CVE-2022-31104

Workarounds

If upgrading is not an option for you at this time, you can avoid the vulnerability by disabling the Wasm simd proposal

config.wasm_simd(false);

Additionally the bug is only present on x86_64 hosts. Other aarch64 hosts are not affected. Note that s390x hosts don't yet implement the simd proposal and are not affected.

References

• The WebAssembly simd proposal
• Original test case showing the erroneous behavior
• Fix for the swizzle instruction
• Fix for the select instruction

For more information

If you have any questions or comments about this advisory:

• Reach out to us on the Bytecode Alliance Zulip chat
• Open an issue in the bytecodealliance/wasmtime repository

Wasmtime's implementation of the SIMD proposal for WebAssembly on x86_64 contained two distinct bugs in the instruction lowerings implemented in Cranelift. The aarch64 implementation of the simd proposal is not affected. The bugs were presented in the i8x16.swizzle and select WebAssembly instructions. The select instruction is only affected when the inputs are of v128 type. The correspondingly affected Cranelift instructions were swizzle and select.

The swizzle instruction lowering in Cranelift erroneously overwrote the mask input register which could corrupt a constant value, for example. This means that future uses of the same constant may see a different value than the constant itself.

The select instruction lowering in Cranelift wasn't correctly implemented for vector types that are 128-bits wide. When the condition was 0 the wrong instruction was used to move the correct input to the output of the instruction meaning that only the low 32 bits were moved and the upper 96 bits of the result were left as whatever the register previously contained (instead of the input being moved from). The select instruction worked correctly if the condition was nonzero, however.

This bug in Wasmtime's implementation of these instructions on x86_64 represents an incorrect implementation of the specified semantics of these instructions according to the WebAssembly specification. The impact of this is benign for hosts running WebAssembly but represents possible vulnerabilities within the execution of a guest program. For example a WebAssembly program could take unintended branches or materialize incorrect values internally which runs the risk of exposing the program itself to other related vulnerabilities which can occur from miscompilations.

CVE-2022-31104 has a CVSS score of 4.8 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.38.1, 0.85.1); upgrading removes the vulnerable code path.

Kodem intelligence

We have released Wasmtime 0.38.1 and cranelift-codegen (and other associated cranelift crates) 0.85.1 which contain the corrected implementations of these two instructions in Cranelift.

1. What is CVE-2022-31104? CVE-2022-31104 is a medium-severity security vulnerability in wasmtime (rust), affecting versions < 0.38.1. It is fixed in 0.38.1, 0.85.1.
2. How severe is CVE-2022-31104? CVE-2022-31104 has a CVSS score of 4.8 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2022-31104?
   • wasmtime (rust) (versions < 0.38.1)
   • cranelift-codegen (rust) (versions < 0.85.1)
4. Is there a fix for CVE-2022-31104? Yes. CVE-2022-31104 is fixed in 0.38.1, 0.85.1. Upgrade to this version or later.
5. Is CVE-2022-31104 exploitable, and should I be worried? Whether CVE-2022-31104 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2022-31104 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2022-31104?
   • Upgrade wasmtime to 0.38.1 or later
   • Upgrade cranelift-codegen to 0.85.1 or later