✍️ Description

When fuzzing vim commit 3c19b5050 (works with latest build and latest commit 65259b5c6 per this time of this report) v8.2.3635 with clang 12 and ASan, I discovered a heap buffer overflow.

Proof of Concept

Here is the poc download link

https://drive.google.com/file/d/16KrhAQ_Feps6uhQGwuc_l8KETMz0eTyz/view?usp=sharing

How to build

LD=lld AS=llvm-as AR=llvm-ar RANLIB=llvm-ranlib CC=clang CXX=clang++ CFLAGS="-fsanitize=address" CXXFLAGS="-fsanitize=address" LDFLAGS="-ldl -fsanitize=address" ./configure --with-features=huge --enable-gui=none
make -j2

Proof of Concept

Run crafted file with this command

./vim -u NONE -X -Z -e -s -S pocBO_min -c :qa!

ASan stack trace:

aldo@vps:~/vim-8.2.3635/src$ ASAN_OPTIONS=symbolize=1 ASAN_SYMBOLIZER_PATH=/usr/bin/llvm-symbolizer ./vim -u NONE -X -Z -e -s -S pocBO_min -c :qa!
=================================================================
==2485525==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x619000000e81 at pc 0x000000486718 bp 0x7ffffffefd60 sp 0x7ffffffef520
WRITE of size 1023 at 0x619000000e81 thread T0
    #0 0x486717 in strcpy (/home/aldo/vim/src/vim+0x486717)
    #1 0xc80184 in find_help_tags /home/aldo/vim/src/help.c:426:6
    #2 0xc7b823 in ex_help /home/aldo/vim/src/help.c:88:9
    #3 0xa06f9d in do_one_cmd /home/aldo/vim/src/ex_docmd.c:2614:2
    #4 0x9eabea in do_cmdline /home/aldo/vim/src/ex_docmd.c:1000:17
    #5 0x1592d2d in do_source /home/aldo/vim/src/scriptfile.c:1406:5
    #6 0x158dd61 in cmd_source /home/aldo/vim/src/scriptfile.c:971:14
    #7 0x158d66e in ex_source /home/aldo/vim/src/scriptfile.c:997:2
    #8 0xa06f9d in do_one_cmd /home/aldo/vim/src/ex_docmd.c:2614:2
    #9 0x9eabea in do_cmdline /home/aldo/vim/src/ex_docmd.c:1000:17
    #10 0x9f1fd1 in do_cmdline_cmd /home/aldo/vim/src/ex_docmd.c:594:12
    #11 0x1f239a1 in exe_commands /home/aldo/vim/src/main.c:3081:2
    #12 0x1f1cd23 in vim_main2 /home/aldo/vim/src/main.c:773:2
    #13 0x1f081ed in main /home/aldo/vim/src/main.c:425:12
    #14 0x7ffff78260b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16
    #15 0x41fe2d in _start (/home/aldo/vim/src/vim+0x41fe2d)

0x619000000e81 is located 0 bytes to the right of 1025-byte region [0x619000000a80,0x619000000e81)
allocated by thread T0 here:
    #0 0x49aced in malloc (/home/aldo/vim/src/vim+0x49aced)
    #1 0x4cc661 in lalloc /home/aldo/vim/src/alloc.c:244:11
    #2 0x4cc4ba in alloc /home/aldo/vim/src/alloc.c:151:12
    #3 0x1f08292 in common_init /home/aldo/vim/src/main.c:910:19
    #4 0x1f068bb in main /home/aldo/vim/src/main.c:179:5
    #5 0x7ffff78260b2 in __libc_start_main /build/glibc-eX1tMB/glibc-2.31/csu/../csu/libc-start.c:308:16

SUMMARY: AddressSanitizer: heap-buffer-overflow (/home/aldo/vim/src/vim+0x486717) in strcpy
Shadow bytes around the buggy address:
  0x0c327fff8180: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff8190: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff81a0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff81b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  0x0c327fff81c0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c327fff81d0:[01]fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff81e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x0c327fff81f0: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c327fff8200: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c327fff8210: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x0c327fff8220: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
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
==2485525==ABORTING

💥 Impact

This vulnerability is capable of crashing software, Bypass Protection Mechanism, Modify Memory, and possible remote execution