CVE-2020-15681: 
NixOS vulnerability analysis and mitigation

CVE-2020-15681 is a vulnerability discovered in Firefox's WebAssembly (WASM) implementation that was disclosed and fixed in October 2020. The vulnerability affects Firefox's handling of WASM threads when multiple threads have references to a module and are looking up exported functions (Mozilla Advisory).

The vulnerability occurs when multiple WASM threads have a reference to a module and are looking up exported functions. One WASM thread could overwrite another's entry in a shared stub table. The issue involves a table of lazy stubs for exported functions where a call to Instance::getExportedFunction would initialize the lazy stub entry to a sentinel, and a later call to callExport would create the actual stub and replace the entry. While the table of created functions is per-instance, the lazy stubs table for a module is shared among all instances that share the module (Mozilla Bug).

The vulnerability could result in a potentially exploitable crash when multiple WASM threads overwrote each other's stub table entries. Mozilla rated this vulnerability as having 'moderate' impact (Mozilla Advisory).

The vulnerability was particularly likely to manifest in heavily threaded code, though it could theoretically occur in non-threaded code as well. While it was confirmed as a serious functionality bug, its direct exploitability was considered uncertain (Mozilla Bug).

The vulnerability was fixed in Firefox 82, released in October 2020. The fix involved ensuring that non-null entries in the jump table are not overwritten by using a compare-and-swap atomic operation. The fix was also backported to Firefox 82 Beta (Mozilla Bug).