CVE-2021-41072 (published 2021-09-14T01:15Z)

squashfs_opendir in unsquash-2.c in Squashfs-Tools 4.5 allows Directory Traversal, a different vulnerability than CVE-2021-40153. A squashfs filesystem that has been crafted to include a symbolic link and then contents under the same filename in a filesystem can cause unsquashfs to first create the symbolic link pointing outside the expected directory, and then the subsequent write operation will cause the unsquashfs process to write through the symbolic link elsewhere in the filesystem.

Severity: MEDIUM
CVSS v2 base score: 5.8 (AV:N/AC:M/Au:N/C:N/I:P/A:P)
CVSS v3 base score: 8.1, HIGH (CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:H)

Weakness information:
cwe_id: CWE-22
cwe_name: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal')
description: The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.
extended_description: 
            Many file operations are intended to take place within a restricted directory. By using special elements such as ".." and "/" separators, attackers can escape outside of the restricted location to access files or directories that are elsewhere on the system. One of the most common special elements is the "../" sequence, which in most modern operating systems is interpreted as the parent directory of the current location. This is referred to as relative path traversal. Path traversal also covers the use of absolute pathnames such as "/usr/local/bin" to access unexpected files. This is referred to as absolute path traversal.
         
         
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/plougher/squashfs-tools
commit_details: repo_url: https://github.com/plougher/squashfs-tools
author: Phillip Lougher
author_date: 2021-09-12 23:50:06+01:00
msg: Unsquashfs: additional write outside destination directory exploit fix

An issue on github (https://github.com/plougher/squashfs-tools/issues/72)
showed how some specially crafted Squashfs filesystems containing
invalid file names (with '/' and '..') can cause Unsquashfs to write
files outside of the destination directory.

Since then it has been shown that specially crafted Squashfs filesystems
that contain a symbolic link pointing outside of the destination directory,
coupled with an identically named file within the same directory, can
cause Unsquashfs to write files outside of the destination directory.

Specifically the symbolic link produces a pathname pointing outside
of the destination directory, which is then followed when writing the
duplicate identically named file within the directory.

This commit fixes this exploit by explictly checking for duplicate
filenames within a directory.  As directories in v2.1, v3.x, and v4.0
filesystems are sorted, this is achieved by checking for consecutively
identical filenames.  Additionally directories are checked to
ensure they are sorted, to avoid attempts to evade the duplicate
check.

Version 1.x and 2.0 filesystems (where the directories were unsorted)
are sorted and then the above duplicate filename check is applied.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
repository: repo_url: https://github.com/plougher/squashfs-tools
repo_name: plougher/squashfs-tools
description: tools to create and extract Squashfs filesystems
repo_language: C
owner: plougher
file_changes: filename: Makefile
change_type: ModificationType.MODIFY
diff: @@ -160,8 +160,8 @@ MKSQUASHFS_OBJS = mksquashfs.o read_fs.o action.o swap.o pseudo.o compressor.o \
 	caches-queues-lists.o reader.o tar.o
 
 UNSQUASHFS_OBJS = unsquashfs.o unsquash-1.o unsquash-2.o unsquash-3.o \
-	unsquash-4.o unsquash-123.o unsquash-34.o unsquash-1234.o swap.o \
-	compressor.o unsquashfs_info.o
+	unsquash-4.o unsquash-123.o unsquash-34.o unsquash-1234.o unsquash-12.o \
+	swap.o compressor.o unsquashfs_info.o
 
 CFLAGS ?= -O2
 CFLAGS += $(EXTRA_CFLAGS) $(INCLUDEDIR) -D_FILE_OFFSET_BITS=64 \
@@ -393,6 +393,8 @@ unsquash-34.o: unsquashfs.h unsquash-34.c unsquashfs_error.h
 
 unsquash-1234.o: unsquash-1234.c unsquashfs_error.h
 
+unsquash-1234.o: unsquash-12.c
+
 unsquashfs_xattr.o: unsquashfs_xattr.c unsquashfs.h squashfs_fs.h xattr.h unsquashfs_error.h
 
 unsquashfs_info.o: unsquashfs.h squashfs_fs.h unsquashfs_error.h

programming_language: Shell
filename: unsquash-1.c
change_type: ModificationType.MODIFY
diff: @@ -370,6 +370,12 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 		}
 	}
 
+	/* check directory for duplicate names.  Need to sort directory first */
+	sort_directory(dir);
+	if(check_directory(dir) == FALSE) {
+		ERROR("File system corrupted: directory has duplicate names\n");
+		goto corrupted;
+	}
 	return dir;
 
 corrupted:

programming_language: C
filename: unsquash-12.c
change_type: ModificationType.ADD
diff: @@ -0,0 +1,110 @@
+/*
+ * Unsquash a squashfs filesystem.  This is a highly compressed read only
+ * filesystem.
+ *
+ * Copyright (c) 2021
+ * Phillip Lougher <phillip@squashfs.org.uk>
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License
+ * as published by the Free Software Foundation; either version 2,
+ * or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
+ *
+ * unsquash-12.c
+ *
+ * Helper functions used by unsquash-1 and unsquash-2.
+ */
+
+#include "unsquashfs.h"
+
+/*
+ * Bottom up linked list merge sort.
+ *
+ */
+void sort_directory(struct dir *dir)
+{
+	struct dir_ent *cur, *l1, *l2, *next;
+	int len1, len2, stride = 1;
+
+	if(dir->dir_count < 2)
+		return;
+
+	/*
+	 * We can consider our linked-list to be made up of stride length
+	 * sublists.  Eacn iteration around this loop merges adjacent
+	 * stride length sublists into larger 2*stride sublists.  We stop
+	 * when stride becomes equal to the entire list.
+	 *
+	 * Initially stride = 1 (by definition a sublist of 1 is sorted), and
+	 * these 1 element sublists are merged into 2 element sublists,  which
+	 * are then merged into 4 element sublists and so on.
+	 */
+	do {
+		l2 = dir->dirs; /* head of current linked list */
+		cur = NULL; /* empty output list */
+
+		/*
+		 * Iterate through the linked list, merging adjacent sublists.
+		 * On each interation l2 points to the next sublist pair to be
+		 * merged (if there's only one sublist left this is simply added
+		 * to the output list)
+		 */
+		while(l2) {
+			l1 = l2;
+			for(len1 = 0; l2 && len1 < stride; len1 ++, l2 = l2->next);
+			len2 = stride;
+
+			/*
+			 * l1 points to first sublist.
+			 * l2 points to second sublist.
+			 * Merge them onto the output list
+			 */
+			while(len1 && l2 && len2) {
+				if(strcmp(l1->name, l2->name) <= 0) {
+					next = l1;
+					l1 = l1->next;
+					len1 --;
+				} else {
+					next = l2;
+					l2 = l2->next;
+					len2 --;
+				}
+
+				if(cur) {
+					cur->next = next;
+					cur = next;
+				} else
+					dir->dirs = cur = next;
+			}
+			/*
+			 * One sublist is now empty, copy the other one onto the
+			 * output list
+			 */
+			for(; len1; len1 --, l1 = l1->next) {
+				if(cur) {
+					cur->next = l1;
+					cur = l1;
+				} else
+					dir->dirs = cur = l1;
+			}
+			for(; l2 && len2; len2 --, l2 = l2->next) {
+				if(cur) {
+					cur->next = l2;
+					cur = l2;
+				} else
+					dir->dirs = cur = l2;
+			}
+		}
+		cur->next = NULL;
+		stride = stride << 1;
+	} while(stride < dir->dir_count);
+}

programming_language: Java
filename: unsquash-1234.c
change_type: ModificationType.MODIFY
diff: @@ -72,3 +72,24 @@ void squashfs_closedir(struct dir *dir)
 
 	free(dir);
 }
+
+
+/*
+ * Check directory for duplicate names.  As the directory should be sorted,
+ * duplicates will be consecutive.  Obviously we also need to check if the
+ * directory has been deliberately unsorted, to evade this check.
+ */
+int check_directory(struct dir *dir)
+{
+	int i;
+	struct dir_ent *ent;
+
+	if(dir->dir_count < 2)
+		return TRUE;
+
+	for(ent = dir->dirs, i = 0; i < dir->dir_count - 1; ent = ent->next, i++)
+		if(strcmp(ent->name, ent->next->name) >= 0)
+			return FALSE;
+
+	return TRUE;
+}

programming_language: C
filename: unsquash-2.c
change_type: ModificationType.MODIFY
diff: @@ -29,6 +29,7 @@
 static squashfs_fragment_entry_2 *fragment_table;
 static unsigned int *uid_table, *guid_table;
 static squashfs_operations ops;
+static int needs_sorting = FALSE;
 
 
 static void read_block_list(unsigned int *block_list, long long start,
@@ -463,6 +464,17 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 		}
 	}
 
+	if(needs_sorting)
+		sort_directory(dir);
+
+	/* check directory for duplicate names and sorting */
+	if(check_directory(dir) == FALSE) {
+		if(needs_sorting)
+			ERROR("File system corrupted: directory has duplicate names\n");
+		else
+			ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
+		goto corrupted;
+	}
 	return dir;
 
 corrupted:
@@ -596,6 +608,10 @@ int read_super_2(squashfs_operations **s_ops, void *s)
 	 * 2.x filesystems use gzip compression.
 	 */
 	comp = lookup_compressor("gzip");
+
+	if(sBlk_3->s_minor == 0)
+		needs_sorting = TRUE;
+
 	return TRUE;
 }
 

programming_language: C
filename: unsquash-3.c
change_type: ModificationType.MODIFY
diff: @@ -497,6 +497,12 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 		}
 	}
 
+	/* check directory for duplicate names and sorting */
+	if(check_directory(dir) == FALSE) {
+		ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
+		goto corrupted;
+	}
+
 	return dir;
 
 corrupted:

programming_language: C
filename: unsquash-4.c
change_type: ModificationType.MODIFY
diff: @@ -434,6 +434,12 @@ static struct dir *squashfs_opendir(unsigned int block_start, unsigned int offse
 		}
 	}
 
+	/* check directory for duplicate names and sorting */
+	if(check_directory(dir) == FALSE) {
+		ERROR("File system corrupted: directory has duplicate names or is unsorted\n");
+		goto corrupted;
+	}
+
 	return dir;
 
 corrupted:

programming_language: C
filename: unsquashfs.h
change_type: ModificationType.MODIFY
diff: @@ -293,4 +293,8 @@ extern long long *alloc_index_table(int);
 /* unsquash-1234.c */
 extern int check_name(char *, int);
 extern void squashfs_closedir(struct dir *);
+extern int check_directory(struct dir *);
+
+/* unsquash-12.c */
+extern void sort_directory(struct dir *);
 #endif

programming_language: C