CVE-2021-31566 (published 2022-08-23T16:15Z)

An improper link resolution flaw can occur while extracting an archive leading to changing modes, times, access control lists, and flags of a file outside of the archive. An attacker may provide a malicious archive to a victim user, who would trigger this flaw when trying to extract the archive. A local attacker may use this flaw to gain more privileges in a system.

CVSS v3 base score: 7.8, HIGH (CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H)

Weakness information:
cwe_id: CWE-59
cwe_name: Improper Link Resolution Before File Access ('Link Following')
description: The product attempts to access a file based on the filename, but it does not properly prevent that filename from identifying a link or shortcut that resolves to an unintended resource.
extended_description: 
            
            
         
         

Fix information:
repo_url: https://github.com/libarchive/libarchive
commit_details: repo_url: https://github.com/libarchive/libarchive
author: Martin Matuska
author_date: 2021-08-20 01:50:27+02:00
msg: Do not follow symlinks when processing the fixup list

Use lchmod() instead of chmod() and tell the remaining functions that the
real file to be modified is a symbolic link.

Fixes #1566
repository: repo_url: https://github.com/libarchive/libarchive
repo_name: libarchive/libarchive
description: Multi-format archive and compression library
repo_language: C
owner: libarchive
file_changes: filename: Makefile.am
change_type: ModificationType.MODIFY
diff: @@ -560,6 +560,7 @@ libarchive_test_SOURCES= \
 	libarchive/test/test_write_disk.c \
 	libarchive/test/test_write_disk_appledouble.c \
 	libarchive/test/test_write_disk_failures.c \
+	libarchive/test/test_write_disk_fixup.c \
 	libarchive/test/test_write_disk_hardlink.c \
 	libarchive/test/test_write_disk_hfs_compression.c \
 	libarchive/test/test_write_disk_lookup.c \

programming_language: Shell
filename: archive_write_disk_posix.c
change_type: ModificationType.MODIFY
diff: @@ -2461,6 +2461,7 @@ _archive_write_disk_close(struct archive *_a)
 {
 	struct archive_write_disk *a = (struct archive_write_disk *)_a;
 	struct fixup_entry *next, *p;
+	struct stat st;
 	int fd, ret;
 
 	archive_check_magic(&a->archive, ARCHIVE_WRITE_DISK_MAGIC,
@@ -2478,6 +2479,20 @@ _archive_write_disk_close(struct archive *_a)
 		    (TODO_TIMES | TODO_MODE_BASE | TODO_ACLS | TODO_FFLAGS)) {
 			fd = open(p->name,
 			    O_WRONLY | O_BINARY | O_NOFOLLOW | O_CLOEXEC);
+			if (fd == -1) {
+				/* If we cannot lstat, skip entry */
+				if (lstat(p->name, &st) != 0)
+					goto skip_fixup_entry;
+				/*
+				 * If we deal with a symbolic link, mark
+				 * it in the fixup mode to ensure no
+				 * modifications are made to its target.
+				 */
+				if (S_ISLNK(st.st_mode)) {
+					p->mode &= ~S_IFMT;
+					p->mode |= S_IFLNK;
+				}
+			}
 		}
 		if (p->fixup & TODO_TIMES) {
 			set_times(a, fd, p->mode, p->name,
@@ -2492,7 +2507,12 @@ _archive_write_disk_close(struct archive *_a)
 				fchmod(fd, p->mode);
 			else
 #endif
-			chmod(p->name, p->mode);
+#ifdef HAVE_LCHMOD
+			lchmod(p->name, p->mode);
+#else
+			if (!S_ISLNK(p->mode))
+				chmod(p->name, p->mode);
+#endif
 		}
 		if (p->fixup & TODO_ACLS)
 			archive_write_disk_set_acls(&a->archive, fd,
@@ -2503,6 +2523,7 @@ _archive_write_disk_close(struct archive *_a)
 		if (p->fixup & TODO_MAC_METADATA)
 			set_mac_metadata(a, p->name, p->mac_metadata,
 					 p->mac_metadata_size);
+skip_fixup_entry:
 		next = p->next;
 		archive_acl_clear(&p->acl);
 		free(p->mac_metadata);
@@ -2643,6 +2664,7 @@ new_fixup(struct archive_write_disk *a, const char *pathname)
 	fe->next = a->fixup_list;
 	a->fixup_list = fe;
 	fe->fixup = 0;
+	fe->mode = 0;
 	fe->name = strdup(pathname);
 	return (fe);
 }

programming_language: C
filename: CMakeLists.txt
change_type: ModificationType.MODIFY
diff: @@ -209,6 +209,7 @@ IF(ENABLE_TEST)
     test_write_disk.c
     test_write_disk_appledouble.c
     test_write_disk_failures.c
+    test_write_disk_fixup.c
     test_write_disk_hardlink.c
     test_write_disk_hfs_compression.c
     test_write_disk_lookup.c

programming_language: Shell
filename: test_write_disk_fixup.c
change_type: ModificationType.ADD
diff: @@ -0,0 +1,77 @@
+/*-
+ * Copyright (c) 2021 Martin Matuska
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE AUTHOR(S) ``AS IS'' AND ANY EXPRESS OR
+ * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+ * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+ * IN NO EVENT SHALL THE AUTHOR(S) BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+ * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+ * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+#include "test.h"
+
+/*
+ * Test fixup entries don't follow symlinks
+ */
+DEFINE_TEST(test_write_disk_fixup)
+{
+	struct archive *ad;
+	struct archive_entry *ae;
+	int r;
+
+	if (!canSymlink()) {
+		skipping("Symlinks not supported");
+		return;
+	}
+
+	/* Write entries to disk. */
+	assert((ad = archive_write_disk_new()) != NULL);
+
+	/*
+	 * Create a file
+	 */
+	assertMakeFile("victim", 0600, "a");
+
+	/*
+	 * Create a directory and a symlink with the same name
+	 */
+
+	/* Directory: dir */
+        assert((ae = archive_entry_new()) != NULL);
+        archive_entry_copy_pathname(ae, "dir");
+        archive_entry_set_mode(ae, AE_IFDIR | 0606);
+	assertEqualIntA(ad, 0, archive_write_header(ad, ae));
+	assertEqualIntA(ad, 0, archive_write_finish_entry(ad));
+        archive_entry_free(ae);
+
+	/* Symbolic Link: dir -> foo */
+	assert((ae = archive_entry_new()) != NULL);
+	archive_entry_copy_pathname(ae, "dir");
+	archive_entry_set_mode(ae, AE_IFLNK | 0777);
+	archive_entry_set_size(ae, 0);
+	archive_entry_copy_symlink(ae, "victim");
+	assertEqualIntA(ad, 0, r = archive_write_header(ad, ae));
+	if (r >= ARCHIVE_WARN)
+		assertEqualIntA(ad, 0, archive_write_finish_entry(ad));
+	archive_entry_free(ae);
+
+	assertEqualInt(ARCHIVE_OK, archive_write_free(ad));
+
+	/* Test the entries on disk. */
+	assertIsSymlink("dir", "victim", 0);
+	assertFileMode("victim", 0600);
+}

programming_language: C