In GenerateJitEntry, the ValType::I32 return case boxed the raw ReturnReg directly with the comment “No widening is required, as the value is boxed.” On a 64-bit CPU the WASM i32 result sits in the low 32 bits of a 64-bit ReturnReg, but the upper 32 bits are not guaranteed to be zero — they can hold leftover bits from prior computation in that register. boxNonDouble(JSVAL_TYPE_INT32, ...) constructs the NaN-boxed JS::Value assuming the payload register holds only a clean 32-bit int in its low half; if the high half contains garbage, those stray bits collide with/corrupt the tag region of the resulting 64-bit JS::Value. The produced value therefore decodes as something other than a clean Int32 — i.e. it can be interpreted as a different JS::Value type, exactly the “treated as a different type” outcome the advisory states. The old assumption (that boxing alone sufficed) was wrong for 64-bit register width.

The patch inserts, guarded by #ifdef JS_64BIT, a masm.widenInt32(ReturnReg) immediately before the boxNonDouble call. This zero-extends the i32 result across the full 64-bit register, clearing the stale high bits so the subsequent box produces a correctly tagged Int32 value. The #ifdef JS_64BIT guard scopes the fix to the architectures where the 64-bit register width creates the exposure. The advisory labels this a logic error; the diff substantiates that the concrete consequence is a mis-typed JS::Value (type confusion) arising from uninitialized/leftover high register bits — I note that distinction rather than smoothing it over.

This is a content-process (renderer) JIT correctness bug reachable with no special privilege by any page that can run WebAssembly, needing only the interaction of visiting the page. The substantiated primitive is a mis-typed JS::Value on 64-bit builds — a type-confusion seed whose exploitability depends on how the confused value is subsequently used (the CVSS vector lists C:L/I:L/A:H and the advisory rates it moderate, consistent with a confusion that is real but not shown here to yield direct full-strength corruption). Escalation to a controlled read/write or RCE is inferred, not proven by this one-line stub change.