Source: https://bugs.chromium.org/p/project-zero/issues/detail?id=716
The ActionScript parameter conversion in the fix for an issue in the December Flash bulletin (https://helpx.adobe.com/security/products/flash-player/apsb15-32.html, most likely one of the UaFs reported by Yuki Chen) can sometimes access a parameter on the native stack that is uninitialized.
If:
var o = {};
o.unwatch();
is called in ActionScript, a parameter array is allocated using alloca(0), which leads to a 16-byte (the minimum size length for alloca in the implementation) that does not get initialized. The conversion function in the UaF check then assumes that at least one parameter has been allocated, and attempts to convert the stack parameter to a string, even though it is a previous value (a UTF string "fffff ... " in the PoC).
A PoC is attached, it is a bit finicky but crashes in the most recent Chrome Flash update. To reproduce, load crasher2.swf?num=15, and then immediately loading crasher2.swf?num=4. The num parameter shifts the stack (for nums between 0 and 31), so changing it around should lead to crashes in different browsers.
Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/39612.zip
