Date: Thu, 9 Jul 2020 18:42:53 +0200
From: X41 D-Sec GmbH Advisories <advisories@...-dsec.de>
To: bugtraq@...urityfocus.com, fulldisclosure@...lists.org
Subject: [FD] X41 D-Sec GmbH Security Advisory X41-2020-006: Memory
 Corruption Vulnerability in bspatch

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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.

- -
- -
-
------------------8<---------------------------------------------------------
    while(newpos<newsize) {
        /* Read control data /
        for(i=0;i<=2;i++) {
            lenread = BZ2bzRead(&cbz2err, cpfbz2, buf, 8);
            if ((lenread < 8) || ((cbz2err != BZOK) &&
                (cbz2err != BZSTREAMEND)))
                errx(1, "Corrupt patch\n");
            ctrl[i]=offtin(buf);
        };
        / Sanity-check */
        if(newpos+ctrl[0]>newsize)
            errx(1,"Corrupt patch\n");
    /* Read diff string */
    lenread = BZ2_bzRead(&dbz2err, dpfbz2, new + newpos, ctrl[0]);
- -
- -
-
------------------8<---------------------------------------------------------

When ctrl[0] takes either negative values or big enough values to
overflow newpos+ctrl[0], the sanity check in place will pass allowing
operations that write out of buffer new boundaries via BZ2_bzRead().
It is worth mentioning that BZ2_bzRead() will truncate ctrl[0] from
64-bit off_t to 32-bit int.

It is expected that an attacker that is able to deliver an specially
crafted patch file can gain remote code execution capabilities when
certain conditions for exploitation are met.

Proof of Concept
================
A crashing reproducer can be downloaded from
https://github.com/x41sec/advisories/blob/master/X41-2020-006/x41-2020-006-bspatch-poc.patch

Fix
===
Please, refer to the FreeBSD advisories
https://www.freebsd.org/security/advisories/FreeBSD-SA-16:25.bspatch.asc
and
https://www.freebsd.org/security/advisories/FreeBSD-SA-16:29.bspatch.asc
for fixes.

Workarounds
===========
As a workaround, only patches passing integrity and authenticity
checks should be applied.

Timeline
========
2016-07-21 CVE-2014-9862 published
2016-07-25 Partial fix for FreeBSD published at FreeBSD-SA-16:25.bspatch
2016-10-10 Complete fix for FreeBSD published at FreeBSD-SA-16:29.bspatch
2020-07-02 X41 Discovers the vulnerability was not or incorrectly
fixed upstream and in prominent forks of the code
2020-07-06 Colin Percival and distros@ notified
2020-07-09 Public disclosure

About X41 D-SEC GmbH
====================
X41 is an expert provider for application security services.
Having extensive industry experience and expertise in the area of
information security, a strong core security team of world class
security experts enables X41 to perform premium security services.
Fields of expertise in the area of application security are security
centered code reviews, binary reverse engineering and vulnerability
discovery.
Custom research and IT security consulting and support services are core
competencies of X41.
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