CVE-2021-39218 (published 2021-09-17T21:15Z)

Wasmtime is an open source runtime for WebAssembly & WASI. In Wasmtime from version 0.26.0 and before version 0.30.0 is affected by a memory unsoundness vulnerability. There was an invalid free and out-of-bounds read and write bug when running Wasm that uses `externref`s in Wasmtime. To trigger this bug, Wasmtime needs to be running Wasm that uses `externref`s, the host creates non-null `externrefs`, Wasmtime performs a garbage collection (GC), and there has to be a Wasm frame on the stack that is at a GC safepoint where there are no live references at this safepoint, and there is a safepoint with live references earlier in this frame's function. Under this scenario, Wasmtime would incorrectly use the GC stack map for the safepoint from earlier in the function instead of the empty safepoint. This would result in Wasmtime treating arbitrary stack slots as `externref`s that needed to be rooted for GC. At the *next* GC, it would be determined that nothing was referencing these bogus `externref`s (because nothing could ever reference them, because they are not really `externref`s) and then Wasmtime would deallocate them and run `<ExternRef as Drop>::drop` on them. This results in a free of memory that is not necessarily on the heap (and shouldn't be freed at this moment even if it was), as well as potential out-of-bounds reads and writes. Even though support for `externref`s (via the reference types proposal) is enabled by default, unless you are creating non-null `externref`s in your host code or explicitly triggering GCs, you cannot be affected by this bug. We have reason to believe that the effective impact of this bug is relatively small because usage of `externref` is currently quite rare. This bug has been patched and users should upgrade to Wasmtime version 0.30.0. If you cannot upgrade Wasmtime at this time, you can avoid this bug by disabling the reference types proposal by passing `false` to `wasmtime::Config::wasm_reference_types`.

Severity: LOW
CVSS v2 base score: 3.3 (AV:L/AC:M/Au:N/C:N/I:P/A:P)
CVSS v3 base score: 6.3, MEDIUM (CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:H/A:H)

Weakness information:
cwe_id: CWE-125
cwe_name: Out-of-bounds Read
description: The product reads data past the end, or before the beginning, of the intended buffer.
extended_description: 
            
            
             
			 
         
         
cwe_id: CWE-787
cwe_name: Out-of-bounds Write
description: The product writes data past the end, or before the beginning, of the intended buffer.
extended_description: 
            
            
             
			 
         
         
cwe_id: CWE-590
cwe_name: Free of Memory not on the Heap
description: The product calls free() on a pointer to memory that was not allocated using associated heap allocation functions such as malloc(), calloc(), or realloc().
extended_description: When free() is called on an invalid pointer, the program's memory management data structures may become corrupted. This corruption can cause the program to crash or, in some circumstances, an attacker may be able to cause free() to operate on controllable memory locations to modify critical program variables or execute code.
         

Fix information:
repo_url: https://github.com/bytecodealliance/wasmtime
commit_details: repo_url: https://github.com/bytecodealliance/wasmtime
author: Nick Fitzgerald
author_date: 2021-09-17 10:28:50-07:00
msg: Merge pull request from GHSA-4873-36h9-wv49

Stop doing fuzzy search for stack maps
repository: repo_url: https://github.com/bytecodealliance/wasmtime
repo_name: bytecodealliance/wasmtime
description: A fast and secure runtime for WebAssembly
repo_language: Rust
owner: bytecodealliance