Showing posts with label AFL. Show all posts
Showing posts with label AFL. Show all posts

Wednesday 4 March 2020

Fuzzing VIM

AAAAAAAAAA....: It's almost a year now I started with fuzzing and discovered multiple bugs. The most commonly software which I've fuzzed so far includes Xpdf, VIM, PuTTY, WebKit, LibreOffice, Glibc etc. In this post I'll be demonstrating fuzzing VIM (Regex engine) through AFL++ a.k.a american fuzzy lop.

Technical Details: VIM a.k.a Vi IMproved has 12 different editing modes which can be utilized for fuzzing. Vim has lots of potential for finding bugs with AFL. One of the bug which I found while fuzzing VIM was CVE-2019-20079, I would also like to thank Dominique Pelle for this.
[+] Git clone VIM
cmd$ git clone ; cd vim

[+] Compile and Make VIM with AFL++ 
cmd$ CC=afl-clang-fast CXX=afl-clang-fast++ ./configure --with-features=huge --enable-gui=none
cmd$ make -j4 ; cd src/

[+] Feed Corpus
cmd$ mkdir corpus ; mkdir output 
cmd$ echo "a*b\+\|[0-9]\|\d{1,9}" > corpus/1 ; echo "^\d{1,10}$" > corpus/2

[+] Fuzzing VIM
cmd$ afl-fuzz -m none -i corpus -o output ./vim -u NONE -X -Z -e -s -S @@ -c ':qa!'
The above options used -u NONE and -X is to speed up vim startup. Options -e -s are used to make vim silent and to avoid 'MORE' prompt which could block VIM, the option -Z disables the external commands which makes fuzzing safer. I've also created a small bash script which automates the above tasks for you [].

While fuzzing, fuzz it on ram file system to avoid making too much I/O something like:  sudo mount -t tmpfs -o size=6g tmpfs /home/afl-fuzz-user/afl-fuzz. Aside you can use [] a script which contains some standard ubuntu packages so you dont get much dependence issues while compiling any target. Keep fuzzing :)

Sunday 11 November 2018

null-pointer dereference in poppler library - CVE-2018-19149

Hi Internet,

Summary: While fuzzing evince v3.28.4, on linux 4.15.0-38-generic (Ubuntu 18.04 LTS), a null-pointer dereference was observed, initially this was reported to evince but the evince team advised that the issue is in poppler, the library used by evince to render PDF, poppler version: 0.62.0-2ubuntu2.2 is vulnerable to null-pointer dereference, however the issue is already fixed in poppler 0.70, but this will still crash your evince v3.28.4 if poppler is not updated to v.0.70. Fuzzing result showing a very important vulnerability in a package currently shipped by a major Linux distribution is still of interest, even if that Linux distribution does not package the latest released upstream version.

Initially, I started fuzzing with evince which is a document reader which comes by default with most of the linux distribution. Also created a malformed PDF files to provide input to AFL, after a successfully compile of evince with afl-gcc, the final command was,

It took three days to get 21 crashes in which 6 unique crashes where observed, while analyzing the crash with triage_crashes which is one of the component which comes with AFL for analyzing crashes, I observed a null-PTR.

So basically a null-PTR is a type of error which causes a SIGSEGV, segmentation fault to the program, and this usually happens when a program or binary try to read or write to the memory with null-PTR.

I went ahead and reported this to GNOME, because evince is one of there asset, the team says "The issue is in Poppler, the library used by Evince to render PDF" arggh!, so stupid am I, I taught `` is one of the shared object in evince but didn't know that poppler is a PDF rendering library which comes by default in most of the PDF reader in linux distribution, and there is a standalone repo out there for poppler.

Also, GNOME evince team says "it seems it has already been addressed. See, Nevertheless, if the issue is still present, please file a bug in"

Okay no worries, I still went ahead and file a bug in poppler, but the team over there asked me what poppler version am i using, and it was version 0.62.0-2ubuntu2.2 and they said the issue is already fixed in poppler version 0.70 After I read this, I was like....
Img Src:

Pheww!, does that mean, my three days of fuzzing just went = to 0 OR am I actually missing something over here ?

I went back to the stack-trace read it again and also check whether am I fuzzing all the latest build of the binary for sure I was fuzzing the latest build of evince but not poppler. Hmmmmmm! I knew my fuzzing system was fully updated but still just to cross check, I did full apt-get update and upgrade but my poppler version remains the same all the time which is 0.62.0-2ubuntu2.2 strange.

I need a guidance over here, and didn't knew what to do ahead, so I contacted MITRE for this and went for a nap, they suggested - "That a fuzzing result showing a very important vulnerability in a package currently shipped by a major Linux distribution is still of interest, even if that Linux distribution does not package the latest released upstream version. For example, an out-of-bounds write finding is still very useful in that case, but not out-of-bounds read, NULL pointer dereference,divide-by-zero, etc."

Ohhh, I see so the latest version of poppler is still not shipped for most of the linux distribution out there, now i understood the entire concept, later MITRE also helped me by assigning a CVE to this issue which is CVE-2018-19149 - Poppler before 0.70.0 has a NULL pointer dereference in _poppler_attachment_new when called from poppler_annot_file_attachment_get_attachment.

An upstream bug is filed in Ubuntu launchpad to track this issue. 

PS: Its not about collecting CVE's, CVE's are just a reference number to an issue you can point for a vulnerability when you show case it somewhere, rather than pointing it to various post. (Personal opinion).

Lessons learned from this:
1. I didn't know poppler is a library which is used by evince and other PDF reader to render PDF's.
2. I understood how to create a malformed PDF to provide input to AFL while fuzzing.
3. The reply from MITRE helped me to resolve my query.
4. During all this, I also got my hands on hongfuzz

Hope you like the read, view this on oss-security mailing list.

Thank you