allow specifying a whitelist of env vars to pass from client

Change-type: major
Signed-off-by: Will Boyce <[email protected]>

Author: wrboyce

Diff for: balena/main.go

@@ -27,6 +27,7 @@ import (
 	"os"
 	"path"
 	"runtime"
+	"strings"
 	"syscall"
 
 	"github.com/balena-io/sshproxy"
@@ -49,7 +50,7 @@ func init() {
 	pflag.CommandLine.Int64P("shell-gid", "g", -1, "Group to run shell as (default: current gid)")
 	pflag.CommandLine.StringP("auth-failed-banner", "b", "", "Path to template displayed after failed authentication")
 	pflag.CommandLine.IntP("max-auth-tries", "m", 0, "Maximum number of authentication attempts per connection (default 0; unlimited)")
-	pflag.CommandLine.BoolP("allow-env", "E", false, "Pass environment from client to shell (default: false) (warning: security implications)")
+	pflag.CommandLine.StringP("allow-env", "E", "", "List of environment variables to pass from client to shell (default: None)")
 	pflag.CommandLine.StringP("sentry-dsn", "S", "", "Sentry DSN for error reporting")
 	pflag.CommandLine.IntP("verbosity", "v", 1, "Set verbosity level (0 = quiet, 1 = normal, 2 = verbose, 3 = debug, default: 1)")
 	pflag.CommandLine.BoolP("version", "", false, "Display version and exit")
@@ -179,7 +180,7 @@ func main() {
 	server, err := sshproxy.New(
 		viper.GetString("dir"),
 		viper.GetString("shell"),
-		viper.GetBool("allow-env"),
+		strings.Split(viper.GetString("allow-env"), ","),
 		shellCreds,
 		viper.GetInt("verbosity"),
 		sshConfig,

Diff for: sshproxy.go

@@ -45,7 +45,7 @@ type Server struct {
 	config       *ssh.ServerConfig
 	shell        string
 	shellCreds   *syscall.Credential
-	passEnv      bool
+	envWhitelist map[string]bool
 	errorHandler ErrorHandler
 	verbosity    int
 }
@@ -57,13 +57,13 @@ type ErrorHandler func(error, map[string]string)
 // and an ssh.ServerConfig. If no ServerConfig is provided, then
 // ServerConfig.NoClientAuth is set to true. ed25519, rsa, ecdsa and dsa
 // keys are loaded, and generated if they do not exist. Returns a new Server.
-func New(keyDir, shell string, passEnv bool, shellCreds *syscall.Credential, verbosity int, sshConfig *ssh.ServerConfig, errorHandler ErrorHandler) (*Server, error) {
+func New(keyDir, shell string, envWhitelist []string, shellCreds *syscall.Credential, verbosity int, sshConfig *ssh.ServerConfig, errorHandler ErrorHandler) (*Server, error) {
 	s := &Server{
 		keyDir:       keyDir,
 		config:       sshConfig,
 		shell:        shell,
 		shellCreds:   shellCreds,
-		passEnv:      passEnv,
+		envWhitelist: make(map[string]bool),
 		errorHandler: errorHandler,
 		verbosity:    verbosity,
 	}
@@ -72,6 +72,9 @@ func New(keyDir, shell string, passEnv bool, shellCreds *syscall.Credential, ver
 			NoClientAuth: true,
 		}
 	}
+	for _, envKey := range envWhitelist {
+		s.envWhitelist[envKey] = true
+	}
 	for _, keyType := range []string{"ed25519", "rsa", "ecdsa", "dsa"} {
 		if err := s.addHostKey(keyType); err != nil {
 			return nil, err
@@ -211,15 +214,16 @@ func (s *Server) handleRequests(reqs <-chan *ssh.Request, channel ssh.Channel, c
 	for req := range reqs {
 		switch req.Type {
 		case "env":
-			if s.passEnv {
-				// append client env to the command environment
-				keyLen := binary.BigEndian.Uint32(req.Payload[:4])
-				valLen := binary.BigEndian.Uint32(req.Payload[keyLen+4 : keyLen+8])
-				key := string(req.Payload[4 : keyLen+4])
+			// append client env to the command environment
+			keyLen := binary.BigEndian.Uint32(req.Payload[:4])
+			valLen := binary.BigEndian.Uint32(req.Payload[keyLen+4 : keyLen+8])
+			key := string(req.Payload[4 : keyLen+4])
+			_, ok := s.envWhitelist[key]
+			if ok {
 				val := string(req.Payload[keyLen+8 : keyLen+valLen+8])
 				env = append(env, fmt.Sprintf("%s=%s", key, val))
 			}
-			if err := req.Reply(s.passEnv, nil); err != nil {
+			if err := req.Reply(ok, nil); err != nil {
 				return err
 			}
 		case "pty-req":