Wasmtime is a standalone runtime for WebAssembly. There is a bug in the Wasmtimes code generator, Cranelift, where functions using reference types may be incorrectly missing metadata required for runtime garbage collection. This means that if a GC happens at runtime then the GC pass will mistakenly think these functions do not have live references to GCd values, reclaiming them and deallocating them. The function will then subsequently continue to use the values assuming they had not been GCd, leading later to a use-after-free. This bug was introduced in the migration to the regalloc2 register allocator that occurred in the Wasmtime 0.37.0 release on 2022-05-20. This bug has been patched and users should upgrade to Wasmtime version 0.38.2. Mitigations for this issue can be achieved by disabling the reference types proposal by passing false to wasmtime::Config::wasm_reference_types or downgrading to Wasmtime 0.36.0 or prior.
Referencing memory after it has been freed can cause a program to crash, use unexpected values, or execute code.
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Cranelift-codegen | Bytecodealliance | 0.84.0 (including) | 0.85.2 (excluding) |
| Wasmtime | Bytecodealliance | 0.37.0 (including) | 0.38.2 (excluding) |
| Firefox | Ubuntu | bionic | * |
| Firefox | Ubuntu | focal | * |
| Firefox | Ubuntu | trusty | * |
| Firefox | Ubuntu | xenial | * |
| Mozjs38 | Ubuntu | bionic | * |
| Mozjs38 | Ubuntu | esm-apps/bionic | * |
| Mozjs38 | Ubuntu | upstream | * |
| Mozjs52 | Ubuntu | bionic | * |
| Mozjs52 | Ubuntu | esm-apps/focal | * |
| Mozjs52 | Ubuntu | esm-infra/bionic | * |
| Mozjs52 | Ubuntu | focal | * |
| Mozjs52 | Ubuntu | upstream | * |
| Mozjs68 | Ubuntu | esm-infra/focal | * |
| Mozjs68 | Ubuntu | focal | * |
| Mozjs68 | Ubuntu | upstream | * |
| Mozjs78 | Ubuntu | esm-apps/jammy | * |
| Mozjs78 | Ubuntu | jammy | * |
| Mozjs78 | Ubuntu | kinetic | * |
| Mozjs78 | Ubuntu | lunar | * |
| Mozjs78 | Ubuntu | upstream | * |
| Mozjs91 | Ubuntu | jammy | * |
| Mozjs91 | Ubuntu | upstream | * |
| Thunderbird | Ubuntu | bionic | * |
| Thunderbird | Ubuntu | devel | * |
| Thunderbird | Ubuntu | focal | * |
| Thunderbird | Ubuntu | jammy | * |
| Thunderbird | Ubuntu | kinetic | * |
| Thunderbird | Ubuntu | lunar | * |
| Thunderbird | Ubuntu | mantic | * |
| Thunderbird | Ubuntu | noble | * |
| Thunderbird | Ubuntu | trusty | * |
| Thunderbird | Ubuntu | xenial | * |
The use of previously-freed memory can have any number of adverse consequences, ranging from the corruption of valid data to the execution of arbitrary code, depending on the instantiation and timing of the flaw. The simplest way data corruption may occur involves the system’s reuse of the freed memory. Use-after-free errors have two common and sometimes overlapping causes:
In this scenario, the memory in question is allocated to another pointer validly at some point after it has been freed. The original pointer to the freed memory is used again and points to somewhere within the new allocation. As the data is changed, it corrupts the validly used memory; this induces undefined behavior in the process. If the newly allocated data happens to hold a class, in C++ for example, various function pointers may be scattered within the heap data. If one of these function pointers is overwritten with an address to valid shellcode, execution of arbitrary code can be achieved.