Episode 25

15 min • 25 mars 2019

Overview

Ghostscript is back to haunt us for another week, plus we look at vulnerabilities in ntfs-3g, snapd, firefox and more.

39 unique CVEs addressed

4 CVEs addressed in Xenial, Bionic, Cosmic
4 DoS (crash) found via fuzzing:
- Stack overflow in readelf
- 2 different OOB read due to failure to NULL terminate a string before processing it
- Read past end of stack due to failing to properly keep track of buffer sizes

8 CVEs addressed in Precise ESM
Covered in Episode 18 and Episode 24 for standard Ubuntu releases (not all CVEs covered in those updates applicable to Precise ESM)

1 CVEs addressed in Xenial
- CVE-2017-12447
Failure to properly validate BMP image palette parameters - leading to OOB when decoding colormap later on

1 CVEs addressed in Xenial, Bionic, Cosmic
- CVE-2019-9755
Discovered recently by Chris Coulson during code-audit of ntfs-3g - actually had been fixed upstream late last year but no CVE assigned
Heap buffer overflow able to be triggered when mounting a filesystem onto a mount point with path name greater than PATH_MAX, and from a current working directory which has a path name also greater than PATH_MAX
Contents of buffers is attacker controlled so heap can be overflown with attacker controlled input - likely to leverage into arbitrary code execution
Contrived example BUT in Debian and Ubuntu ntfs-3g is setuid root - which then leads to root privilege escalation with arbitrary code execution
Update was released within hours of the bug being made public to fix the heap buffer overflow
Currently testing ntfs-3g as not-setuid root to release in a future update to avoid any other possible privilege escalation bugs in the future

2 CVEs addressed in Trusty, Xenial, Bionic, Cosmic
- CVE-2019-3838
- CVE-2019-3835
Similar to previous CVE, forceput operator could be extracted from the DefineResource method to allow access to the file-system outside of the -dSAFER sandbox
superexec operator was available in the internal dictionary - also able to be extracted and hence used to access files outside the sandbox

2 CVEs addressed in Xenial
- CVE-2017-17969
- CVE-2016-2335
Heap based OOB write when decompressing a crafted ZIP file (crash -> DoS, possible code execution)
Heap based OOB read when decompressing a UDF file (universal disk format - used for DVD images) - crash, DoS

1 CVEs addressed in Trusty, Xenial, Bionic, Cosmic
- CVE-2019-7303
Jann Horn reported the seccomp blacklist for TIOCSTI can be bypassed
snapd creates a seccomp filter for each snap which is designed to block TIOCSTI (as this can be used to fake input to other processes outside of the sandbox)
This is a 32-bit value to the ioctl system call, but on 64-bit architectures the kernel does this comparison as a 64-bit integer - so can be circumvented by using a 64-bit value to ioctl systemcall which has other bits set in the upper 32 bits - since when seccomp does comparison it uses the full 64 bits - so it won’t match the 32-bit value of TIOCSTI and so will be allowed - but then when used as the ioctl() argument it will correctly be truncated to 32-bits and the ioctl will proceed
Fixed in snapd to add a second seccomp filter to disallow anything in the upper 32-bits
Initially seemed like a kernel or libseccomp issue but both currently document this as a limitation already so treated in the end as a vulnerability in snapd

3 CVEs addressed in Cosmic
Dependency solver used by packaging systems to resolve dependencies between packages etc
2 NULL pointer dereferences and 1 invalid memory read due to mishandling of variable length function arguments - all crash -> DoS

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