CVE-2021-32629 : Exploit Details and Defense Strategies

Cranelift, an open-source code generator maintained by Bytecode Alliance, has a vulnerability in version 0.73 affecting Wasmtime. This bug, found in the Cranelift x64 backend, could lead to a potential sandbox escape in a Wasm program. The impact is significant, allowing access to memory addresses before the start of the Wasm program heap, potentially leading to memory leaks. The bug arises from a sign-extend instead of a zero-extend on a value loaded from the stack under specific circumstances.

Understanding CVE-2021-32629

This section delves into what CVE-2021-32629 is, its impact, technical details, and mitigation strategies.

What is CVE-2021-32629?

The vulnerability in Cranelift 0.73 enables sandbox escape by allowing access to memory addresses before the program heap starts, posing a risk of memory leaks in Wasm programs.

The Impact of CVE-2021-32629

The bug, triggered by specific circumstances, can potentially expose memory before the heap's start, affecting Wasm program security. The exploit relies on a code generation vulnerability, impacting heap implementations and sandbox protections.

Technical Details of CVE-2021-32629

This section focuses on the technical details of the vulnerability, affected systems, exploitation mechanisms, and how to mitigate and prevent it.

Vulnerability Description

The bug arises from a code generation flaw in the Cranelift x64 backend, leading to sign-extend instead of zero-extend on stack values under specific conditions, potentially allowing memory access before the Wasm program heap's start.

Affected Systems and Versions

Versions up to 0.73.0 of Cranelift used by Wasmtime are impacted, especially when the new backend is enabled. Lucet and Wasmtime may also be exploitable under specific circumstances.

Exploitation Mechanism

The vulnerability allows the exploitation of memory addresses before the Wasm program heap starts by mishandling stack values, particularly in scenarios involving integer values wider than 32 bits.

Mitigation and Prevention

Here, we outline the steps to address the CVE-2021-32629 vulnerability and enhance system security.

Immediate Steps to Take

Users are advised to update Cranelift to versions beyond 0.73.0, disable the new backend if in use, and apply relevant patches promptly to prevent sandbox escape on Wasm programs.

Long-Term Security Practices

Implement proactive security measures, including regular software updates, security assessments, and code review to mitigate code generation flaws and prevent memory access vulnerabilities.

Patching and Updates

Stay informed about security advisories, apply patches promptly, and ensure system components are regularly updated to address vulnerabilities like the code generation flaw in Cranelift.