Freerdp CVE-2026-56297

Summary

Remote code execution and denial of service in FreeRDP before 3.22.0 stem from a use-after-free in the dynamic virtual channel manager (drdynvc), where dvcman_channel_close and dvcman_call_on_receive access channel_callback without proper synchronization (CWE-362). A malicious or compromised RDP server can race DYNVC_DATA and DYNVC_CLOSE messages to free a channel callback while another thread still uses it, corrupting the client heap. This is a client-side flaw affecting anyone connecting to an attacker-controlled server; no public exploit identified at time of analysis, and it is not listed in CISA KEV.

Technical Context

FreeRDP is a widely used open-source implementation of the Microsoft Remote Desktop Protocol, embedded in clients such as Remmina, GNOME Connections, Weston, and numerous commercial and VDI products. The bug lives in the drdynvc (Dynamic Virtual Channel) subsystem, which multiplexes optional channels (clipboard, audio, device redirection, etc.) over the RDP connection and processes them on a dedicated client thread. Per CWE-362 (concurrent execution using shared resource with improper synchronization / race condition), dvcman_channel_close (handling DYNVC_CLOSE) can free the channel_callback structure while dvcman_call_on_receive (handling DYNVC_DATA) is concurrently dereferencing it, producing a heap use-after-free. Because the RDP server dictates the timing and ordering of DYNVC_DATA and DYNVC_CLOSE PDUs, a hostile server controls the race window.

Risk Assessment

The published CVSS 4.0 vector (AV:N/AC:H/AT:N/PR:N/UI:N/VC:L/VI:L/VA:H) yields 8.3 (High), driven mainly by high availability impact with lower confidentiality/integrity impact — consistent with a heap corruption that reliably crashes the client (DoS) but only potentially yields RCE. AC:H correctly reflects that success depends on winning a non-deterministic thread race, which is unreliable and may require many attempts. There is no EPSS score, no KEV listing, and no confirmed public POC in the provided data, so exploitation pressure is currently low-to-unknown; the RCE and Denial Of Service tags reflect potential impact, not observed attacks. One nuance to verify: the vector uses UI:N/PR:N, but this is a client-side vulnerability requiring the victim to connect to a malicious server — a real-world limiting factor that raw CVSS understates. Net: a legitimate patch priority for fleets whose users connect to untrusted or reachable RDP endpoints, but not an emergency mass-exploitation scenario given AC:H and no evidence of exploitation.

Affected Products

FreeRDP (the open-source RDP client/library and its FreeRDP-based clients) at all versions prior to 3.22.0 are affected; the flaw is fixed in FreeRDP 3.22.0. No CPE strings were provided in the input, so exact downstream package ranges (e.g., distribution builds of Remmina or the freerdp2/freerdp3 packages) should be confirmed against vendor trackers. Authoritative details are in the FreeRDP GitHub Security Advisory GHSA-3mv2-5q57-2v8h (https://github.com/FreeRDP/FreeRDP/security/advisories/GHSA-3mv2-5q57-2v8h) and the VulnCheck advisory (https://www.vulncheck.com/advisories/freerdp-use-after-free-via-race-condition-in-drdynvc-channel-callback).

Remediation

Vendor-released patch: FreeRDP 3.22.0 — upgrade all FreeRDP libraries and any FreeRDP-based clients (Remmina, GNOME Connections, xrdp clients, etc.) to 3.22.0 or later, rebuilding or updating downstream distribution packages that bundle the library. Until patched, reduce exposure by only connecting to trusted, known-good RDP servers and avoiding connections to untrusted or attacker-influenced hosts, since the malicious behavior originates server-side; where feasible, restrict outbound RDP (TCP 3389) from client machines to an allowlist of approved servers via egress firewall rules (trade-off: breaks ad-hoc connections to new servers). If specific dynamic virtual channels are not required, disabling optional channel redirections (clipboard, drive, audio) narrows the attack surface of drdynvc but does not fully eliminate the race and reduces functionality. Follow guidance in GHSA-3mv2-5q57-2v8h (https://github.com/FreeRDP/FreeRDP/security/advisories/GHSA-3mv2-5q57-2v8h) and the VulnCheck advisory (https://www.vulncheck.com/advisories/freerdp-use-after-free-via-race-condition-in-drdynvc-channel-callback).

Exploit Scenario

An attacker stands up or compromises an RDP server and lures a victim into connecting with a vulnerable FreeRDP client (e.g., via a malicious .rdp file, phishing link, or a man-in-the-middle position). Once connected, the server rapidly interleaves DYNVC_DATA and DYNVC_CLOSE messages on a dynamic virtual channel to win the race and trigger the heap use-after-free in the client's drdynvc thread, crashing the client or, with careful heap grooming, potentially executing code in the client process. No public POC is identified at this time, and the AC:H race condition makes reliable code execution difficult.

Exploitation Conditions

Exploitation requires the victim to establish an RDP session to an attacker-controlled or compromised server running against a FreeRDP client before 3.22.0, and the connection must use the drdynvc dynamic virtual channel path. The attacker must then concurrently send DYNVC_DATA and DYNVC_CLOSE messages on a dynamic virtual channel to win a timing race in channel_callback access. Limiting factors: the attacker must control the RDP server side (this is a client-side vulnerability, so it cannot be triggered against an unwilling client that never connects), the exploit depends on winning a non-deterministic thread race (CVSS AC:H) so it is unreliable and may require repeated attempts, and reliable code execution beyond a crash requires heap-layout control. No authentication to the malicious server is needed, but the user must initiate the outbound connection.

Attack Chain

Lure victim to malicious RDP server → Client opens dynamic virtual channel (drdynvc) → Server races DYNVC_DATA and DYNVC_CLOSE → Trigger heap use-after-free on channel_callback → Crash client or execute code in client process

Confidence Notes

Affected/fixed version (before 3.22.0; fixed in 3.22.0), root cause, and functions are confirmed by the FreeRDP GitHub Security Advisory GHSA-3mv2-5q57-2v8h and the VulnCheck advisory. CVSS 4.0 vector, 8.3 score, and CWE-362 are provided in the input; RCE is described as potential, not demonstrated. Not listed in CISA KEV and no EPSS score or public POC was provided, so real-world exploitation status is unknown; downstream package version ranges are not confirmed due to absent CPE data.

Prevalence

high

Prevalence Basis

core open-source RDP library embedded in many Linux remote-desktop clients

Assessed CVSS Vector

CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:H

Assessed CVSS Rationale

Client-side flaw needing the victim to connect to a malicious server (UI:R) and to win a thread race (AC:H); heap corruption gives high availability impact with limited C/I.

Assessed CVSS 4.0 Vector

CVSS:4.0/AV:N/AC:H/AT:N/PR:N/UI:P/VC:L/VI:L/VA:H/SC:N/SI:N/SA:N