reject hostname option injection via bracketed mount source

A source like [-oProxyCommand=CMD]:/path passes the bracket-parsing
check in find_base_path() and ends up as -oProxyCommand=CMD in the
ssh argv.  When sftp_server is a path, ssh gets a destination argument
and executes the injected ProxyCommand before connecting.

Reject hostnames starting with - after bracket stripping, and add --
before the hostname in the ssh command line so positional args can't
be misread as options.

sshfs.c

@@ -4019,6 +4019,11 @@ static char *find_base_path(void)
 	*d++ = '\0';
 	s++;
 
+	if (sshfs.host[0] == '-') {
+		fprintf(stderr, "invalid hostname '%s'\n", sshfs.host);
+		exit(1);
+	}
+
 	return s;
 }
 
@@ -4410,7 +4415,6 @@ int main(int argc, char *argv[])
 	tmp = g_strdup_printf("-%i", sshfs.ssh_ver);
 	ssh_add_arg(tmp);
 	g_free(tmp);
-	ssh_add_arg(sshfs.host);
 	if (sshfs.sftp_server)
 		sftp_server = sshfs.sftp_server;
 	else if (sshfs.ssh_ver == 1)
@@ -4421,6 +4425,8 @@ int main(int argc, char *argv[])
 	if (sshfs.ssh_ver != 1 && strchr(sftp_server, '/') == NULL)
 		ssh_add_arg("-s");
 
+	ssh_add_arg("--");
+	ssh_add_arg(sshfs.host);
 	ssh_add_arg(sftp_server);
 	free(sshfs.sftp_server);
 

test/meson.build

@@ -1,5 +1,5 @@
 test_scripts = [ 'conftest.py', 'pytest.ini', 'test_sshfs.py',
-                 'util.py' ]
+                 'test_hostname_validation.py', 'util.py' ]
 custom_target('test_scripts', input: test_scripts,
               output: test_scripts, build_by_default: true,
               command: ['cp', '-fPp',

test/test_hostname_validation.py

@@ -0,0 +1,60 @@
+#!/usr/bin/env python3
+"""Tests for hostname validation — no FUSE mount required."""
+
+if __name__ == "__main__":
+    import pytest
+    import sys
+
+    sys.exit(pytest.main([__file__] + sys.argv[1:]))
+
+import subprocess
+from util import base_cmdline, basename
+from os.path import join as pjoin
+
+
+def test_reject_option_injection_in_hostname(tmpdir):
+    """Bracketed source that resolves to a dash-prefixed host must be rejected."""
+
+    mnt_dir = str(tmpdir.mkdir("mnt"))
+    malicious = "[-oProxyCommand=echo pwned]:/path"
+
+    cmdline = base_cmdline + [
+        pjoin(basename, "sshfs"),
+        "-f",
+        malicious,
+        mnt_dir,
+    ]
+    res = subprocess.run(
+        cmdline,
+        stdin=subprocess.DEVNULL,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        timeout=10,
+        text=True,
+    )
+    assert res.returncode != 0
+    assert "invalid hostname" in res.stderr
+
+
+def test_reject_dash_host_after_doubledash(tmpdir):
+    """Non-bracketed dash-prefixed source after -- must also be rejected."""
+
+    mnt_dir = str(tmpdir.mkdir("mnt"))
+
+    cmdline = base_cmdline + [
+        pjoin(basename, "sshfs"),
+        "-f",
+        "--",
+        "-oProxyCommand=echo pwned:/path",
+        mnt_dir,
+    ]
+    res = subprocess.run(
+        cmdline,
+        stdin=subprocess.DEVNULL,
+        stdout=subprocess.PIPE,
+        stderr=subprocess.PIPE,
+        timeout=10,
+        text=True,
+    )
+    assert res.returncode != 0
+    assert "invalid hostname" in res.stderr