CVE-2020-36448: Rust Cache Crate Send/Sync Vulnerability

CVE-2020-36448 Overview

CVE-2020-36448 is a race condition vulnerability discovered in the cache crate for Rust. The vulnerability stems from unconditional implementations of the Send and Sync traits for the Cache<K> type, regardless of whether the underlying type K is safe to share across threads. This allows non-thread-safe types to be sent between threads, potentially leading to data races, memory corruption, and undefined behavior.

Critical Impact
Applications using the affected cache crate may experience data races that could lead to memory corruption, information disclosure, or arbitrary code execution when Cache<K> is used with non-thread-safe types in concurrent contexts.

Affected Products

cache crate for Rust (through 2020-11-24)

Discovery Timeline

2021-08-08 - CVE CVE-2020-36448 published to NVD
2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-36448

Vulnerability Analysis

The cache crate implements Send and Sync traits for Cache<K> without proper bounds checking on the generic type K. In Rust's concurrency model, Send indicates that a type is safe to transfer ownership to another thread, while Sync indicates that a type is safe to share references between threads. These traits should only be implemented when the underlying data types also implement these traits.

By unconditionally implementing these traits, the cache crate allows types that are not thread-safe (i.e., do not implement Send or Sync) to be used across thread boundaries. This breaks Rust's memory safety guarantees and can lead to data races when multiple threads access the cache concurrently.

The vulnerability requires specific conditions to exploit: an attacker would need to craft a scenario where a Cache<K> instance containing a non-thread-safe type is shared between threads, and those threads perform concurrent operations that trigger a data race.

Root Cause

The root cause is the unsound implementation of Send and Sync traits without proper trait bounds. In safe Rust code, these auto-traits should only be implemented when the contained types also implement these traits. The cache crate bypasses this safety mechanism by using unconditional implementations, effectively telling the Rust compiler that Cache<K> is always thread-safe, even when K is not.

This pattern violates Rust's safety contracts and is classified as "unsound" in the Rust security ecosystem. The correct implementation would require trait bounds such as impl<K: Send> Send for Cache<K> and impl<K: Sync> Sync for Cache<K>.

Attack Vector

Exploitation requires network access and involves high complexity. An attacker would need to:

Identify an application using the vulnerable cache crate with a non-thread-safe type parameter
Trigger concurrent access patterns that exploit the data race condition
Leverage the resulting undefined behavior for potential memory corruption, information disclosure, or code execution

The attack is opportunistic and depends heavily on how the target application uses the cache, making exploitation challenging but potentially severe when conditions align.

Detection Methods for CVE-2020-36448

Indicators of Compromise

Unexpected application crashes or segmentation faults in multi-threaded Rust applications using the cache crate
Memory corruption indicators such as corrupted data structures or unexpected values in cached data
Race condition symptoms including intermittent failures that only occur under concurrent load

Detection Strategies

Audit Cargo.toml and Cargo.lock files for dependencies on the cache crate (versions through 2020-11-24)
Use cargo audit to scan for known vulnerabilities in Rust dependencies including RUSTSEC-2020-0128
Review code for usage patterns where Cache<K> is shared across threads with non-thread-safe types
Implement thread sanitizer testing (TSAN) to detect data races during development and testing

Monitoring Recommendations

Monitor application logs for crash dumps and memory-related errors in production systems
Implement runtime monitoring for unusual thread behavior or synchronization issues
Set up dependency scanning in CI/CD pipelines to catch vulnerable crate versions before deployment

How to Mitigate CVE-2020-36448

Immediate Actions Required

Audit all Rust projects for usage of the cache crate and determine if concurrent access patterns exist
Review whether Cache<K> is used with types that do not implement Send and Sync
Consider migrating to alternative caching solutions that properly implement thread safety traits
If migration is not immediately possible, ensure the cache is only accessed from a single thread

Patch Information

The cache crate has been flagged as unmaintained with no fix available. Organizations should consult the RustSec Advisory RUSTSEC-2020-0128 for the latest guidance and consider migrating to maintained alternatives.

Workarounds

Wrap all cache access in a single-threaded executor or use explicit synchronization primitives (Mutex, RwLock) around cache operations
Replace the cache crate with a maintained alternative that properly implements Send and Sync bounds
If the cache must be used, ensure all type parameters implement Send and Sync traits to satisfy the implied contract
Implement architectural changes to isolate cache usage to a single thread, eliminating concurrent access scenarios