CVE-2026-56297 in FreeRDP
Summary
by MITRE • 07/08/2026
FreeRDP before 3.22.0 contains a use-after-free vulnerability in dvcman_channel_close and dvcman_call_on_receive due to improper synchronization of channel_callback access. A malicious RDP server can trigger a race condition by sending DYNVC_DATA and DYNVC_CLOSE messages concurrently, causing heap-use-after-free in the drdynvc client thread and potentially enabling remote code execution or denial of service.
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Analysis
by VulDB Data Team • 07/08/2026
The vulnerability under discussion represents a critical use-after-free condition affecting FreeRDP versions prior to 3.22.0, specifically within the dynamic virtual channel management subsystem. This flaw exists in the dvcman_channel_close and dvcman_call_on_receive functions where improper synchronization mechanisms fail to protect access to channel_callback structures. The issue manifests as a race condition that can be exploited by malicious RDP servers through carefully crafted concurrent messaging patterns involving DYNVC_DATA and DYNVC_CLOSE operations, creating a scenario where memory previously freed by one thread becomes accessible to another thread before proper deallocation occurs.
The technical exploitation of this vulnerability leverages the inherent timing issues within the dynamic channel management code path. When a malicious RDP server simultaneously sends DYNVC_DATA and DYNVC_CLOSE messages, it creates a window where the dvcman_channel_close function may attempt to free memory structures while dvcman_call_on_receive is still accessing them through channel_callback references. This synchronization failure results in heap-use-after-free conditions that can be reliably triggered by an attacker controlling the RDP server endpoint. The vulnerability maps directly to CWE-416, which specifically addresses use-after-free errors in memory management, and demonstrates poor thread synchronization practices that violate fundamental security principles for concurrent programming environments.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to potentially enable remote code execution capabilities. When heap corruption occurs due to the use-after-free condition, attackers can manipulate memory contents to redirect program execution flow or inject malicious payloads into the drdynvc client thread context. The vulnerability affects the core RDP client functionality where dynamic virtual channels are managed, making it particularly dangerous in enterprise environments where RDP connections are frequently used for administrative access and remote desktop sessions. Attackers with control over the RDP server can leverage this flaw to compromise systems running affected FreeRDP versions without requiring additional authentication mechanisms or local privileges.
Mitigation strategies for this vulnerability should focus on implementing proper synchronization primitives throughout the dynamic channel management subsystem. The most effective approach involves adding mutex locks or other thread-safe mechanisms around critical sections that access channel_callback structures in both dvcman_channel_close and dvcman_call_on_receive functions. Additionally, developers should implement proper reference counting mechanisms to ensure memory structures remain valid for the duration of their intended use. The recommended solution aligns with ATT&CK technique T1059.007 for command and scripting interpreter usage patterns, as attackers may attempt to exploit this vulnerability through indirect code execution pathways. Organizations should prioritize upgrading to FreeRDP version 3.22.0 or later where these synchronization issues have been resolved, while also implementing network segmentation and monitoring to detect anomalous RDP traffic patterns that might indicate exploitation attempts.
The vulnerability demonstrates how seemingly simple concurrency issues can lead to severe security implications in widely-used networking libraries. The race condition exploits fundamental flaws in memory management synchronization that are common in complex multi-threaded applications. This particular flaw highlights the importance of thorough testing for concurrent access patterns and proper implementation of memory safety mechanisms in client-server protocol implementations. Security practitioners should consider this vulnerability as part of broader RDP security assessments, particularly focusing on dynamic channel behavior and potential attack vectors that leverage timing-based exploits. The remediation process requires careful code review of all synchronization points and implementation of comprehensive testing for race condition scenarios to prevent similar vulnerabilities from emerging in future development cycles.