A Segmentation fault caused by null pointer dereference vulnerability eists in Gpac through 1.0.2 via the avc_parse_slice function in av_parsers.c when using mp4box, which causes a denial of service.
- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). 

Hi, there.

There is a segmentation fault caused by null pointer dereference in avc_parse_slice, av_parsers.c:5678 in commit  592ba2689a3.

Here is my environment, compiler info and gpac version:
~~~~
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.6 LTS
Release:	16.04
Codename:	xenial
gcc: 5.4.0

MP4Box - GPAC version 1.1.0-DEV-rev1170-g592ba26-master
(c) 2000-2021 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
	MINI build (encoders, decoders, audio and video output disabled)

Please cite our work in your research:
	GPAC Filters: https://doi.org/10.1145/3339825.3394929
	GPAC: https://doi.org/10.1145/1291233.1291452

GPAC Configuration: --static-bin --enable-debug
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_FREETYPE GPAC_HAS_JPEG GPAC_HAS_PNG  GPAC_DISABLE_3D
~~~~

To reproduce, run
~~~~
./MP4Box -info poc
~~~~

POC:
[poc.zip](https://github.com/gpac/gpac/files/7059392/mp4box-seg-npd-avc_parse_slice5678.zip)
(unzip first)

Here is the trace reported by gdb:
~~~~
Stopped reason: SIGSEGV
gef➤  bt
#0  0x0000000000bcd59d in avc_parse_slice (svc_idr_flag=GF_FALSE, si=0x7fffffff5020, avc=0x24ae050, bs=0x248df40) at /mnt/data/playground/gpac/src/media_tools/av_parsers.c:5678
#1  gf_avc_parse_nalu (bs=0x248df40, avc=0x24ae050) at /mnt/data/playground/gpac/src/media_tools/av_parsers.c:6087
#2  0x000000000144109d in naludmx_parse_nal_avc (is_islice=<synthetic pointer>, is_slice=<synthetic pointer>, skip_nal=<synthetic pointer>, nal_type=0x4, size=0x4f, data=0x2491e5b "$1\200", ctx=0x24ada70) at /mnt/data/playground/gpac/src/filters/reframe_nalu.c:2348
#3  naludmx_process (filter=0x24a0bd0) at /mnt/data/playground/gpac/src/filters/reframe_nalu.c:2874
#4  0x0000000000fe4c18 in gf_filter_process_task (task=0x248d520) at /mnt/data/playground/gpac/src/filter_core/filter.c:2441
#5  0x0000000000f7b909 in gf_fs_thread_proc (sess_thread=sess_thread@entry=0x248c2b0) at /mnt/data/playground/gpac/src/filter_core/filter_session.c:1640
#6  0x0000000000f93558 in gf_fs_run (fsess=fsess@entry=0x248c220) at /mnt/data/playground/gpac/src/filter_core/filter_session.c:1877
#7  0x0000000000c18b4b in gf_media_import (importer=importer@entry=0x7fffffff5bf0) at /mnt/data/playground/gpac/src/media_tools/media_import.c:1178
#8  0x0000000000497345 in convert_file_info (inName=0x7fffffffe159 "tmp", trackID=0x0) at /mnt/data/playground/gpac/applications/mp4box/fileimport.c:128
#9  0x0000000000456aaa in mp4boxMain (argc=<optimized out>, argv=<optimized out>) at /mnt/data/playground/gpac/applications/mp4box/main.c:5925
#10 0x0000000001f06bb6 in generic_start_main ()
#11 0x0000000001f071a5 in __libc_start_main ()
#12 0x000000000041c4e9 in _start ()
~~~~

The reason for this bug is that the program does not check the nullity of the pointer.
<img width="655" alt="image" src="https://user-images.githubusercontent.com/7632714/130955606-eefd9dbd-049a-48b2-ab9f-fa46197e9d99.png">


Segmentation fault caused by null pointer dereference using mp4box in avc_parse_slice, av_parsers.c:5678
This is a GitHub Issue
repo:gpac
owner:gpac
Title : Segmentation fault caused by null pointer dereference using mp4box in avc_parse_slice, av_parsers.c:5678
Issue date:

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- [x] I looked for a similar issue and couldn't find any.
- [x] I tried with the latest version of GPAC. Installers available at http://gpac.io/downloads/gpac-nightly-builds/
- [x] I give enough information for contributors to reproduce my issue (meaningful title, github labels, platform and compiler, command-line ...). 

Hi, there.

There is a segmentation fault caused by null pointer dereference in avc_parse_slice, av_parsers.c:5678 in commit  592ba2689a3.

Here is my environment, compiler info and gpac version:
~~~~
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.6 LTS
Release:	16.04
Codename:	xenial
gcc: 5.4.0

MP4Box - GPAC version 1.1.0-DEV-rev1170-g592ba26-master
(c) 2000-2021 Telecom Paris distributed under LGPL v2.1+ - http://gpac.io
	MINI build (encoders, decoders, audio and video output disabled)

Please cite our work in your research:
	GPAC Filters: https://doi.org/10.1145/3339825.3394929
	GPAC: https://doi.org/10.1145/1291233.1291452

GPAC Configuration: --static-bin --enable-debug
Features: GPAC_CONFIG_LINUX GPAC_64_BITS GPAC_HAS_SOCK_UN GPAC_MINIMAL_ODF GPAC_HAS_QJS GPAC_HAS_FREETYPE GPAC_HAS_JPEG GPAC_HAS_PNG  GPAC_DISABLE_3D
~~~~

To reproduce, run
~~~~
./MP4Box -info poc
~~~~

POC:
[poc.zip](https://github.com/gpac/gpac/files/7059392/mp4box-seg-npd-avc_parse_slice5678.zip)
(unzip first)

Here is the trace reported by gdb:
~~~~
Stopped reason: SIGSEGV
gef➤  bt
#0  0x0000000000bcd59d in avc_parse_slice (svc_idr_flag=GF_FALSE, si=0x7fffffff5020, avc=0x24ae050, bs=0x248df40) at /mnt/data/playground/gpac/src/media_tools/av_parsers.c:5678
#1  gf_avc_parse_nalu (bs=0x248df40, avc=0x24ae050) at /mnt/data/playground/gpac/src/media_tools/av_parsers.c:6087
#2  0x000000000144109d in naludmx_parse_nal_avc (is_islice=<synthetic pointer>, is_slice=<synthetic pointer>, skip_nal=<synthetic pointer>, nal_type=0x4, size=0x4f, data=0x2491e5b "$1\200", ctx=0x24ada70) at /mnt/data/playground/gpac/src/filters/reframe_nalu.c:2348
#3  naludmx_process (filter=0x24a0bd0) at /mnt/data/playground/gpac/src/filters/reframe_nalu.c:2874
#4  0x0000000000fe4c18 in gf_filter_process_task (task=0x248d520) at /mnt/data/playground/gpac/src/filter_core/filter.c:2441
#5  0x0000000000f7b909 in gf_fs_thread_proc (sess_thread=sess_thread@entry=0x248c2b0) at /mnt/data/playground/gpac/src/filter_core/filter_session.c:1640
#6  0x0000000000f93558 in gf_fs_run (fsess=fsess@entry=0x248c220) at /mnt/data/playground/gpac/src/filter_core/filter_session.c:1877
#7  0x0000000000c18b4b in gf_media_import (importer=importer@entry=0x7fffffff5bf0) at /mnt/data/playground/gpac/src/media_tools/media_import.c:1178
#8  0x0000000000497345 in convert_file_info (inName=0x7fffffffe159 "tmp", trackID=0x0) at /mnt/data/playground/gpac/applications/mp4box/fileimport.c:128
#9  0x0000000000456aaa in mp4boxMain (argc=<optimized out>, argv=<optimized out>) at /mnt/data/playground/gpac/applications/mp4box/main.c:5925
#10 0x0000000001f06bb6 in generic_start_main ()
#11 0x0000000001f071a5 in __libc_start_main ()
#12 0x000000000041c4e9 in _start ()
~~~~

The reason for this bug is that the program does not check the nullity of the pointer.
<img width="655" alt="image" src="https://user-images.githubusercontent.com/7632714/130955606-eefd9dbd-049a-48b2-ab9f-fa46197e9d99.png">


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