From 812f934e77700afedbf5e929b282f29a47b2d9c6 Mon Sep 17 00:00:00 2001
From: Samanta Navarro <ferivoz@riseup.net>
Date: Thu, 18 May 2023 11:56:17 +0000
Subject: [PATCH] process_prefix_flag: Drop privileges

Using --prefix in a setuid binary is quite dangerous. An unprivileged
user could prepare a custom shadow file in home directory. During a data
race the user could exchange directories with links which could lead to
exchange of shadow file in system's /etc directory.

This could be used for local privilege escalation.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>
---
 libmisc/prefix_flag.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/libmisc/prefix_flag.c b/libmisc/prefix_flag.c
index 56243f2e..d7acb9ca 100644
--- a/libmisc/prefix_flag.c
+++ b/libmisc/prefix_flag.c
@@ -85,6 +85,15 @@ extern const char* process_prefix_flag (const char* short_opt, int argc, char **
 
 
 	if (prefix != NULL) {
+		/* Drop privileges */
+		if (   (setregid (getgid (), getgid ()) != 0)
+		    || (setreuid (getuid (), getuid ()) != 0)) {
+			fprintf (shadow_logfd,
+			         _("%s: failed to drop privileges (%s)\n"),
+			         Prog, strerror (errno));
+			exit (EXIT_FAILURE);
+		}
+
 		if ( prefix[0] == '\0' || !strcmp(prefix, "/"))
 			return ""; /* if prefix is "/" then we ignore the flag option */
 		/* should we prevent symbolic link from being used as a prefix? */
-- 
2.27.0

