ssh/tailssh: filter accepted environment variables

Noted by @danderson

Updates #3802

Change-Id: Iac70717ed57f11726209ac1ea93ddc6696605f94
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ssh/tailssh/incubator.go

@@ -220,7 +220,11 @@ func (ss *sshSession) launchProcess() error {
 	cmd := ss.cmd
 	cmd.Dir = ss.conn.localUser.HomeDir
 	cmd.Env = append(cmd.Env, envForUser(ss.conn.localUser)...)
-	cmd.Env = append(cmd.Env, ss.Environ()...)
+	for _, kv := range ss.Environ() {
+		if acceptEnvPair(kv) {
+			cmd.Env = append(cmd.Env, kv)
+		}
+	}
 
 	ci := ss.conn.info
 	cmd.Env = append(cmd.Env,
@@ -493,3 +497,14 @@ func updateStringInSlice(ss []string, a, b string) {
 		}
 	}
 }
+
+// acceptEnvPair reports whether the environment variable key=value pair
+// should be accepted from the client. It uses the same default as OpenSSH
+// AcceptEnv.
+func acceptEnvPair(kv string) bool {
+	k, _, ok := strings.Cut(kv, "=")
+	if !ok {
+		return false
+	}
+	return k == "TERM" || k == "LANG" || strings.HasPrefix(k, "LC_")
+}

ssh/tailssh/tailssh_test.go

@@ -431,3 +431,22 @@ func TestExpandPublicKeyURL(t *testing.T) {
 		t.Errorf("on empty: got %q; want %q", got, want)
 	}
 }
+
+func TestAcceptEnvPair(t *testing.T) {
+	tests := []struct {
+		in   string
+		want bool
+	}{
+		{"TERM=x", true},
+		{"term=x", false},
+		{"TERM", false},
+		{"LC_FOO=x", true},
+		{"LD_PRELOAD=naah", false},
+		{"TERM=screen-256color", true},
+	}
+	for _, tt := range tests {
+		if got := acceptEnvPair(tt.in); got != tt.want {
+			t.Errorf("for %q, got %v; want %v", tt.in, got, tt.want)
+		}
+	}
+}