An issue was discovered in libredwg through v0.10.1.3751. A NULL pointer dereference exists in the function bit_read_BB() located in bits.c. It allows an attacker to cause Denial of Service.
## System info

Ubuntu X64, gcc (Ubuntu 5.5.0-12ubuntu1), dwg2dxf (latest master [bacd017](https://github.com/LibreDWG/libredwg/commit/bacd01778d20b7b79f28541ea7a60cafe25175aa))
## Configure

CFLAGS="-g -fsanitize=address" LDFLAGS="-fsanitize=address" ./configure

## Command line

./programs/dwg2dxf -b -m ./SEGV-bit_read_BB-bits-186 -o /dev/null

## Output

```
Segmentation fault (core dumped)
```

## AddressSanitizer output

```
ASAN:SIGSEGV
=================================================================
==33413==ERROR: AddressSanitizer: SEGV on unknown address 0x63001eedeec1 (pc 0x56001ae1aa1c bp 0x7ffc0f008820 sp 0x7ffc0f008570 T0)
    #0 0x56001ae1aa1b in bit_read_BB /home/seviezhou/libredwg/src/bits.c:186
    #1 0x56001ae3d11b in bit_read_BS /home/seviezhou/libredwg/src/bits.c:525
    #2 0x56001ae3d11b in bit_read_TU /home/seviezhou/libredwg/src/bits.c:1891
    #3 0x56001ae4d52b in bit_read_CMC /home/seviezhou/libredwg/src/bits.c:2610
    #4 0x56001a5261c2 in dwg_decode_VIEWPORT_private /home/seviezhou/libredwg/src/dwg.spec:1674
    #5 0x56001aef02ff in dwg_decode_VIEWPORT /home/seviezhou/libredwg/src/dwg.spec:1607
    #6 0x56001aef02ff in dwg_decode_add_object /home/seviezhou/libredwg/src/decode.c:5583
    #7 0x56001aef77b8 in read_2004_section_handles /home/seviezhou/libredwg/src/decode.c:2843
    #8 0x56001aef77b8 in decode_R2004 /home/seviezhou/libredwg/src/decode.c:3682
    #9 0x56001af060ba in dwg_decode /home/seviezhou/libredwg/src/decode.c:242
    #10 0x56001adfcd94 in dwg_read_file /home/seviezhou/libredwg/src/dwg.c:251
    #11 0x56001adfa242 in main /home/seviezhou/libredwg/programs/dwg2dxf.c:258
    #12 0x7f13152a8b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #13 0x56001adfb2b9 in _start (/home/seviezhou/libredwg/programs/dwg2dxf+0xa8d2b9)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/seviezhou/libredwg/src/bits.c:186 bit_read_BB
==33413==ABORTING
```

## POC

[SEGV-bit_read_BB-bits-186.zip](https://github.com/LibreDWG/libredwg/files/5016490/SEGV-bit_read_BB-bits-186.zip)

A Segmentation fault in bits.c:186
This is a GitHub Issue
repo:libredwg
owner:LibreDWG
Title : A Segmentation fault in bits.c:186
Issue date:

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## System info

Ubuntu X64, gcc (Ubuntu 5.5.0-12ubuntu1), dwg2dxf (latest master [bacd017](https://github.com/LibreDWG/libredwg/commit/bacd01778d20b7b79f28541ea7a60cafe25175aa))
## Configure

CFLAGS="-g -fsanitize=address" LDFLAGS="-fsanitize=address" ./configure

## Command line

./programs/dwg2dxf -b -m ./SEGV-bit_read_BB-bits-186 -o /dev/null

## Output

```
Segmentation fault (core dumped)
```

## AddressSanitizer output

```
ASAN:SIGSEGV
=================================================================
==33413==ERROR: AddressSanitizer: SEGV on unknown address 0x63001eedeec1 (pc 0x56001ae1aa1c bp 0x7ffc0f008820 sp 0x7ffc0f008570 T0)
    #0 0x56001ae1aa1b in bit_read_BB /home/seviezhou/libredwg/src/bits.c:186
    #1 0x56001ae3d11b in bit_read_BS /home/seviezhou/libredwg/src/bits.c:525
    #2 0x56001ae3d11b in bit_read_TU /home/seviezhou/libredwg/src/bits.c:1891
    #3 0x56001ae4d52b in bit_read_CMC /home/seviezhou/libredwg/src/bits.c:2610
    #4 0x56001a5261c2 in dwg_decode_VIEWPORT_private /home/seviezhou/libredwg/src/dwg.spec:1674
    #5 0x56001aef02ff in dwg_decode_VIEWPORT /home/seviezhou/libredwg/src/dwg.spec:1607
    #6 0x56001aef02ff in dwg_decode_add_object /home/seviezhou/libredwg/src/decode.c:5583
    #7 0x56001aef77b8 in read_2004_section_handles /home/seviezhou/libredwg/src/decode.c:2843
    #8 0x56001aef77b8 in decode_R2004 /home/seviezhou/libredwg/src/decode.c:3682
    #9 0x56001af060ba in dwg_decode /home/seviezhou/libredwg/src/decode.c:242
    #10 0x56001adfcd94 in dwg_read_file /home/seviezhou/libredwg/src/dwg.c:251
    #11 0x56001adfa242 in main /home/seviezhou/libredwg/programs/dwg2dxf.c:258
    #12 0x7f13152a8b96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
    #13 0x56001adfb2b9 in _start (/home/seviezhou/libredwg/programs/dwg2dxf+0xa8d2b9)

AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV /home/seviezhou/libredwg/src/bits.c:186 bit_read_BB
==33413==ABORTING
```

## POC

[SEGV-bit_read_BB-bits-186.zip](https://github.com/LibreDWG/libredwg/files/5016490/SEGV-bit_read_BB-bits-186.zip)

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