CVE-2020-36451 in rcu_cell Crate
Summary
by MITRE • 08/08/2021
An issue was discovered in the rcu_cell crate through 2020-11-14 for Rust. There are unconditional implementations of Send and Sync for RcuCell.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 08/14/2021
The vulnerability identified as CVE-2020-36451 resides within the rcu_cell crate version 2020-11-14 and earlier, representing a critical flaw in the Rust programming ecosystem that undermines memory safety guarantees. This issue specifically affects the implementation of concurrent programming primitives within the Rust language, where the crate provides a thread-safe cell implementation using Read-Copy-Update (RCU) mechanisms. The problem manifests through improper trait implementations that violate fundamental concurrency safety principles in Rust's type system.
The technical flaw involves unconditional implementations of Send and Sync traits for the RcuCell type, which are critical markers in Rust indicating whether a type can be safely transferred between threads or shared across multiple threads without data races. This improper implementation directly contravenes the core safety guarantees that Rust provides, as it allows potentially unsafe operations to proceed without proper compile-time checks. The Send trait indicates that a type can be safely transferred from one thread to another, while Sync indicates that a type can be safely shared between threads. When these traits are implemented unconditionally for RcuCell, it creates a scenario where the compiler cannot verify that the type adheres to the safety requirements necessary for concurrent access.
The operational impact of this vulnerability extends beyond simple memory safety issues, as it can lead to data races and undefined behavior when multiple threads attempt to access or modify the shared RcuCell instances. This vulnerability particularly affects applications that rely on the rcu_cell crate for concurrent data sharing, potentially exposing systems to memory corruption, application crashes, or more severe security implications when the compromised crate is used in production environments. The flaw essentially bypasses Rust's ownership system and concurrency model, allowing code that should be rejected at compile time to pass through successfully and execute with potentially dangerous behavior.
The vulnerability aligns with CWE-472 Unprotected Race Conditions and CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization, both of which relate to improper handling of shared resources in concurrent programming environments. From an ATT&CK perspective, this vulnerability could be leveraged in initial access or privilege escalation scenarios where malicious actors exploit the unsafe concurrency patterns to gain unauthorized access to system resources or manipulate shared data structures. The issue demonstrates how seemingly minor trait implementations can have catastrophic effects on system security and reliability, particularly in systems where Rust's memory safety guarantees are critical for security.
Mitigation strategies for CVE-2020-36451 require immediate action to upgrade to patched versions of the rcu_cell crate, as the vulnerability cannot be resolved through code changes alone due to its fundamental nature in trait implementations. Developers should conduct thorough code reviews to identify all usages of the affected crate and replace them with safer alternatives or updated versions. Additionally, organizations should implement dependency scanning tools that can detect vulnerable crate versions and establish policies for maintaining up-to-date security patches. The fix typically involves proper conditional implementation of Send and Sync traits based on the actual safety characteristics of the RcuCell type, ensuring that only types that truly satisfy the safety requirements can be marked as Send or Sync.