CVE-2022-39394 - Wasmtime C API Vulnerability: Potential Buffer Overflow in `wasmtime_trap_code` Function

Wasmtime is a popular standalone WebAssembly runtime, enabling developers to build and run WebAssembly applications efficiently. Recently, a vulnerability (CVE-2022-39394) has been discovered that affects Wasmtime's C API and could potentially lead to a buffer overflow. This blog post will provide the details of the vulnerability and recommended mitigation strategies to ensure the security of your Wasmtime applications.

Details on the Issue

The vulnerability lies in the implementation of the wasmtime_trap_code function in Wasmtime's C API. The definition of this function in the wasmtime/trap.h header file does not match its declared signature, causing a 4-byte write into a 1-byte buffer provided by the caller.

Here's a code snippet highlighting the discrepancy

// In the header file (wasmtime/trap.h):
uint8_t wasmtime_trap_code(const wasmtime_trap_t *trap);

// In the implementation:
uint32_t wasmtime_trap_code(const wasmtime_trap_t *trap) {
    ...
}

This mismatch can cause three zero bytes to be written beyond the 1-byte location supplied by the caller, potentially leading to a buffer overflow. This could result in undefined behavior, such as crashes or data corruption, depending on the layout of the affected program's memory.

Original references

* Wasmtime GitHub - Pull Request that fixed the issue: https://github.com/bytecodealliance/wasmtime/pull/3695
* Wasmtime Advisory: https://github.com/bytecodealliance/wasmtime/security/advisories/GHSA-3ccw-2qh3-77rc

Exploit details

While there is no known exploit for this specific vulnerability, a potential attacker could theoretically craft a malicious input that would trigger the buffer overflow in affected Wasmtime applications using the C API. By carefully controlling the layout of the program's memory, an attacker might be able to execute arbitrary code, taking advantage of this vulnerability.

There are several ways to mitigate the risk caused by this vulnerability

1. Upgrade to Wasmtime 2..2: The Wasmtime team has released a patch for this bug, and users should upgrade their Wasmtime installations to version 2..2 or later.

2. Workaround with a 4-byte buffer: If upgrading is not an option, you can work around this bug by providing a 4-byte buffer casted to a 1-byte buffer when calling wasmtime_trap_code. This will prevent the overflow.

uint32_t trap_code_buffer;
uint8_t trap_code = (uint8_t)wasmtime_trap_code((uint8_t *)&trap_code_buffer);

3. Internal users of the wasmtime crate are not affected: The vulnerability only affects users of the C API function wasmtime_trap_code. If you are using other Wasmtime APIs or the internal Rust crate, you are not at risk.

In conclusion, the CVE-2022-39394 vulnerability represents a critical security issue for Wasmtime applications utilizing the C API. However, by taking the proper mitigation steps – upgrading to Wasmtime 2..2 and/or implementing the suggested workaround – you can substantially reduce the risk posed by this vulnerability. Stay informed and be proactive to maintain a secure development environment.

Timeline

Published on: 11/10/2022 20:15:00 UTC
Last modified on: 11/16/2022 13:31:00 UTC