From 7ef3787c84b6b524501211b11a26c742f829af1a Mon Sep 17 00:00:00 2001
From: "djm@openbsd.org" <djm@openbsd.org>
Date: Mon, 18 Dec 2023 14:47:44 +0000
Subject: [PATCH] upstream: ban user/hostnames with most shell metacharacters
 This makes ssh(1) refuse user or host names provided on the commandline that
 contain most shell metacharacters.

Some programs that invoke ssh(1) using untrusted data do not filter
metacharacters in arguments they supply. This could create
interactions with user-specified ProxyCommand and other directives
that allow shell injection attacks to occur.

It's a mistake to invoke ssh(1) with arbitrary untrusted arguments,
but getting this stuff right can be tricky, so this should prevent
most obvious ways of creating risky situations. It however is not
and cannot be perfect: ssh(1) has no practical way of interpreting
what shell quoting rules are in use and how they interact with the
user's specified ProxyCommand.

To allow configurations that use strange user or hostnames to
continue to work, this strictness is applied only to names coming
from the commandline. Names specified using User or Hostname
directives in ssh_config(5) are not affected.

feedback/ok millert@ markus@ dtucker@ deraadt@

OpenBSD-Commit-ID: 3b487348b5964f3e77b6b4d3da4c3b439e94b2d9

CVE: CVE-2023-51385

Upstream-Status: Backport
[https://github.com/openssh/openssh-portable/commit/7ef3787c84b6b524501211b11a26c742f829af1a]

Signed-off-by: Archana Polampalli <archana.polampalli@windriver.com>
---
 ssh.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/ssh.c b/ssh.c
index 8ff9788..82ed15f 100644
--- a/ssh.c
+++ b/ssh.c
@@ -611,6 +611,41 @@ ssh_conn_info_free(struct ssh_conn_info *cinfo)
	free(cinfo);
 }

+static int
+valid_hostname(const char *s)
+{
+	size_t i;
+
+	if (*s == '-')
+		return 0;
+	for (i = 0; s[i] != 0; i++) {
+		if (strchr("'`\"$\\;&<>|(){}", s[i]) != NULL ||
+		    isspace((u_char)s[i]) || iscntrl((u_char)s[i]))
+			return 0;
+	}
+	return 1;
+}
+
+static int
+valid_ruser(const char *s)
+{
+	size_t i;
+
+	if (*s == '-')
+		return 0;
+	for (i = 0; s[i] != 0; i++) {
+		if (strchr("'`\";&<>|(){}", s[i]) != NULL)
+			return 0;
+		/* Disallow '-' after whitespace */
+		if (isspace((u_char)s[i]) && s[i + 1] == '-')
+			return 0;
+		/* Disallow \ in last position */
+		if (s[i] == '\\' && s[i + 1] == '\0')
+			return 0;
+	}
+	return 1;
+}
+
 /*
  * Main program for the ssh client.
  */
@@ -1097,6 +1132,10 @@ main(int ac, char **av)
	if (!host)
		usage();

+	if (!valid_hostname(host))
+		fatal("hostname contains invalid characters");
+	if (options.user != NULL && !valid_ruser(options.user))
+		fatal("remote username contains invalid characters");
	host_arg = xstrdup(host);

	/* Initialize the command to execute on remote host. */
--
2.40.0
