LibreDWG v0.12.4.4608 was discovered to contain a heap-use-after-free via the function dwg_add_handleref at dwg.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
==8997==ERROR: AddressSanitizer: heap-use-after-free on address 0x604000000730 at pc 0x000000517369 bp 0x7fffffffc7d0 sp 0x7fffffffc7c8
READ of size 8 at 0x604000000730 thread T0
    #0 0x517368 in dwg_add_handleref /testcase/libredwg/src/dwg.c:2014:21
    #1 0x7ea615 in dwg_add_BLOCK_HEADER /testcase/libredwg/src/dwg_api.c:24588:3
    #2 0x70baf6 in decode_preR13_section /testcase/libredwg/src/decode_r11.c:325:20
    #3 0x705d0a in decode_preR13 /testcase/libredwg/src/decode_r11.c:830:12
    #4 0x53245a in dwg_decode /testcase/libredwg/src/decode.c:209:23
    #5 0x50d759 in dwg_read_file /testcase/libredwg/src/dwg.c:254:11
    #6 0x50c454 in main /testcase/libredwg/programs/dwg2dxf.c:258:15
    #7 0x7ffff6e22c86 in __libc_start_main /build/glibc-CVJwZb/glibc-2.27/csu/../csu/libc-start.c:310
    #8 0x419ee9 in _start (/testcase/libredwg/programs/dwg2dxf+0x419ee9)

0x604000000730 is located 32 bytes inside of 48-byte region [0x604000000710,0x604000000740)
freed by thread T0 here:
    #0 0x4d23a0 in __interceptor_cfree.localalias.0 /fuzzer/build/llvm_tools/llvm-4.0.0.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:55
    #1 0x7070a2 in decode_preR13_header_variables /testcase/libredwg/src/./header_variables_r11.spec:65:3
    #2 0x232900001100144d  (<unknown module>)

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 0x54900c in dwg_new_ref /testcase/libredwg/src/decode.c:4027:43

SUMMARY: AddressSanitizer: heap-use-after-free /testcase/libredwg/src/dwg.c:2014:21 in dwg_add_handleref
Shadow bytes around the buggy address:
  0x0c087fff8090: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 fa
  0x0c087fff80a0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff80b0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff80c0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff80d0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
=>0x0c087fff80e0: fa fa fd fd fd fd[fd]fd fa fa 00 00 00 00 00 00
  0x0c087fff80f0: fa fa 00 00 00 00 00 00 fa fa fd fd fd fd fd fd
  0x0c087fff8100: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff8110: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff8120: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff8130: fa fa 00 00 00 00 00 00 fa fa 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
==8997==ABORTING

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


heap-use-after-free exists in the function dwg_add_handleref in dwg.c 
This is a GitHub Issue
repo:libredwg
owner:LibreDWG
Title : heap-use-after-free exists in the function dwg_add_handleref in dwg.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
==8997==ERROR: AddressSanitizer: heap-use-after-free on address 0x604000000730 at pc 0x000000517369 bp 0x7fffffffc7d0 sp 0x7fffffffc7c8
READ of size 8 at 0x604000000730 thread T0
    #0 0x517368 in dwg_add_handleref /testcase/libredwg/src/dwg.c:2014:21
    #1 0x7ea615 in dwg_add_BLOCK_HEADER /testcase/libredwg/src/dwg_api.c:24588:3
    #2 0x70baf6 in decode_preR13_section /testcase/libredwg/src/decode_r11.c:325:20
    #3 0x705d0a in decode_preR13 /testcase/libredwg/src/decode_r11.c:830:12
    #4 0x53245a in dwg_decode /testcase/libredwg/src/decode.c:209:23
    #5 0x50d759 in dwg_read_file /testcase/libredwg/src/dwg.c:254:11
    #6 0x50c454 in main /testcase/libredwg/programs/dwg2dxf.c:258:15
    #7 0x7ffff6e22c86 in __libc_start_main /build/glibc-CVJwZb/glibc-2.27/csu/../csu/libc-start.c:310
    #8 0x419ee9 in _start (/testcase/libredwg/programs/dwg2dxf+0x419ee9)

0x604000000730 is located 32 bytes inside of 48-byte region [0x604000000710,0x604000000740)
freed by thread T0 here:
    #0 0x4d23a0 in __interceptor_cfree.localalias.0 /fuzzer/build/llvm_tools/llvm-4.0.0.src/projects/compiler-rt/lib/asan/asan_malloc_linux.cc:55
    #1 0x7070a2 in decode_preR13_header_variables /testcase/libredwg/src/./header_variables_r11.spec:65:3
    #2 0x232900001100144d  (<unknown module>)

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 0x54900c in dwg_new_ref /testcase/libredwg/src/decode.c:4027:43

SUMMARY: AddressSanitizer: heap-use-after-free /testcase/libredwg/src/dwg.c:2014:21 in dwg_add_handleref
Shadow bytes around the buggy address:
  0x0c087fff8090: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 fa
  0x0c087fff80a0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff80b0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff80c0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff80d0: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
=>0x0c087fff80e0: fa fa fd fd fd fd[fd]fd fa fa 00 00 00 00 00 00
  0x0c087fff80f0: fa fa 00 00 00 00 00 00 fa fa fd fd fd fd fd fd
  0x0c087fff8100: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff8110: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff8120: fa fa 00 00 00 00 00 00 fa fa 00 00 00 00 00 00
  0x0c087fff8130: fa fa 00 00 00 00 00 00 fa fa 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
==8997==ABORTING

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


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