CVE-2025-24898: rust-openssl Use-After-Free Vulnerability

CVE-2025-24898 Overview

A Use After Free vulnerability has been identified in rust-openssl, a set of OpenSSL bindings for the Rust programming language. The vulnerability exists in the ssl::select_next_proto function, which can return a slice pointing into the server argument's buffer but with a lifetime bound to the client argument. In situations where the server buffer's lifetime is shorter than the client buffer's, this can cause a use after free condition.

Critical Impact

This vulnerability could cause affected servers to crash or return arbitrary memory contents to clients, potentially exposing sensitive data or enabling denial of service attacks.

Affected Products

• rust-openssl crate versions prior to 0.10.70
• Applications using ssl::select_next_proto in ALPN callbacks
• Systems with Rust applications implementing custom ALPN protocol selection

Discovery Timeline

• 2025-02-03 - CVE CVE-2025-24898 published to NVD
• 2025-02-11 - Last updated in NVD database

Technical Details for CVE-2025-24898

Vulnerability Analysis

The vulnerability is a classic Use After Free (CWE-416) memory safety issue that occurs due to improper lifetime management in Rust's OpenSSL bindings. The ssl::select_next_proto function is designed to assist with Application-Layer Protocol Negotiation (ALPN) during TLS handshakes. When called, it should return a reference to data within one of its input buffers.

The core issue lies in how the function's return value lifetime is constrained. The returned slice points to memory in the server argument's buffer, but the Rust compiler's borrow checker only enforces that the returned reference lives as long as the client argument. This creates a window where the server buffer can be deallocated while references to its contents still exist.

This vulnerability requires network access to exploit but involves complex attack prerequisites. The vulnerable pattern is most likely to occur when the server buffer is constructed within the ALPN selection callback itself, rather than being passed in from an outer scope with a longer lifetime.

Root Cause

The root cause is an incorrect lifetime annotation in the ssl::select_next_proto function signature. In Rust, lifetime parameters are used to ensure that references remain valid for as long as they are used. The original function signature failed to properly constrain the output buffer's lifetime to both input buffers, allowing the compiler to accept code that could access freed memory.

Specifically, the function's signature allowed the returned slice lifetime to be tied only to the client parameter, when it should have been tied to both server and client parameters since the returned data could originate from either buffer.

Attack Vector

The attack vector for this vulnerability involves triggering the use after free condition during TLS ALPN negotiation. An attacker would need to:

1. Establish a TLS connection with a vulnerable server implementing custom ALPN selection
2. Send crafted ALPN protocol lists that trigger the vulnerable code path
3. Exploit the race between buffer deallocation and access to cause either a crash (denial of service) or memory disclosure

The vulnerability is exploited over the network during the TLS handshake process. The specific impact depends on timing and memory layout - successful exploitation could reveal arbitrary memory contents from the server process or cause the server to crash.

For technical details on the vulnerability and its fix, refer to the GitHub Security Advisory GHSA-rpmj-rpgj-qmpm and the associated pull request #2360.

Detection Methods for CVE-2025-24898

Indicators of Compromise

• Unexpected server crashes during TLS handshake operations
• Anomalous memory access patterns in application logs
• Unusual ALPN negotiation failures or errors in TLS connection logs
• Memory corruption indicators in crash dumps from Rust applications using OpenSSL bindings

Detection Strategies

• Review application dependencies for rust-openssl versions prior to 0.10.70
• Audit code using ssl::select_next_proto within ALPN selection callbacks for local buffer construction patterns
• Monitor for abnormal process terminations in services using rust-openssl for TLS
• Implement cargo-audit or similar dependency scanning tools to identify vulnerable crate versions

Monitoring Recommendations

• Enable detailed TLS handshake logging to detect anomalous ALPN negotiation patterns
• Configure memory sanitizers in development and staging environments to catch use after free conditions
• Set up alerting for unexpected service restarts or crashes in production systems using rust-openssl
• Monitor network traffic for unusual TLS handshake patterns that could indicate exploitation attempts

How to Mitigate CVE-2025-24898

Immediate Actions Required

• Upgrade the openssl crate to version 0.10.70 or later immediately
• Review all usages of ssl::select_next_proto in ALPN selection callbacks
• If the server buffer is constructed within the callback, refactor to use a buffer with appropriate lifetime
• Run cargo update to pull the latest patched version and rebuild affected applications

Patch Information

The vulnerability has been fixed in rust-openssl crate version 0.10.70. The fix properly constrains the output buffer's lifetime to that of both input buffers in the ssl::select_next_proto function signature. Users should update their Cargo.toml to require at least version 0.10.70 of the openssl crate.

The patch is available via Crates.io and detailed in GitHub Pull Request #2360. Debian users should also reference the Debian LTS Announcement for distribution-specific updates.

Workarounds

• Ensure the server buffer passed to ssl::select_next_proto has a lifetime that extends beyond the callback scope
• Move server buffer construction outside of the ALPN selection callback
• If immediate upgrade is not possible, consider implementing ALPN selection using alternative methods
• Apply network-level controls to limit TLS connection sources to trusted endpoints while awaiting patch deployment

# Configuration example
# Update Cargo.toml to require patched version
# In Cargo.toml:
# [dependencies]
# openssl = ">=0.10.70"

# Update dependencies and rebuild
cargo update -p openssl
cargo build --release

# Verify the installed version
cargo tree -p openssl