CVE-2025-65503 Overview
CVE-2025-65503 is a use-after-free vulnerability [CWE-416] affecting Redboltz async_mqtt version 10.2.5. The flaw resides in endpoint destructors and is triggered when SSL initialization fails. The failure causes an incorrect destruction order between the io_context and endpoint objects, leading to memory access on freed resources. A local, authenticated user can exploit this condition to cause a denial of service in applications built on the affected library.
Critical Impact
Local users with low privileges can crash applications embedding async_mqtt 10.2.5 by forcing SSL initialization to fail, terminating MQTT-dependent services.
Affected Products
- Redboltz async_mqtt version 10.2.5
- Applications and services embedding the affected async_mqtt C++ MQTT library
- Downstream MQTT clients and brokers built against the vulnerable release
Discovery Timeline
- 2025-11-24 - CVE-2025-65503 published to NVD
- 2026-06-17 - Last updated in NVD database
Technical Details for CVE-2025-65503
Vulnerability Analysis
The vulnerability exists in the endpoint destructor logic of async_mqtt, a C++17 asynchronous MQTT library built on Boost.Asio. Endpoint objects in the library hold references to a shared io_context responsible for scheduling asynchronous operations. When an SSL/TLS session is established, initialization can fail before the endpoint is fully constructed or registered with the io_context.
When this failure path executes, object destruction proceeds in an order that violates the dependency between io_context and the endpoint. The endpoint destructor references memory associated with io_context state that has already been released. This results in a use-after-free condition during teardown of the failed SSL session.
The issue is tracked in GitHub Issue #436 and remediated in GitHub Pull Request #437.
Root Cause
The root cause is incorrect object lifetime management between the io_context and the endpoint objects it services. The destructors do not enforce an ordering guarantee, so a failed SSL handshake path can release the underlying context resources before the dependent endpoint object completes destruction. Accessing the released memory from the endpoint destructor produces the use-after-free.
Attack Vector
Exploitation requires local access with low privileges but no user interaction. An attacker triggers repeated SSL initialization failures against an application that uses async_mqtt for MQTT-over-TLS connections. Malformed certificates, unreachable trust stores, or misconfigured cipher parameters can force the failure path. Once the destructor executes on the corrupted state, the process crashes, producing a denial of service. The confidentiality and integrity impact is none; only availability is affected.
No public proof-of-concept exploit is available, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.
Detection Methods for CVE-2025-65503
Indicators of Compromise
- Unexpected process crashes in applications linked against async_mqtt 10.2.5, particularly during MQTT-over-TLS session establishment
- Core dumps or crash reports referencing endpoint destructor frames and io_context teardown
- Repeated SSL/TLS handshake failure log entries immediately preceding application termination
Detection Strategies
- Inventory build artifacts and container images for the async_mqtt library and flag any use of version 10.2.5
- Enable AddressSanitizer or Valgrind in test environments to surface the use-after-free during SSL failure scenarios
- Correlate MQTT broker or client crash events with concurrent TLS handshake failure records from the same host
Monitoring Recommendations
- Monitor systemd, container orchestrator, or supervisor restart counters for MQTT services to detect abnormal termination patterns
- Collect and centralize process crash telemetry with stack traces from hosts running MQTT workloads
- Alert on sustained TLS handshake failures targeting MQTT listeners, which may indicate exploitation attempts
How to Mitigate CVE-2025-65503
Immediate Actions Required
- Identify all applications and services that link against async_mqtt 10.2.5 and prioritize them for update
- Restrict local access to hosts running vulnerable MQTT clients or brokers using standard operating system access controls
- Validate TLS configuration to reduce the frequency of legitimate SSL initialization failures that could be leveraged for exploitation
Patch Information
The fix is delivered through GitHub Pull Request #437, which corrects the destruction order between io_context and endpoint objects. Rebuild affected applications against a patched release of async_mqtt that incorporates the fix from the upstream repository. Redistribute the resulting binaries or container images to all production hosts.
Workarounds
- Pre-validate TLS certificates, trust chains, and cipher parameters before instantiating async_mqtt endpoints to avoid the SSL failure path
- Wrap endpoint construction in exception handling that terminates the process cleanly before the vulnerable destructor executes
- Restrict which local users can influence MQTT service configuration or certificate stores until the patched build is deployed
# Configuration example: identify vulnerable async_mqtt builds in a Linux environment
find / -type f \( -name "libasync_mqtt*" -o -name "async_mqtt*.so*" \) 2>/dev/null
ldd /path/to/your/mqtt/binary | grep -i async_mqtt
# Rebuild against a patched async_mqtt release that includes PR #437
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
