CVE-2026-109257 Overview
CVE-2026-109257 is a double-free vulnerability [CWE-415] in FreeRDP versions before 3.32.0. The flaw resides in the nsc_context_initialize_encode() function, which leaves dangling plane pointers after a partial buffer growth failure. A remote attacker using a crafted Remote Desktop Protocol (RDP) client can negotiate NSCodec with a large desktop geometry to trigger an allocation failure. The subsequent context teardown frees the dangling pointers, crashing the server. The vulnerability affects server-side FreeRDP deployments and results in a denial-of-service condition.
Critical Impact
Unauthenticated remote attackers can crash FreeRDP-based RDP servers by triggering an allocation failure during NSCodec negotiation, leading to service disruption.
Affected Products
- FreeRDP versions before 3.32.0
- Applications and server implementations embedding vulnerable FreeRDP libraries
- RDP server deployments using NSCodec negotiation
Discovery Timeline
- 2026-10-12 - CVE-2026-109257 published to NVD
- 2026-10-15 - Last updated in NVD database
Technical Details for CVE-2026-109257
Vulnerability Analysis
The vulnerability resides in the NSCodec encoder initialization path within FreeRDP. During session negotiation, an RDP client specifies desktop geometry that drives buffer allocation inside nsc_context_initialize_encode(). When the function attempts to grow internal plane buffers for a large desktop size and one of the allocations fails partway through, it exits without clearing the plane pointers that referenced the partially freed memory.
When the server later tears down the NSCodec context, the cleanup routine iterates over those plane pointers and frees them again. This produces a classic double-free condition on heap-allocated buffers, corrupting allocator metadata and terminating the server process. See the GitHub Security Advisory for additional technical detail.
Root Cause
The root cause is improper error handling after a partial buffer growth failure. The function releases memory without resetting the owning pointers to NULL, violating the contract expected by the context destructor. Any subsequent free operation on those dangling pointers triggers undefined behavior consistent with CWE-415.
Attack Vector
Exploitation requires network access to an RDP service built on vulnerable FreeRDP code. An attacker connects with a modified RDP client, negotiates the NSCodec capability, and advertises a desktop geometry large enough to exhaust available memory during plane buffer allocation. No authentication is required. The crash occurs during normal connection teardown, enabling repeatable denial of service against the target. See the VulnCheck Advisory for FreeRDP for exploitation context.
Detection Methods for CVE-2026-109257
Indicators of Compromise
- Repeated FreeRDP server process crashes or restarts with heap corruption signatures in core dumps.
- RDP sessions terminating during capability negotiation shortly after client-advertised desktop geometry exceeds normal values.
- Abnormally large desktop geometry parameters in RDP Client Core Data captured on the wire.
Detection Strategies
- Monitor FreeRDP server logs for abnormal termination during NSCodec initialization and context teardown.
- Inspect network telemetry for RDP connections advertising unusually large desktop dimensions during channel negotiation.
- Alert on repeated short-lived RDP sessions from the same source that cause server-side faults.
Monitoring Recommendations
- Enable process crash telemetry and core dump collection on hosts running FreeRDP-based services.
- Baseline typical RDP desktop geometry values across your environment and alert on outliers.
- Correlate RDP connection anomalies with host-level crash events to identify active exploitation attempts.
How to Mitigate CVE-2026-109257
Immediate Actions Required
- Upgrade FreeRDP to version 3.32.0 or later on all affected servers and dependent applications.
- Inventory third-party products that embed FreeRDP and apply vendor updates when available.
- Restrict RDP service exposure to trusted networks until patches are deployed.
Patch Information
The FreeRDP maintainers resolved the issue in version 3.32.0 by ensuring plane pointers are nulled after any partial allocation failure in nsc_context_initialize_encode(). Patch details are documented in the GitHub Security Advisory.
Workarounds
- Disable NSCodec support on the server if the deployment allows alternative codecs.
- Place vulnerable RDP servers behind a VPN or RDP gateway that enforces authentication before protocol negotiation.
- Apply network-level rate limiting on RDP ports to reduce repeated crash attempts from a single source.
# Verify installed FreeRDP version
freerdp --version
# Example firewall rule to restrict RDP exposure to a trusted range
iptables -A INPUT -p tcp --dport 3389 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 3389 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
