CVE-2026-121962 in FreeRDP
Summary
by MITRE • 11/04/2026
FreeRDP before 3.32.2 contains a heap buffer overflow in transport_default_read_pdu when handling partial Early User Authorization reads during PROTOCOL_HYBRID_EX CredSSP. A malicious RDP server can split the four-byte EUA result into short reads that advance the stream pointer beyond the 16 KiB allocation, causing an out-of-bounds write and client denial of service.
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Analysis
by VulDB Data Team • 11/04/2026
The vulnerability identified in FreeRDP versions prior to 3.32.2 represents a critical heap buffer overflow within the transport layer's protocol data unit handling logic. Specifically, the flaw resides in the function responsible for reading Protocol Data Units during the Remote Desktop Protocol connection establishment phase. This issue manifests when processing Early User Authorization results under the PROTOCOL_HYBRID_EX CredSSP authentication method. The core technical deficiency involves an improper validation of stream pointer advancement relative to the allocated buffer size. When a malicious RDP server intentionally splits the four-byte EUA result into multiple short reads, it triggers a logic error that allows the internal stream pointer to advance beyond the boundaries of the 16 KiB heap allocation. This condition results in an out-of-bounds write operation, which corrupts adjacent memory structures and leads directly to client-side denial of service conditions through application crashes or potential arbitrary code execution if the corrupted memory is subsequently utilized by other system components.
From a technical perspective, this vulnerability aligns with CWE-122, Heap-based Buffer Overflow, as it involves writing data beyond the allocated heap region due to insufficient boundary checks during input processing. The attack vector leverages the complexity of the CredSSP authentication handshake, specifically targeting the hybrid extension protocol which is commonly used in modern Windows Remote Desktop environments. By manipulating the fragmentation of the EUA result, an attacker can exploit the state machine's failure to correctly account for partial reads against the fixed buffer limit. This exploitation technique does not require user interaction beyond establishing a connection to a rogue server, classifying it as a network-based attack that targets the initial authentication phase before any desktop session is established.
The operational impact of this vulnerability is severe, primarily resulting in denial of service for clients attempting to connect via RDP using hybrid authentication methods. The immediate consequence is the termination of the FreeRDP client process due to memory corruption detected by operating system security mechanisms or standard runtime checks. However, given that heap buffer overflows can potentially be leveraged for remote code execution depending on the specific allocator state and subsequent use-after-free scenarios, there remains a significant risk beyond simple service disruption. Organizations relying on FreeRDP as their primary RDP client face substantial operational risks if they connect to untrusted or compromised servers, such as those found in public Wi-Fi networks or maliciously configured enterprise environments.
Mitigation strategies must prioritize immediate software updates and defensive configuration adjustments. The most effective remediation is upgrading FreeRDP to version 3.32.2 or later, where the stream pointer validation logic has been corrected to ensure that partial reads do not cause buffer overruns. In scenarios where updating is not immediately feasible, organizations should restrict RDP client usage to trusted networks and implement network-level authentication controls to prevent connections from unverified servers. Additionally, deploying intrusion detection systems capable of identifying anomalous packet fragmentation patterns during the CredSSP handshake can provide an additional layer of defense against exploitation attempts targeting this specific vulnerability class.