LibreDWG v0.12.4.4608 was discovered to contain a heap-use-after-free via the function decode_preR13_section at decode_r11.c.
### system info
Ubuntu x86_64, clang 6.0, dwg2dxf([0.12.4.4608](https://github.com/LibreDWG/libredwg/releases/tag/0.12.4.4608))

### Command line
./programs/dwg2dxf -b -m @@ -o /dev/null

### AddressSanitizer output
==8989==ERROR: AddressSanitizer: heap-use-after-free on address 0x7ffff7e35838 at pc 0x0000007106ca bp 0x7fffffffc8b0 sp 0x7fffffffc8a8
READ of size 8 at 0x7ffff7e35838 thread T0
    #0 0x7106c9 in decode_preR13_section /testcase/libredwg/src/decode_r11.c:339:35
    #1 0x705d0a in decode_preR13 /testcase/libredwg/src/decode_r11.c:830:12
    #2 0x53245a in dwg_decode /testcase/libredwg/src/decode.c:209:23
    #3 0x50d759 in dwg_read_file /testcase/libredwg/src/dwg.c:254:11
    #4 0x50c454 in main /testcase/libredwg/programs/dwg2dxf.c:258:15
    #5 0x7ffff6e22c86 in __libc_start_main /build/glibc-CVJwZb/glibc-2.27/csu/../csu/libc-start.c:310
    #6 0x419ee9 in _start (/testcase/libredwg/programs/dwg2dxf+0x419ee9)

0x7ffff7e35838 is located 56 bytes inside of 172032-byte region [0x7ffff7e35800,0x7ffff7e5f800)
freed by thread T0 here:
    #0 0x4d2968 in realloc /fuzzer/build/llvm_tools/llvm-4.0.0.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:79
    #1 0x5a05b5 in dwg_add_object /testcase/libredwg/src/decode.c:4730:35

previously allocated by thread T0 here:
    #0 0x4d2750 in calloc /fuzzer/build/llvm_tools/llvm-4.0.0.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:74
    #1 0x5a0465 in dwg_add_object /testcase/libredwg/src/decode.c:4719:35

SUMMARY: AddressSanitizer: heap-use-after-free /testcase/libredwg/src/decode_r11.c:339:35 in decode_preR13_section
Shadow bytes around the buggy address:
  0x10007efbeab0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbeac0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbead0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbeae0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbeaf0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x10007efbeb00: fd fd fd fd fd fd fd[fd]fd fd fd fd fd fd fd fd
  0x10007efbeb10: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb20: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb30: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb40: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb50: 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
==8989==ABORTING

### poc
https://gitee.com/cxlzff/fuzz-poc/raw/master/libredwg/decode_preR13_uaf


heap-use-after-free exists in the function decode_preR13_section in decode_r11.c
This is a GitHub Issue
repo:libredwg
owner:LibreDWG
Title : heap-use-after-free exists in the function decode_preR13_section in decode_r11.c
Issue date:

--- start body ---
### system info
Ubuntu x86_64, clang 6.0, dwg2dxf([0.12.4.4608](https://github.com/LibreDWG/libredwg/releases/tag/0.12.4.4608))

### Command line
./programs/dwg2dxf -b -m @@ -o /dev/null

### AddressSanitizer output
==8989==ERROR: AddressSanitizer: heap-use-after-free on address 0x7ffff7e35838 at pc 0x0000007106ca bp 0x7fffffffc8b0 sp 0x7fffffffc8a8
READ of size 8 at 0x7ffff7e35838 thread T0
    #0 0x7106c9 in decode_preR13_section /testcase/libredwg/src/decode_r11.c:339:35
    #1 0x705d0a in decode_preR13 /testcase/libredwg/src/decode_r11.c:830:12
    #2 0x53245a in dwg_decode /testcase/libredwg/src/decode.c:209:23
    #3 0x50d759 in dwg_read_file /testcase/libredwg/src/dwg.c:254:11
    #4 0x50c454 in main /testcase/libredwg/programs/dwg2dxf.c:258:15
    #5 0x7ffff6e22c86 in __libc_start_main /build/glibc-CVJwZb/glibc-2.27/csu/../csu/libc-start.c:310
    #6 0x419ee9 in _start (/testcase/libredwg/programs/dwg2dxf+0x419ee9)

0x7ffff7e35838 is located 56 bytes inside of 172032-byte region [0x7ffff7e35800,0x7ffff7e5f800)
freed by thread T0 here:
    #0 0x4d2968 in realloc /fuzzer/build/llvm_tools/llvm-4.0.0.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:79
    #1 0x5a05b5 in dwg_add_object /testcase/libredwg/src/decode.c:4730:35

previously allocated by thread T0 here:
    #0 0x4d2750 in calloc /fuzzer/build/llvm_tools/llvm-4.0.0.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:74
    #1 0x5a0465 in dwg_add_object /testcase/libredwg/src/decode.c:4719:35

SUMMARY: AddressSanitizer: heap-use-after-free /testcase/libredwg/src/decode_r11.c:339:35 in decode_preR13_section
Shadow bytes around the buggy address:
  0x10007efbeab0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbeac0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbead0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbeae0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
  0x10007efbeaf0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
=>0x10007efbeb00: fd fd fd fd fd fd fd[fd]fd fd fd fd fd fd fd fd
  0x10007efbeb10: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb20: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb30: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb40: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
  0x10007efbeb50: 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
==8989==ABORTING

### poc
https://gitee.com/cxlzff/fuzz-poc/raw/master/libredwg/decode_preR13_uaf


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