Bug Hunter Diary

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4.1 Vulnerability DiscoveryTo find the vulnerability I did the following:• Step 1: List the demuxers of FFmpeg.• Step 2: Identify the input data.• Step 3: Trace the input data.Step 1: List the Demuxers of FFmpegAfter getting the latest source code revision from the FFmpeg SVNrepository, I generated a list of the demuxers that are available in thelibavformat library, which is included with FFmpeg (see Figure 4-1).I noticed that FFmpeg separates most demuxers in different C filesunder the directory libavformat/.Figure 4-1: FFmpeg libavformat demuxersNoteFFmpeg development has moved to a Git repository, 2 and the SVNrepository is no longer updated. The vulnerable source code revision(SVN-r16556) of FFmpeg can now be downloaded from this book’swebsite. 3Step 2: Identify the Input DataNext, I tried to identify the input data processed by the demuxers.While reading the source code, I discovered that most demuxersdeclare a function called demuxername_read_header(), which usually52 Chapter 4
takes a parameter of the type AVFormatContext. This function declaresand initializes a pointer that looks like this:[..]ByteIOContext *pb = s->pb;[..]Many different get_something functions (e.g., get_le32(), get_buffer())and special macros (e.g., AV_RL32, AV_RL16) are then used to extractportions of the data pointed to by pb. At this point, I was pretty surethat pb had to be a pointer to the input data of the media files beingprocessed.Step 3: Trace the Input DataI decided to search for bugs by tracing the input data of eachdemuxer at the source code level. I started with the first demuxerfile from the list, called 4xm.c. While auditing the demuxer of the4X movie file format, 4 I found the vulnerability shown in the listingbelow.Source code filelibavformat/4xm.cFunctionfourxm_read_header()[..]93 static int fourxm_read_header(AVFormatContext *s,94 AVFormatParameters *ap)95 {96 ByteIOContext *pb = s->pb;..101 unsigned char *header;..103 int current_track = -1;..106 fourxm->track_count = 0;107 fourxm->tracks = NULL;..120 /* allocate space for the header and load the whole thing */121 header = av_malloc(header_size);122 if (!header)123 return AVERROR(ENOMEM);124 if (get_buffer(pb, header, header_size) != header_size)125 return AVERROR(EIO);..160 } else if (fourcc_tag == strk_TAG) {161 /* check that there is enough data */162 if (size != strk_SIZE) {163 av_free(header);164 return AVERROR_INVALIDDATA;165 }166 current_track = AV_RL32(&header[i + 8]);NULL Pointer FTW 53
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A Bug Hunter’s Diary. Copyright
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Chapter 3: Escape from the WWW Zone
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B.2 The Windows Debugger (WinDbg) .
Page 15 and 16: IntroductionWelcome to A Bug Hunter
Page 17 and 18: 1Bug HuntingBug hunting is the proc
Page 19 and 20: After I’ve found a bug, I want to
Page 21 and 22: Table 1-1: Debuggers Used in This B
Page 23: 2Back to the ’90sSunday, October
Page 26 and 27: 16361637 /* clear the SEQ table */1
Page 28 and 29: If a TiVo file is loaded by VLC, th
Page 30 and 31: Get the →vulnerableWindows versio
Page 32 and 33: EIP = 41414141 . . . Mission EIP co
Page 34 and 35: NoteTo configure Process Explorer t
Page 36 and 37: See the following excerpt from the
Page 38 and 39: 3. Immunity Debugger is a great Win
Page 40 and 41: NoteInput/output controls (IOCTLs)
Page 42 and 43: msgbb_datapdatabb_rptrdb_basedatabu
Page 44 and 45: 8180 /*8181 * Null terminate the st
Page 46 and 47: 26736 /*26737 * SIOC[GS]TUNPARAM ap
Page 48 and 49: 19165 * are also rejected as they i
Page 50 and 51: 01 #include 02 #include 03 #include
Page 52 and 53: fffffe8000f7e4b0 unix:die+da ()ffff
Page 54 and 55: zero pagenot mappedzero pageis mapp
Page 56 and 57: gaining full control over EIP/RIP a
Page 58 and 59: 112 return 1;113 }114115 printf ("O
Page 60 and 61: solaris$ isainfo -b64I then compile
Page 62 and 63: ip_output+0x10()ip_wput+0x37()putne
Page 64 and 65: Notes1. The source code of OpenSola
Page 68 and 69: 167 if (current_track + 1 > fourxm-
Page 71 and 72: Step 1: Find a Sample 4X Movie File
Page 73 and 74: Next, I modified the values of trac
Page 75 and 76: Table 4-2: List of the Assembler In
Page 77 and 78: 63 #elif defined (HAVE_MEMALIGN)64
Page 79 and 80: linux$ gdb -q ./ffmpeg_g(gdb) run -
Page 81 and 82: (UINT_MAX / sizeof(AudioTrack) - 1)
Page 83 and 84: 7ebe000-b7ec0000 r--p 0015c000 08:0
Page 85 and 86: 5Browse and You’re OwnedSunday, A
Page 87 and 88: {SWI.ClassId_q.ClassId clsid = new
Page 89 and 90: 27 except KeyboardInterrupt:28 prin
Page 91 and 92: Figure 5-4: Defining a breakpoint a
Page 93 and 94: Figure 5-7: User-controlled argumen
Page 95 and 96: Next, I tried to retrieve the size
Page 97 and 98: 05 06 arg = String(232, "A") + Stri
Page 99: iDefense VCPnotifiedVulnerability d
Page 102 and 103: 6.1 Vulnerability DiscoveryI used t
Page 104 and 105: Step 3: Check the Device Security S
Page 106 and 107: Below, the elements of the MajorFun
Page 108 and 109: At address .text:00010748, a pointe
Page 110 and 111: [..][..]//// Current stack location
Page 112 and 113: The transfer type is specified usin
Page 114 and 115: If the requested IOCTL code matches
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Figure 6-5: Graph view of the vulne
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.text:00010DBA push 1 ; _DWORD.text
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99 // .text:00010DEF cmp dword ptr
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nt!RtlpBreakWithStatusInstruction:8
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After I gained control over EIP, I
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7A Bug Older Than 4.4BSDSaturday, M
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Here are some examples:Source code
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1094 return (ENXIO);1095 if (t != t
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4. The assumed address of l_open()
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osx$ cat /Library/Logs/panic.logSat
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Back on the Linux host, I started t
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0x35574c, the value is used to calc
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48 define search_memloc49 set $max_
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As the debugger output shows, the E
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ApplenotifiedApple asks formore det
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I performed the following steps whe
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I then began fuzzing files of the A
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Figure 8-1: Playing the unmodifiedA
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• Line 40 copies the current proc
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As the debugger output shows, media
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The manipulated byte (0xff) that ca
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(gdb) x/12x 0x1301bfc0x1301bfc: 0x0
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ApplenotifiedApple confirmsthe vuln
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Every function of a process that is
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eip 0x43434343 0x43434343eflags 0x1
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The access violation is caused when
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is less than 12. If it is, the stri
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The program in Listing A-4 calls th
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0x080483f3 : mov DWORD PTR [esp+0x4
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BDebuggingThis appendix contains in
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Information CommandsCommandDescript
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Information CommandsCommandrkbu add
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Figure B-3: Configuration settings
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Figure B-6: Attaching the kernel de
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Other CommandsCommandset disassembl
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linux# pwd/home/tk/gdb-292/srclinux
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Notes1. See the Solaris Modular Deb
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The many mitigation techniques coul
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Figure C-1: DEP and ASLR status sho
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08 p[0] = 0x41414141;09 printf (“
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ConclusionIn case of a buffer overf
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zonecfg:wwwzone> set autoboot=truez
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Notes1. See Rob King, “New Leopar
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COMRaider, 72coordinated disclosure
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Rreadelf, 161RELRO, 67-69, 183-186r
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A Bug Hunter’s Diary is set in Ne
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