CVE-2026-109253 in FreeRDP
Summary
by MITRE • 10/12/2026
FreeRDP from 3.9.0 before 3.32.0 contains an out-of-bounds read vulnerability in the RDPEAR NDR scalar readers, which check value length before consuming alignment padding. Malicious or compromised RDP servers can send crafted unaligned RDPEAR payloads, such as CreateApReqAuthenticatorReq messages, to clients using Remote Credential Guard, causing over-reads and potentially crashing the client process.
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Analysis
by VulDB Data Team • 10/12/2026
The vulnerability identified in FreeRDP versions prior to 3.32.0 represents a critical memory safety flaw within the RDPEAR NDR scalar readers component of the Remote Desktop Protocol implementation. This specific issue stems from an incorrect sequence of operations during the parsing of network data streams, specifically involving how alignment padding is handled relative to value length validation. In compliant implementations of the Microsoft Network Data Representation (NDR) protocol used by RDPEAR, parsers must strictly adhere to byte-alignment rules where certain fields require padding bytes to ensure subsequent data structures are aligned on specific memory boundaries. The flaw arises because the software checks the declared value length before properly consuming or skipping these required alignment padding bytes. This logical error creates a discrepancy between the expected stream position and the actual cursor location within the received buffer, leading directly to an out-of-bounds read condition when the parser attempts to access data based on misaligned offsets.
From a technical perspective, this vulnerability is classified under CWE-125, which denotes Out-of-Bounds Read vulnerabilities. The root cause lies in the failure of the input validation logic to account for structural padding requirements inherent in the NDR serialization format. When an RDPEAR payload arrives, such as a CreateApReqAuthenticatorReq message utilized during authentication processes like Remote Credential Guard, the parser reads metadata that dictates how much data follows. However, because it fails to advance its internal pointer past necessary alignment bytes before validating or processing the subsequent value length, it calculates incorrect memory addresses for reading scalar values. This results in the application attempting to read from memory locations outside the bounds of the allocated buffer associated with the current packet payload. Such behavior is a classic example of improper input validation leading to memory corruption risks, even if the immediate manifestation is often limited to information disclosure or denial of service rather than arbitrary code execution due to the read-only nature of the flaw.
The operational impact of this vulnerability is significant for organizations relying on FreeRDP as their client-side implementation for Remote Desktop Protocol connections. An attacker controlling a malicious or compromised RDP server can exploit this weakness by crafting specially designed RDPEAR payloads that trigger the out-of-bounds read condition upon receipt by the vulnerable client. The primary consequence observed in such scenarios is the crash of the FreeRDP client process, resulting in a denial of service for legitimate users attempting to establish remote sessions. While the immediate effect may appear as a simple application failure, out-of-bounds reads can potentially leak sensitive information from adjacent memory regions if the attacker can control subsequent read operations or if the vulnerability interacts with other memory management routines. This poses a risk not only to system availability but also to confidentiality, particularly in environments where Remote Credential Guard is enabled and authentication tokens are processed through these vulnerable code paths.
In terms of threat modeling, this exploitation vector aligns with ATT&CK technique T1078, Valid Accounts, as it involves the manipulation of authentication-related messages within a remote access protocol. It also reflects aspects of T1203, Exploitation for Client Execution, if the crash leads to unstable states that could be leveraged in more complex attack chains involving memory corruption primitives. The vulnerability specifically targets clients using Remote Credential Guard, which is designed to enhance security by offloading credential handling to a secure environment on the client machine. By disrupting this process through crashes or potential data leakage, attackers can degrade the security posture of remote access infrastructure that relies heavily on FreeRDP for connectivity.
Mitigation strategies must prioritize immediate software updates and defensive configuration changes. The most effective remediation is upgrading FreeRDP to version 3.32.0 or later, where this parsing logic has been corrected to properly consume alignment padding before validating value lengths. Organizations should implement strict patch management policies to ensure all client endpoints running FreeRDP are updated promptly upon the release of security fixes. Additionally, network segmentation and firewall rules can be employed to restrict RDP traffic to trusted sources only, reducing the attack surface presented by untrusted or potentially compromised servers. Deploying intrusion detection systems capable of identifying anomalous RDPEAR packet structures may also provide an additional layer of defense against exploitation attempts targeting this specific memory safety flaw until all vulnerable instances are remediated.