From 9d47b8d3f50c3a6282896df8274147e3b9a38c56 Mon Sep 17 00:00:00 2001
From: Damien Miller <djm@mindrot.org>
Date: Thu, 10 Mar 2016 05:03:39 +1100
Subject: [PATCH] sanitise characters destined for xauth(1)

reported by github.com/tintinweb
---
 session.c | 32 ++++++++++++++++++++++++++++++--
 1 file changed, 30 insertions(+), 2 deletions(-)

diff --git a/session.c b/session.c
index 7a02500..87fddfc 100644
--- a/session.c
+++ b/session.c
@@ -46,6 +46,7 @@
 
 #include <arpa/inet.h>
 
+#include <ctype.h>
 #include <errno.h>
 #include <fcntl.h>
 #include <grp.h>
@@ -274,6 +275,21 @@ do_authenticated(Authctxt *authctxt)
 	do_cleanup(authctxt);
 }
 
+/* Check untrusted xauth strings for metacharacters */
+static int
+xauth_valid_string(const char *s)
+{
+	size_t i;
+
+	for (i = 0; s[i] != '\0'; i++) {
+		if (!isalnum((u_char)s[i]) &&
+		    s[i] != '.' && s[i] != ':' && s[i] != '/' &&
+		    s[i] != '-' && s[i] != '_')
+		return 0;
+	}
+	return 1;
+}
+
 /*
  * Prepares for an interactive session.  This is called after the user has
  * been successfully authenticated.  During this message exchange, pseudo
@@ -347,7 +363,13 @@ do_authenticated1(Authctxt *authctxt)
 				s->screen = 0;
 			}
 			packet_check_eom();
-			success = session_setup_x11fwd(s);
+			if (xauth_valid_string(s->auth_proto) &&
+			    xauth_valid_string(s->auth_data))
+				success = session_setup_x11fwd(s);
+			else {
+				success = 0;
+				error("Invalid X11 forwarding data");
+			}
 			if (!success) {
 				free(s->auth_proto);
 				free(s->auth_data);
@@ -2178,7 +2200,13 @@ session_x11_req(Session *s)
 	s->screen = packet_get_int();
 	packet_check_eom();
 
-	success = session_setup_x11fwd(s);
+	if (xauth_valid_string(s->auth_proto) &&
+	    xauth_valid_string(s->auth_data))
+		success = session_setup_x11fwd(s);
+	else {
+		success = 0;
+		error("Invalid X11 forwarding data");
+	}
 	if (!success) {
 		free(s->auth_proto);
 		free(s->auth_data);