CVE-2020-14315

X41 D-Sec GmbH Security Advisory X41-2020-006: Memory Corruption Vulnerability
in bspatch
From: X41 D-Sec GmbH Advisories 
Date: Thu, 9 Jul 2020 18:41:37 +0200

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X41 D-SEC GmbH Security Advisory: X41-2020-006

Advisory X41-2020-006: Memory Corruption Vulnerability in bspatch
=================================================================
Severity Rating: High
Confirmed Affected Versions: Colin Percival's bsdiff 4.3
Confirmed Patched Versions: FreeBSD's bsdiff
(https://svnweb.freebsd.org/base/head/usr.bin/bsdiff/bspatch/bspatch.c)
Vendor: Colin Percival
Vendor URL: https://www.daemonology.net/bsdiff/
Vendor Reference: None
Vector: Patch file
Credit: X41 D-SEC GmbH, Luis Merino
Status: Public
CVE: CVE-2020-14315
CWE: 119
CVSS Score: N/A
CVSS Vector: N/A
Advisory-URL: https://www.x41-dsec.de/lab/advisories/x41-2020-006-bspatch/

Summary and Impact
==================
A memory corruption vulnerability is present in bspatch as shipped in
Colin Percival���s bsdiff tools version 4.3. Insufficient checks when
handling external inputs allows an attacker to bypass the sanity
checks in place and write out of a dynamically allocated buffer
boundaries.

Even though the patching procedure is usually combined with integrity
and authenticity checks, an attacker that is able to deliver a
malicious patch can cause heap corruption in the process running
bspatch code, when the authenticity checks happen after applying the
patches. Depending on their ability to control and shape the heap
state before and during the processing of a malicious patch file,
remote code execution may be achieved. This has already been
demonstrated
(https://gist.github.com/anonymous/e48209b03f1dd9625a992717e7b89c4f#file-freebsd-txt-L1192)
as a proof-of-concept exploit in 2016 by an anonymous author against
the FreeBSD bspatch implementation on 32bit architectures.

This issue was initially reported for bspatch in bsdiff ���as used in
Apple OS X before 10.11.6 and other products��� with CVE-2014-9862 by an
anonymous researcher and was partially addressed by several projects,
including Android
(https://android.googlesource.com/platform/external/bsdiff/+/4d054795b673855e3a7556c6f2f7ab99ca509998%5E%21/#F0),
ChromiumOS
(https://bugs.chromium.org/p/chromium/issue/detail?id=372525) and
FreeBSD
(https://www.freebsd.org/security/advisories/FreeBSD-SA-16:25.bspatch.asc)
during 2016. This initial batch of fixes prevented the attack via
negative control values.

Nevertheless, huge control values that would integer overflow the
sanity checks and allow an attacker writing out of bounds were not
fixed. A subsequent patch was released by FreeBSD
(https://www.freebsd.org/security/advisories/FreeBSD-SA-16:29.bspatch.asc)
addressing the remaining issues together with additional hardening.
Unfortunately, most of bspatch copies didn���t port this fix.

It is worth mentioning that bsdiff 4.3, as hosted at Colin Percival���s
bsdiff website https://www.daemonology.net/bsdiff/, still ships a copy
of bspatch.c vulnerable to these issues via both negative and huge
control values. All the Linux distributions we have checked shipping
bsdiff are building from this sources, with some of them applying the
partial fix initially released.

Product Description
===================
bsdiff and bspatch are tools for building and applying patches to
binary files. They provide an efficient way to apply binary patches
for applications update mechanisms.

Analysis
========
Insufficient checks when calculating the buffer offset and size of
write operations allows writing out of a heap allocated buffer boundaries.

- -
-
------------------8newsize)
            errx(1,"Corrupt patch\n");
    /* Read diff string */
    lenread = BZ2_bzRead(&dbz2err, dpfbz2, new + newpos, ctrl[0]);
- -
-
------------------8

References:
http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2020-14315
http://seclists.org/oss-sec/2020/q3/18
https://www.freebsd.org/security/advisories/FreeBSD-SA-16:29.bspatch.asc
https://www.freebsd.org/security/advisories/FreeBSD-SA-16:25.bspatch.asc
https://svnweb.freebsd.org/base/head/usr.bin/bsdiff/bspatch/bspatch.c
https://www.x41-dsec.de/lab/advisories/x41-2020-006-bspatch/
https://bugs.chromium.org/p/chromium/issue/detail?id=372525