envPair)
 			continue
 		}
 	}
 
-	return acceptedPairs
+	return acceptedPairs, nil
 }
 
 // matchAcceptEnv is a convenience function that wraps calling matchAcceptEnvPattern

ssh/tailssh/accept_env_test.go

@@ -108,6 +108,7 @@ func TestFilterEnv(t *testing.T) {
 		acceptEnv        []string
 		environ          []string
 		expectedFiltered []string
+		wantErrMessage   string
 	}{
 		{
 			name:             "simple direct matches",
@@ -127,11 +128,26 @@ func TestFilterEnv(t *testing.T) {
 			environ:          []string{"FOO=BAR", "FOO2=BAZ", "FOO_3=123", "FOOOO4-2=AbCdEfG", "FO1-kmndGamc79567=ABC", "FO57=BAR2"},
 			expectedFiltered: []string{"FOO=BAR", "FOOOO4-2=AbCdEfG", "FO1-kmndGamc79567=ABC"},
 		},
+		{
+			name:             "environ format invalid",
+			acceptEnv:        []string{"FO?", "FOOO*", "FO*5?7"},
+			environ:          []string{"FOOBAR"},
+			expectedFiltered: nil,
+			wantErrMessage:   `invalid environment variable: "FOOBAR". Variables must be in "KEY=VALUE" format`,
+		},
 	}
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			filtered := filterEnv(tc.acceptEnv, tc.environ)
+			filtered, err := filterEnv(tc.acceptEnv, tc.environ)
+			if err == nil && tc.wantErrMessage != "" {
+				t.Errorf("wanted error with message %q but error was nil", tc.wantErrMessage)
+			}
+
+			if err != nil && err.Error() != tc.wantErrMessage {
+				t.Errorf("err = %v; want %v", err, tc.wantErrMessage)
+			}
+
 			if diff := cmp.Diff(tc.expectedFiltered, filtered); diff != "" {
 				t.Errorf("unexpected filter result (-got,+want): \n%s", diff)
 			}

ssh/tailssh/incubator.go

@@ -12,6 +12,7 @@
 package tailssh
 
 import (
+	"encoding/json"
 	"errors"
 	"flag"
 	"fmt"
@@ -154,6 +155,22 @@ func (ss *sshSession) newIncubatorCommand(logf logger.Logf) (cmd *exec.Cmd, err
 		incubatorArgs = append(incubatorArgs, "--cmd="+ss.RawCommand())
 	}
 
+	allowSendEnv := nm.HasCap(tailcfg.NodeAttrSSHEnvironmentVariables)
+	if allowSendEnv {
+		env, err := filterEnv(ss.conn.acceptEnv, ss.Session.Environ())
+		if err != nil {
+			return nil, err
+		}
+
+		if len(env) > 0 {
+			encoded, err := json.Marshal(env)
+			if err != nil {
+				return nil, fmt.Errorf("failed to encode environment: %w", err)
+			}
+			incubatorArgs = append(incubatorArgs, fmt.Sprintf("--encoded-env=%q", encoded))
+		}
+	}
+
 	return exec.CommandContext(ss.ctx, ss.conn.srv.tailscaledPath, incubatorArgs...), nil
 }
 
@@ -192,6 +209,9 @@ type incubatorArgs struct {
 	forceV1Behavior    bool
 	debugTest          bool
 	isSELinuxEnforcing bool
+	encodedEnv         string
+	allowListEnvKeys   string
+	forwardedEnviron   []string
 }
 
 func parseIncubatorArgs(args []string) (incubatorArgs, error) {
@@ -215,6 +235,7 @@ func parseIncubatorArgs(args []string) (incubatorArgs, error) {
 	flags.BoolVar(&ia.forceV1Behavior, "force-v1-behavior", false, "allow falling back to the su command if login is unavailable")
 	flags.BoolVar(&ia.debugTest, "debug-test", false, "should debug in test mode")
 	flags.BoolVar(&ia.isSELinuxEnforcing, "is-selinux-enforcing", false, "whether SELinux is in enforcing mode")
+	flags.StringVar(&ia.encodedEnv, "encoded-env", "", "JSON encoded array of environment variables in '['key=value']' format")
 	flags.Parse(args)
 
 	for _, g := range strings.Split(groups, ",") {
@@ -225,6 +246,30 @@ func parseIncubatorArgs(args []string) (incubatorArgs, error) {
 		ia.gids = append(ia.gids, gid)
 	}
 
+	ia.forwardedEnviron = os.Environ()
+	// pass through SSH_AUTH_SOCK environment variable to support ssh agent forwarding
+	ia.allowListEnvKeys = "SSH_AUTH_SOCK"
+
+	if ia.encodedEnv != "" {
+		unquoted, err := strconv.Unquote(ia.encodedEnv)
+		if err != nil {
+			return ia, fmt.Errorf("unable to parse encodedEnv %q: %w", ia.encodedEnv, err)
+		}
+
+		var extraEnviron []string
+
+		err = json.Unmarshal([]byte(unquoted), &extraEnviron)
+		if err != nil {
+			return ia, fmt.Errorf("unable to parse encodedEnv %q: %w", ia.encodedEnv, err)
+		}
+
+		ia.forwardedEnviron = append(ia.forwardedEnviron, extraEnviron...)
+
+		for _, v := range extraEnviron {
+			ia.allowListEnvKeys = fmt.Sprintf("%s,%s", ia.allowListEnvKeys, strings.Split(v, "=")[0])
+		}
+	}
+
 	return ia, nil
 }
 
@@ -406,7 +451,7 @@ func tryExecLogin(dlogf logger.Logf, ia incubatorArgs) error {
 	dlogf("logging in with %+v", loginArgs)
 
 	// If Exec works, the Go code will not proceed past this:
-	err = unix.Exec(loginCmdPath, loginArgs, os.Environ())
+	err = unix.Exec(loginCmdPath, loginArgs, ia.forwardedEnviron)
 
 	// If we made it here, Exec failed.
 	return err
@@ -441,7 +486,7 @@ func trySU(dlogf logger.Logf, ia incubatorArgs) (handled bool, err error) {
 
 	loginArgs := []string{
 		su,
-		"-w", "SSH_AUTH_SOCK", // pass through SSH_AUTH_SOCK environment variable to support ssh agent forwarding
+		"-w", ia.allowListEnvKeys,
 		"-l",
 		ia.localUser,
 	}
@@ -453,7 +498,7 @@ func trySU(dlogf logger.Logf, ia incubatorArgs) (handled bool, err error) {
 	dlogf("logging in with %+v", loginArgs)
 
 	// If Exec works, the Go code will not proceed past this:
-	err = unix.Exec(su, loginArgs, os.Environ())
+	err = unix.Exec(su, loginArgs, ia.forwardedEnviron)
 
 	// If we made it here, Exec failed.
 	return true, err
@@ -482,11 +527,11 @@ func findSU(dlogf logger.Logf, ia incubatorArgs) string {
 		return ""
 	}
 
-	// First try to execute su -w SSH_AUTH_SOCK -l <user> -c true
+	// First try to execute su -w <allow listed env> -l <user> -c true
 	// to make sure su supports the necessary arguments.
 	err = exec.Command(
 		su,
-		"-w", "SSH_AUTH_SOCK",
+		"-w", ia.allowListEnvKeys,
 		"-l",
 		ia.localUser,
 		"-c", "true",
@@ -515,7 +560,7 @@ func handleSSHInProcess(dlogf logger.Logf, ia incubatorArgs) error {
 
 	args := shellArgs(ia.isShell, ia.cmd)
 	dlogf("running %s %q", ia.loginShell, args)
-	cmd := newCommand(ia.hasTTY, ia.loginShell, args)
+	cmd := newCommand(ia.hasTTY, ia.loginShell, ia.forwardedEnviron, args)
 	err := cmd.Run()
 	if ee, ok := err.(*exec.ExitError); ok {
 		ps := ee.ProcessState
@@ -532,12 +577,12 @@ func handleSSHInProcess(dlogf logger.Logf, ia incubatorArgs) error {
 	return err
 }
 
-func newCommand(hasTTY bool, cmdPath string, cmdArgs []string) *exec.Cmd {
+func newCommand(hasTTY bool, cmdPath string, cmdEnviron []string, cmdArgs []string) *exec.Cmd {
 	cmd := exec.Command(cmdPath, cmdArgs...)
 	cmd.Stdin = os.Stdin
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr
-	cmd.Env = os.Environ()
+	cmd.Env = cmdEnviron
 
 	if hasTTY {
 		// If we were launched with a tty then we should mark that as the ctty

ssh/tailssh/tailssh.go

@@ -238,6 +238,7 @@ type conn struct {
 	localUser    *userMeta       // set by doPolicyAuth
 	userGroupIDs []string        // set by doPolicyAuth
 	pubKey       gossh.PublicKey // set by doPolicyAuth
+	acceptEnv    []string
 
 	// mu protects the following fields.
 	//
@@ -377,7 +378,7 @@ func (c *conn) doPolicyAuth(ctx ssh.Context, pubKey ssh.PublicKey) error {
 		c.logf("failed to get conninfo: %v", err)
 		return errDenied
 	}
-	a, localUser, err := c.evaluatePolicy(pubKey)
+	a, localUser, acceptEnv, err := c.evaluatePolicy(pubKey)
 	if err != nil {
 		if pubKey == nil && c.havePubKeyPolicy() {
 			return errPubKeyRequired
@@ -387,6 +388,7 @@ func (c *conn) doPolicyAuth(ctx ssh.Context, pubKey ssh.PublicKey) error {
 	c.action0 = a
 	c.currentAction = a
 	c.pubKey = pubKey
+	c.acceptEnv = acceptEnv
 	if a.Message != "" {
 		if err := ctx.SendAuthBanner(a.Message); err != nil {
 			return fmt.Errorf("SendBanner: %w", err)
@@ -619,16 +621,16 @@ func (c *conn) setInfo(ctx ssh.Context) error {
 
 // evaluatePolicy returns the SSHAction and localUser after evaluating
 // the SSHPolicy for this conn. The pubKey may be nil for "none" auth.
-func (c *conn) evaluatePolicy(pubKey gossh.PublicKey) (_ *tailcfg.SSHAction, localUser string, _ error) {
+func (c *conn) evaluatePolicy(pubKey gossh.PublicKey) (_ *tailcfg.SSHAction, localUser string, acceptEnv []string, _ error) {
 	pol, ok := c.sshPolicy()
 	if !ok {
-		return nil, "", fmt.Errorf("tailssh: rejecting connection; no SSH policy")
+		return nil, "", nil, fmt.Errorf("tailssh: rejecting connection; no SSH policy")
 	}
-	a, localUser, ok := c.evalSSHPolicy(pol, pubKey)
+	a, localUser, acceptEnv, ok := c.evalSSHPolicy(pol, pubKey)
 	if !ok {
-		return nil, "", fmt.Errorf("tailssh: rejecting connection; no matching policy")
+		return nil, "", nil, fmt.Errorf("tailssh: rejecting connection; no matching policy")
 	}
-	return a, localUser, nil
+	return a, localUser, acceptEnv, nil
 }
 
 // pubKeyCacheEntry is the cache value for an HTTPS URL of public keys (like
@@ -892,7 +894,7 @@ func (c *conn) newSSHSession(s ssh.Session) *sshSession {
 
 // isStillValid reports whether the conn is still valid.
 func (c *conn) isStillValid() bool {
-	a, localUser, err := c.evaluatePolicy(c.pubKey)
+	a, localUser, _, err := c.evaluatePolicy(c.pubKey)
 	c.vlogf("stillValid: %+v %v %v", a, localUser, err)
 	if err != nil {
 		return false
@@ -1275,13 +1277,13 @@ func (c *conn) ruleExpired(r *tailcfg.SSHRule) bool {
 	return r.RuleExpires.Before(c.srv.now())
 }
 
-func (c *conn) evalSSHPolicy(pol *tailcfg.SSHPolicy, pubKey gossh.PublicKey) (a *tailcfg.SSHAction, localUser string, ok bool) {
+func (c *conn) evalSSHPolicy(pol *tailcfg.SSHPolicy, pubKey gossh.PublicKey) (a *tailcfg.SSHAction, localUser string, acceptEnv []string, ok bool) {
 	for _, r := range pol.Rules {
-		if a, localUser, err := c.matchRule(r, pubKey); err == nil {
+		if a, localUser, acceptEnv, err := c.matchRule(r, pubKey); err == nil {
-			return a, localUser, true
+			return a, localUser, acceptEnv, true
 		}
 	}
-	return nil, "", false
+	return nil, "", nil, false
 }
 
 // internal errors for testing; they don't escape to callers or logs.
@@ -1294,26 +1296,26 @@ var (
 	errInvalidConn    = errors.New("invalid connection state")
 )
 
-func (c *conn) matchRule(r *tailcfg.SSHRule, pubKey gossh.PublicKey) (a *tailcfg.SSHAction, localUser string, err error) {
+func (c *conn) matchRule(r *tailcfg.SSHRule, pubKey gossh.PublicKey) (a *tailcfg.SSHAction, localUser string, acceptEnv []string, err error) {
 	defer func() {
 		c.vlogf("matchRule(%+v): %v", r, err)
 	}()
 
 	if c == nil {
-		return nil, "", errInvalidConn
+		return nil, "", nil, errInvalidConn
 	}
 	if c.info == nil {
 		c.logf("invalid connection state")
-		return nil, "", errInvalidConn
+		return nil, "", nil, errInvalidConn
 	}
 	if r == nil {
-		return nil, "", errNilRule
+		return nil, "", nil, errNilRule
 	}
 	if r.Action == nil {
-		return nil, "", errNilAction
+		return nil, "", nil, errNilAction
 	}
 	if c.ruleExpired(r) {
-		return nil, "", errRuleExpired
+		return nil, "", nil, errRuleExpired
 	}
 	if !r.Action.Reject {
 		// For all but Reject rules, SSHUsers is required.
@@ -1321,15 +1323,15 @@ func (c *conn) matchRule(r *tailcfg.SSHRule, pubKey gossh.PublicKey) (a *tailcfg
 		// empty string anyway.
 		localUser = mapLocalUser(r.SSHUsers, c.info.sshUser)
 		if localUser == "" {
-			return nil, "", errUserMatch
+			return nil, "", nil, errUserMatch
 		}
 	}
 	if ok, err := c.anyPrincipalMatches(r.Principals, pubKey); err != nil {
-		return nil, "", err
+		return nil, "", nil, err
 	} else if !ok {
-		return nil, "", errPrincipalMatch
+		return nil, "", nil, errPrincipalMatch
 	}
-	return r.Action, localUser, nil
+	return r.Action, localUser, r.AcceptEnv, nil
 }
 
 func mapLocalUser(ruleSSHUsers map[string]string, reqSSHUser string) (localUser string) {

ssh/tailssh/tailssh_integration_test.go

@@ -108,6 +108,7 @@ func TestIntegrationSSH(t *testing.T) {
 		want            []string
 		forceV1Behavior bool
 		skip            bool
+		allowSendEnv    bool
 	}{
 		{
 			cmd:             "id",
@@ -131,6 +132,18 @@ func TestIntegrationSSH(t *testing.T) {
 			skip:            os.Getenv("SKIP_FILE_OPS") == "1" || !fallbackToSUAvailable(),
 			forceV1Behavior: false,
 		},
+		{
+			cmd:             `echo "${GIT_ENV_VAR:-unset1} ${EXACT_MATCH:-unset2} ${TESTING:-unset3} ${NOT_ALLOWED:-unset4}"`,
+			want:            []string{"working1 working2 working3 unset4"},
+			forceV1Behavior: false,
+			allowSendEnv:    true,
+		},
+		{
+			cmd:             `echo "${GIT_ENV_VAR:-unset1} ${EXACT_MATCH:-unset2} ${TESTING:-unset3} ${NOT_ALLOWED:-unset4}"`,
+			want:            []string{"unset1 unset2 unset3 unset4"},
+			forceV1Behavior: false,
+			allowSendEnv:    false,
+		},
 	}
 
 	for _, test := range tests {
@@ -151,7 +164,13 @@ func TestIntegrationSSH(t *testing.T) {
 			}
 
 			t.Run(fmt.Sprintf("%s_%s_%s", test.cmd, shellQualifier, versionQualifier), func(t *testing.T) {
-				s := testSession(t, test.forceV1Behavior)
+				sendEnv := map[string]string{
+					"GIT_ENV_VAR": "working1",
+					"EXACT_MATCH": "working2",
+					"TESTING":     "working3",
+					"NOT_ALLOWED": "working4",
+				}
+				s := testSession(t, test.forceV1Behavior, test.allowSendEnv, sendEnv)
 
 				if shell {
 					err := s.RequestPty("xterm", 40, 80, ssh.TerminalModes{
@@ -201,7 +220,7 @@ func TestIntegrationSFTP(t *testing.T) {
 			}
 			wantText := "hello world"
 
-			cl := testClient(t, forceV1Behavior)
+			cl := testClient(t, forceV1Behavior, false)
 			scl, err := sftp.NewClient(cl)
 			if err != nil {
 				t.Fatalf("can't get sftp client: %s", err)
@@ -233,7 +252,7 @@ func TestIntegrationSFTP(t *testing.T) {
 				t.Fatalf("unexpected file contents (-got +want):\n%s", diff)
 			}
 
-			s := testSessionFor(t, cl)
+			s := testSessionFor(t, cl, nil)
 			got := s.run(t, "ls -l "+filePath, false)
 			if !strings.Contains(got, "testuser") {
 				t.Fatalf("unexpected file owner user: %s", got)
@@ -262,7 +281,7 @@ func TestIntegrationSCP(t *testing.T) {
 			}
 			wantText := "hello world"
 
-			cl := testClient(t, forceV1Behavior)
+			cl := testClient(t, forceV1Behavior, false)
 			scl, err := scp.NewClientBySSH(cl)
 			if err != nil {
 				t.Fatalf("can't get sftp client: %s", err)
@@ -291,7 +310,7 @@ func TestIntegrationSCP(t *testing.T) {
 				t.Fatalf("unexpected file contents (-got +want):\n%s", diff)
 			}
 
-			s := testSessionFor(t, cl)
+			s := testSessionFor(t, cl, nil)
 			got := s.run(t, "ls -l "+filePath, false)
 			if !strings.Contains(got, "testuser") {
 				t.Fatalf("unexpected file owner user: %s", got)
@@ -349,7 +368,7 @@ func TestSSHAgentForwarding(t *testing.T) {
 
 	// Run tailscale SSH server and connect to it
 	username := "testuser"
-	tailscaleAddr := testServer(t, username, false)
+	tailscaleAddr := testServer(t, username, false, false)
 	tcl, err := ssh.Dial("tcp", tailscaleAddr, &ssh.ClientConfig{
 		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
 	})
@@ -465,11 +484,11 @@ readLoop:
 	return string(_got)
 }
 
-func testClient(t *testing.T, forceV1Behavior bool, authMethods ...ssh.AuthMethod) *ssh.Client {
+func testClient(t *testing.T, forceV1Behavior bool, allowSendEnv bool, authMethods ...ssh.AuthMethod) *ssh.Client {
 	t.Helper()
 
 	username := "testuser"
-	addr := testServer(t, username, forceV1Behavior)
+	addr := testServer(t, username, forceV1Behavior, allowSendEnv)
 
 	cl, err := ssh.Dial("tcp", addr, &ssh.ClientConfig{
 		HostKeyCallback: ssh.InsecureIgnoreHostKey(),
@@ -483,9 +502,9 @@ func testClient(t *testing.T, forceV1Behavior bool, authMethods ...ssh.AuthMetho
 	return cl
 }
 
-func testServer(t *testing.T, username string, forceV1Behavior bool) string {
+func testServer(t *testing.T, username string, forceV1Behavior bool, allowSendEnv bool) string {
 	srv := &server{
-		lb:             &testBackend{localUser: username, forceV1Behavior: forceV1Behavior},
+		lb:             &testBackend{localUser: username, forceV1Behavior: forceV1Behavior, allowSendEnv: allowSendEnv},
 		logf:           log.Printf,
 		tailscaledPath: os.Getenv("TAILSCALED_PATH"),
 		timeNow:        time.Now,
@@ -509,16 +528,20 @@ func testServer(t *testing.T, username string, forceV1Behavior bool) string {
 	return l.Addr().String()
 }
 
-func testSession(t *testing.T, forceV1Behavior bool) *session {
+func testSession(t *testing.T, forceV1Behavior bool, allowSendEnv bool, sendEnv map[string]string) *session {
-	cl := testClient(t, forceV1Behavior)
+	cl := testClient(t, forceV1Behavior, allowSendEnv)
-	return testSessionFor(t, cl)
+	return testSessionFor(t, cl, sendEnv)
 }
 
-func testSessionFor(t *testing.T, cl *ssh.Client) *session {
+func testSessionFor(t *testing.T, cl *ssh.Client, sendEnv map[string]string) *session {
 	s, err := cl.NewSession()
 	if err != nil {
 		t.Fatal(err)
 	}
+	for k, v := range sendEnv {
+		s.Setenv(k, v)
+	}
+
 	t.Cleanup(func() { s.Close() })
 
 	stdinReader, stdinWriter := io.Pipe()
@@ -564,6 +587,7 @@ func generateClientKey(t *testing.T, privateKeyFile string) (ssh.Signer, *rsa.Pr
 type testBackend struct {
 	localUser       string
 	forceV1Behavior bool
+	allowSendEnv    bool
 }
 
 func (tb *testBackend) GetSSH_HostKeys() ([]gossh.Signer, error) {
@@ -597,6 +621,9 @@ func (tb *testBackend) NetMap() *netmap.NetworkMap {
 	if tb.forceV1Behavior {
 		capMap[tailcfg.NodeAttrSSHBehaviorV1] = struct{}{}
 	}
+	if tb.allowSendEnv {
+		capMap[tailcfg.NodeAttrSSHEnvironmentVariables] = struct{}{}
+	}
 	return &netmap.NetworkMap{
 		SSHPolicy: &tailcfg.SSHPolicy{
 			Rules: []*tailcfg.SSHRule{
@@ -604,6 +631,7 @@ func (tb *testBackend) NetMap() *netmap.NetworkMap {
 					Principals: []*tailcfg.SSHPrincipal{{Any: true}},
 					Action:     &tailcfg.SSHAction{Accept: true, AllowAgentForwarding: true},
 					SSHUsers:   map[string]string{"*": tb.localUser},
+					AcceptEnv:  []string{"GIT_*", "EXACT_MATCH", "TEST?NG"},
 				},
 			},
 		},

ssh/tailssh/tailssh_test.go

@@ -24,6 +24,7 @@ import (
 	"os/user"
 	"reflect"
 	"runtime"
+	"slices"
 	"strconv"
 	"strings"
 	"sync"
@@ -61,6 +62,7 @@ func TestMatchRule(t *testing.T) {
 		ci            *sshConnInfo
 		wantErr       error
 		wantUser      string
+		wantAcceptEnv []string
 	}{
 		{
 			name: "invalid-conn",
@@ -153,6 +155,21 @@ func TestMatchRule(t *testing.T) {
 			ci:       &sshConnInfo{sshUser: "alice"},
 			wantUser: "thealice",
 		},
+		{
+			name: "ok-with-accept-env",
+			rule: &tailcfg.SSHRule{
+				Action:     someAction,
+				Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+				SSHUsers: map[string]string{
+					"*":     "ubuntu",
+					"alice": "thealice",
+				},
+				AcceptEnv: []string{"EXAMPLE", "?_?", "TEST_*"},
+			},
+			ci:            &sshConnInfo{sshUser: "alice"},
+			wantUser:      "thealice",
+			wantAcceptEnv: []string{"EXAMPLE", "?_?", "TEST_*"},
+		},
 		{
 			name: "no-users-for-reject",
 			rule: &tailcfg.SSHRule{
@@ -210,7 +227,7 @@ func TestMatchRule(t *testing.T) {
 				info: tt.ci,
 				srv:  &server{logf: t.Logf},
 			}
-			got, gotUser, err := c.matchRule(tt.rule, nil)
+			got, gotUser, gotAcceptEnv, err := c.matchRule(tt.rule, nil)
 			if err != tt.wantErr {
 				t.Errorf("err = %v; want %v", err, tt.wantErr)
 			}
@@ -220,6 +237,128 @@ func TestMatchRule(t *testing.T) {
 			if err == nil && got == nil {
 				t.Errorf("expected non-nil action on success")
 			}
+			if !slices.Equal(gotAcceptEnv, tt.wantAcceptEnv) {
+				t.Errorf("acceptEnv = %v; want %v", gotAcceptEnv, tt.wantAcceptEnv)
+			}
+		})
+	}
+}
+
+func TestEvalSSHPolicy(t *testing.T) {
+	someAction := new(tailcfg.SSHAction)
+	tests := []struct {
+		name          string
+		policy        *tailcfg.SSHPolicy
+		ci            *sshConnInfo
+		wantMatch     bool
+		wantUser      string
+		wantAcceptEnv []string
+	}{
+		{
+			name: "multiple-matches-picks-first-match",
+			policy: &tailcfg.SSHPolicy{
+				Rules: []*tailcfg.SSHRule{
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"other": "other1",
+						},
+					},
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"*":     "ubuntu",
+							"alice": "thealice",
+						},
+						AcceptEnv: []string{"EXAMPLE", "?_?", "TEST_*"},
+					},
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"other2": "other3",
+						},
+					},
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"*":     "ubuntu",
+							"alice": "thealice",
+							"mark":  "markthe",
+						},
+						AcceptEnv: []string{"*"},
+					},
+				},
+			},
+			ci:            &sshConnInfo{sshUser: "alice"},
+			wantUser:      "thealice",
+			wantAcceptEnv: []string{"EXAMPLE", "?_?", "TEST_*"},
+			wantMatch:     true,
+		},
+		{
+			name: "no-matches-returns-failure",
+			policy: &tailcfg.SSHPolicy{
+				Rules: []*tailcfg.SSHRule{
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"other": "other1",
+						},
+					},
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"fedora": "ubuntu",
+						},
+						AcceptEnv: []string{"EXAMPLE", "?_?", "TEST_*"},
+					},
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"other2": "other3",
+						},
+					},
+					{
+						Action:     someAction,
+						Principals: []*tailcfg.SSHPrincipal{{Any: true}},
+						SSHUsers: map[string]string{
+							"mark": "markthe",
+						},
+						AcceptEnv: []string{"*"},
+					},
+				},
+			},
+			ci:            &sshConnInfo{sshUser: "alice"},
+			wantUser:      "",
+			wantAcceptEnv: nil,
+			wantMatch:     false,
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			c := &conn{
+				info: tt.ci,
+				srv:  &server{logf: t.Logf},
+			}
+			got, gotUser, gotAcceptEnv, match := c.evalSSHPolicy(tt.policy, nil)
+			if match != tt.wantMatch {
+				t.Errorf("match = %v; want %v", match, tt.wantMatch)
+			}
+			if gotUser != tt.wantUser {
+				t.Errorf("user = %q; want %q", gotUser, tt.wantUser)
+			}
+			if tt.wantMatch == true && got == nil {
+				t.Errorf("expected non-nil action on success")
+			}
+			if !slices.Equal(gotAcceptEnv, tt.wantAcceptEnv) {
+				t.Errorf("acceptEnv = %v; want %v", gotAcceptEnv, tt.wantAcceptEnv)
+			}
 		})
 	}
 }