Sergei Glazunov, Project Zero (Originally posted on Project Zero blog 2021-02-04)

The Basics

Disclosure or Patch Date: 19 October 2020

Product: Google Chrome/ Freetype

Advisory: https://chromereleases.googleblog.com/2020/10/stable-channel-update-for-desktop_20.html

Affected Versions: 86.0.4240.80 and previous

First Patched Version: 86.0.4240.111

Issue/Bug Report:

Patch CL:

Bug-Introducing CL: http://git.savannah.gnu.org/cgit/freetype/freetype2.git/commit/?id=01705395b08167b654a24f26673ee6e75a84f2be

Reporter(s): Sergei Glazunov of Google Project Zero

The Code

Proof-of-concept: A font file which will reproduce the crash in an ASAN build is attached here.

Exploit sample: N/A

Did you have access to the exploit sample when doing the analysis? Yes

The Vulnerability

Bug class: Heap buffer overflow

Vulnerability details:

FreeType is a popular software development library used to render text onto bitmaps, and provides support for other font-related operations. The vulnerability exists in the function Load_SBit_Png, which processes PNG images that are embedded into fonts. Load_SBit_Png truncates the image height and width to 16-bit integers when calculating the bitmap size. This size is used to allocate the buffer. However, later the code passes the original 32-bit values for the height and width along with the allocated buffer to libpng for further processing. Therefore, if the original width and/or height are greater than 65535, the allocated buffer won’t be able to fit the bitmap.

Patch analysis: N/A

Thoughts on how this vuln might have been found (fuzzing, code auditing, variant analysis, etc.):

The issue is relatively straightforward to be found during a manual code review. It's also quite possible that it has been discovered as a result of variant analysis of https://bugs.chromium.org/p/project-zero/issues/detail?id=168.

(Historical/present/future) context of bug:

The vulnerability was used by the actor in two exploit chains:

  • together with a OS kernel issue (CVE-2020-17087) on Windows,
  • together with a Chrome-specific UAF (CVE-2020-16010) in the browser process on Android.

The Exploit

(The terms exploit primitive, exploit strategy, exploit technique, and exploit flow are defined here.)

Exploit strategy (or strategies): Still under analysis

Exploit flow: Still under analysis

Known cases of the same exploit flow: Still under analysis

Part of an exploit chain? Yes, paired with CVE-2020-17087 on Windows and CVE-2020-16010 on Android. The attacker also had iOS

The Next Steps

Variant analysis

Areas/approach for variant analysis (and why):

Fuzzing for interoperability issues between FreeType and its dependencies (e.g., libpng).

Found variants: N/A

Structural improvements

The issue is a textbook buffer overflow, so while generic solutions like memory tagging apply, it's unlikely there's a potential structural improvement that's specific to the bug/area.

0-day detection methods

Other References