upstream: Improve rules for %-expansion of username.

Usernames passed on the commandline will no longer be subject to
% expansion. Some tools invoke ssh with connection information
(i.e. usernames and host names) supplied from untrusted sources.
These may contain % expansion sequences which could yield
unexpected results.

Since openssh-9.6, all usernames have been subject to validity
checking. This change tightens the validity checks by refusing
usernames that include control characters (again, these can cause
surprises when supplied adversarially).

This change also relaxes the validity checks in one small way:
usernames supplied via the configuration file as literals (i.e.
include no % expansion characters) are not subject to these
validity checks. This allows usernames that contain arbitrary
characters to be used, but only via configuration files. This
is done on the basis that ssh's configuration is trusted.

Pointed out by David Leadbeater, ok deraadt@

OpenBSD-Commit-ID: e2f0c871fbe664aba30607321575e7c7fc798362

diff --git a/ssh.c b/ssh.c
index 1eaa5986ed47a8cc302dc48b54cb1d4408138463..6bbb82874e456be97a2cf0e0b6cbbd950d82edde 100644 (file)
--- a/ssh.c
+++ b/ssh.c
@@ -1,4 +1,4 @@
-/* $OpenBSD: ssh.c,v 1.616 2025/08/29 03:50:38 djm Exp $ */
+/* $OpenBSD: ssh.c,v 1.617 2025/09/04 00:29:09 djm Exp $ */
 /*
  * Author: Tatu Ylonen <ylo@cs.hut.fi>
  * Copyright (c) 1995 Tatu Ylonen <ylo@cs.hut.fi>, Espoo, Finland
@@ -668,6 +668,8 @@ valid_ruser(const char *s)
        if (*s == '-')
                return 0;
        for (i = 0; s[i] != 0; i++) {
+               if (iscntrl((u_char)s[i]))
+                       return 0;
                if (strchr("'`\";&<>|(){}", s[i]) != NULL)
                        return 0;
                /* Disallow '-' after whitespace */
@@ -689,6 +691,7 @@ main(int ac, char **av)
        struct ssh *ssh = NULL;
        int i, r, opt, exit_status, use_syslog, direct, timeout_ms;
        int was_addr, config_test = 0, opt_terminated = 0, want_final_pass = 0;
+       int user_on_commandline = 0, user_was_default = 0, user_expanded = 0;
        char *p, *cp, *line, *argv0, *logfile, *args;
        char cname[NI_MAXHOST], thishost[NI_MAXHOST];
        struct stat st;
@@ -1037,8 +1040,10 @@ main(int ac, char **av)
                        }
                        break;
                case 'l':
-                       if (options.user == NULL)
+                       if (options.user == NULL) {
                                options.user = xstrdup(optarg);
+                               user_on_commandline = 1;
+                       }
                        break;
 
                case 'L':
@@ -1141,6 +1146,7 @@ main(int ac, char **av)
                        if (options.user == NULL) {
                                options.user = tuser;
                                tuser = NULL;
+                               user_on_commandline = 1;
                        }
                        free(tuser);
                        if (options.port == -1 && tport != -1)
@@ -1155,6 +1161,7 @@ main(int ac, char **av)
                                if (options.user == NULL) {
                                        options.user = p;
                                        p = NULL;
+                                       user_on_commandline = 1;
                                }
                                *cp++ = '\0';
                                host = xstrdup(cp);
@@ -1314,8 +1321,10 @@ main(int ac, char **av)
        if (fill_default_options(&options) != 0)
                cleanup_exit(255);
 
-       if (options.user == NULL)
+       if (options.user == NULL) {
+               user_was_default = 1;
                options.user = xstrdup(pw->pw_name);
+       }
 
        /*
         * If ProxyJump option specified, then construct a ProxyCommand now.
@@ -1464,20 +1473,30 @@ main(int ac, char **av)
            "" : options.jump_host);
 
        /*
-        * Expand User. It cannot contain %r (itself) or %C since User is
+        * If the user was specified via a configuration directive then attempt
+        * to expand it. It cannot contain %r (itself) or %C since User is
         * a component of the hash.
         */
-       if (options.user != NULL) {
+       if (!user_on_commandline && !user_was_default) {
                if ((p = percent_dollar_expand(options.user,
                    DEFAULT_CLIENT_PERCENT_EXPAND_ARGS_NOUSER(cinfo),
                    (char *)NULL)) == NULL)
                        fatal("invalid environment variable expansion");
+               user_expanded = strcmp(p, options.user) != 0;
                free(options.user);
                options.user = p;
-               if (!valid_ruser(options.user))
-                       fatal("remote username contains invalid characters");
        }
 
+       /*
+        * Usernames specified on the commandline or expanded from the
+        * configuration file must be validated.
+        * Conversely, usernames from getpwnam(3) or specified as literals
+        * via configuration (i.e. not expanded) are not subject to validation.
+        */
+       if ((user_on_commandline || user_expanded) &&
+           !valid_ruser(options.user))
+               fatal("remote username contains invalid characters");
+
        /* Now User is expanded, store it and calculate hash. */
        cinfo->remuser = xstrdup(options.user);
        cinfo->conn_hash_hex = ssh_connection_hash(cinfo->thishost,