Bringing unallocated data back: the FAT12/16/32 case

~ Maxim Suhanov ~ 2 Comments

Modern operating systems provide a way to increase the size of a given file without writing to it.

In Unix-like operating systems, this is achieved through the truncate() and ftruncate() system calls. These calls allow programs to decrease or increase the file size.

If the file size is decreased, data beyond the new end-of-file position is discarded (it can survive as deleted data, but there is no way to bring these bytes back by restoring the original file size).

If the file size is increased, extra data (data after the old end-of-file position) is filled with null bytes. Internally, many modern file systems don’t write those extra null bytes to the drive, they make a sparse data range instead (so, if a program increases the file size by several gigabytes, there is no need to actually write that amount of null bytes to the drive).

Some file systems may reserve sectors to hold that amount of extra data and attach those sectors to the file, but their contents are left intact. This is useful to quickly preallocate some space to avoid fragmentation: a file system driver reserves the requested amount of sectors, but it doesn’t clear them (and an attempt to read from that extra space returns null bytes, despite the fact that these extra sectors may contain something different).

Treating the extra space as containing null bytes is essential for security, there is a POSIX requirement for that:

If the file size is increased, the extended area shall appear as if it were zero-filled.

https://pubs.opengroup.org/onlinepubs/9699919799/functions/ftruncate.html

It’s a vulnerability if deleted (or uninitialized) data is returned when reading that extra area (for example, see CVE-2021-4155).

Surprisingly, such vulnerabilities are still discovered in file system drivers.

Continue reading “Bringing unallocated data back: the FAT12/16/32 case” →

CVE-2023-45897: a vulnerability in the Linux exFAT userspace tools

~ Maxim Suhanov ~ 1 Comment

Some vulnerabilities tend to appear in independently written code. And CVE-2023-4273 isn’t an exception here…

A very similar vulnerability, CVE-2023-45897, has been discovered in the exfatprogs package: in particular, in the fsck tool. This vulnerability has exactly the same mechanics (even the same proof-of-concept file system image would work), but the overflow occurs in the heap memory, not in the stack.

Continue reading “CVE-2023-45897: a vulnerability in the Linux exFAT userspace tools” →

CVE-2023-4692, CVE-2023-4693: vulnerabilities in the GRUB boot manager

~ Maxim Suhanov ~ 1 Comment

The GRUB boot manager is more an operating system than a boot loader. For example, it has more than 20 file system types supported!

This is a really wide attack surface… And, currently, GRUB is the default choice in the Secure Boot implementation using Microsoft-signed shims (but things are moving forward).

Some time ago, I discovered two vulnerabilities (or three vulnerabilities, if we count a security issue which is almost unexploitable) in the NTFS driver of the GRUB boot manager. And here are some technical details…

Continue reading “CVE-2023-4692, CVE-2023-4693: vulnerabilities in the GRUB boot manager” →

CVE-2023-4273: a vulnerability in the Linux exFAT driver

~ Maxim Suhanov ~ 5 Comments

According to the exFAT file system specification, the maximum length of a file name is 255 characters (UTF-16LE):

The FileName field shall contain a Unicode string, which is a portion of the file name. In the order File Name directory entries exist in a File directory entry set, FileName fields concatenate to form the file name for the File directory entry set. Given the length of the FileName field, 15 characters, and the maximum number of File Name directory entries, 17, the maximum length of the final, concatenated file name is 255.

This limit wasn’t really enforced in the exFAT driver of the Linux kernel, so it could be possible to craft a File directory entry set containing more than 17 File Name directory entries resulting in a concatenated file name much longer than 255 characters. And all of these characters were copied into a stack-allocated array of 258 characters.

So, here comes a stack overflow. But how to weaponize it?

Continue reading “CVE-2023-4273: a vulnerability in the Linux exFAT driver” →

Do researchers handle exFAT volumes correctly?

~ Maxim Suhanov ~ 1 Comment

Let’s conduct a simple experiment. In the Ext4 file system, I create two files (“1.txt” and “2.txt”).

touch 1.txt 2.txt

Then, I gather file system metadata (including timestamps) for these files:

sudo debugfs -R 'stat /path_to/the_file/1.txt' /dev/block_device
sudo debugfs -R 'stat /path_to/the_file/2.txt' /dev/block_device

In my case, the output is:

For “1.txt”:

Inode: 23918802   Type: regular    Mode:  0644   Flags: 0x80000
Generation: 2882061095    Version: 0x00000000:00000001
User:  1000   Group:  1000   Project:     0   Size: 0
File ACL: 0
Links: 1   Blockcount: 0
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
 atime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
 mtime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
crtime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
Size of extra inode fields: 32
Inode checksum: 0x9906b43e
EXTENTS:

For “2.txt”:

Inode: 23921678   Type: regular    Mode:  0644   Flags: 0x80000
Generation: 2369567438    Version: 0x00000000:00000001
User:  1000   Group:  1000   Project:     0   Size: 0
File ACL: 0
Links: 1   Blockcount: 0
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
 atime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
 mtime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
crtime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
Size of extra inode fields: 32
Inode checksum: 0x9b052019
EXTENTS:

Now, I’m going to store something in these files. I will use the bash shell to append some text to the first file (using the output redirection syntax) and the gedit text editor to modify the second one.

echo 123 >> 1.txt
gedit 2.txt # In the gedit window, I will type "456" and hit Ctrl-S.

And let’s get the timestamps for these files again…

For “1.txt”:

Inode: 23918802   Type: regular    Mode:  0644   Flags: 0x80000
Generation: 2882061095    Version: 0x00000000:00000001
User:  1000   Group:  1000   Project:     0   Size: 4
File ACL: 0
Links: 1   Blockcount: 8
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x6399eed2:68debfb8 -- Wed Dec 14 18:42:10 2022
 atime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
 mtime: 0x6399eed2:68debfb8 -- Wed Dec 14 18:42:10 2022
crtime: 0x6399ee07:e1dbba28 -- Wed Dec 14 18:38:47 2022
Size of extra inode fields: 32
Inode checksum: 0x4a517d18
EXTENTS:
(0):95716043

For “2.txt”:

Inode: 23860507   Type: regular    Mode:  0644   Flags: 0x80000
Generation: 534549923    Version: 0x00000000:00000001
User:  1000   Group:  1000   Project:     0   Size: 4
File ACL: 0
Links: 1   Blockcount: 8
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x6399eed8:16d4b85c -- Wed Dec 14 18:42:16 2022
 atime: 0x6399eed8:14ec7284 -- Wed Dec 14 18:42:16 2022
 mtime: 0x6399eed8:16d4b85c -- Wed Dec 14 18:42:16 2022
crtime: 0x6399eed8:14ec7284 -- Wed Dec 14 18:42:16 2022
Size of extra inode fields: 32
Inode checksum: 0x2b67e3c3
EXTENTS:
(0):96358507

Do you see anything unusual?

Continue reading “Do researchers handle exFAT volumes correctly?” →

exFAT: orphan file name entries

~ Maxim Suhanov ~ 2 Comments

The exFAT file system was designed with Unicode file names and optional vendor-specific extensions in mind. To keep things simple, the file system specification allows the usage of multiple directory entries to describe a single file (so, additional file metadata is described in additional directory entries). This solution is similar to the VFAT extension for the FAT12/16/32 file systems, which was designed as a hack to the original file system format (originally, only one directory entry was used to describe a single file, so long file names were implemented as additional directory entries, which are “invisible” to operating systems without the VFAT support).

In the exFAT file system, a typical file consists of these entries (in this order, with no other entries between):

  • one file entry,
  • one stream extension entry,
  • one or more file name entries (as needed to store the file name),
  • zero, one or more vendor-specific entries (which can be ignored if not supported).

The first two entries describe all file metadata (its attributes, timestamps, data size, first cluster, etc.), while the file name entries contain strings to form the file name (each file name entry stores no more than 15 Unicode characters and the file name is no longer than 255 characters). Together, these entries are called a directory entry set (and it must contain at least three entries).

When a file is deleted, its directory entry set is marked as free. This process is very similar to what happens to a deleted file in the FAT12/16/32 file systems: the first byte of a directory entry is changed to mark it as free.

And, of course, it is possible to recover a deleted file when its directory set and data clusters are not overwritten. If the directory entry set is partially overwritten (with new directory entries), the following can be observed:

Continue reading “exFAT: orphan file name entries” →

macOS & FAT directories

~ Maxim Suhanov ~ 1 Comment

Previously, I wrote about things you probably didn’t know about FAT. Now, let’s continue the story!

In FAT12/16/32 file systems, each directory (except the root directory) contains two special entries:

  • dot (“.”);
  • dot-dot (“..”).

The first one (dot) refers to the directory itself, while the second one (dot-dot) refers to the parent directory. Apparently, these entries were introduced to keep file system implementations simple, so there is no need to generate those entries on the fly (this is what happens to file systems not wasting their space to store dot and dot-dot entries in every directory).

Since two special entries are regular 8.3 directory entries, they contain timestamps (Created, Modified, Last Accessed). According to Microsoft, these timestamps should be set like this:

[…] and all of the date and time fields in both of these entries are set to the same values as they were in the directory entry for the directory that you just created.

FATGEN, 1.03 (DOC)

So, the timestamps in the dot-dot directory entry have no connection to the parent directory of that directory (they aren’t copied from the corresponding timestamp fields of the parent directory). Both special entries have their timestamps set to the same values as in the directory itself.

For example, all these entries are expected to share the same timestamps:

  • E:\test
  • E:\test\.
  • E:\test\..

But are these timestamps always synchronized?

Continue reading “macOS & FAT directories” →

The uppercased hell

~ Maxim Suhanov ~ 1 Comment

Recently, Microsoft warned users about compatibility issues with applications using some non-ASCII characters in names of their registry keys. According to Microsoft:

Compatibility issues have been found between apps using some non-ASCII characters in their registry keys or subkeys and Windows 11. Affected apps might be unable to open and might cause other issues or errors in Windows, including the possibility of receiving an error with a blue screen. Important Affected registry keys with non-ASCII characters might not be able to be repaired.

Sounds interesting!

Before we start, here are some useful links:

  1. Windows registry file format specification
  2. Measured Boot and Malware Signatures: exploring two vulnerabilities found in the Windows loader (archived copy)
  3. Playing with case-insensitive file names (a registry hive is similar to a file system)
Continue reading “The uppercased hell” →

Playing with case-insensitive file names

~ Maxim Suhanov ~ 4 Comments

Although NTFS has been designed with case-sensitivity in mind, it’s used mostly in the case-insensitive environment. One can natively store, within the same directory, two or more files with their names differing only in case, but Windows-based tools won’t deal with them correctly. To provide true case-sensitivity, Microsoft implemented an additional layer, per-directory case-sensitivity, as described here, here, and here.

But there are several issues with usual, case-insensitive, operations…

Continue reading “Playing with case-insensitive file names” →