Executive Summary

CVE-2026-44420 is a heap-buffer-overflow vulnerability in FreeRDP's server-side clipboard (cliprdr) channel. A malicious Remote Desktop Protocol (RDP) client can exploit this by sending a CB_CLIP_CAPS PDU with an undersized capabilitySetLength value, which causes a heap overflow write.

Specifically, the vulnerability occurs because the server allocates a heap buffer based on the client-controlled capabilitySetLength but then writes a fixed-size structure larger than the allocated buffer, leading to an out-of-bounds write. This can crash the server process or potentially allow code execution by corrupting heap memory.

This issue affects FreeRDP versions up to 3.25.0 and was fixed in version 3.26.0.

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Impact Analysis

This vulnerability can have serious impacts including crashing the FreeRDP server process remotely, resulting in a denial of service (DoS).

More critically, because the heap memory is corrupted, an attacker may be able to execute arbitrary code on the server, potentially gaining control over the affected system.

The vulnerability has a high CVSS score of 8.8, indicating a high severity with impacts on confidentiality, integrity, and availability.

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Detection Guidance

This vulnerability is triggered by a malicious RDP client sending a CB_CLIP_CAPS PDU with an undersized capabilitySetLength value to a FreeRDP server. Detection would involve monitoring RDP traffic for anomalous or malformed CB_CLIP_CAPS PDUs, specifically those with unusually small capabilitySetLength values.

Since the vulnerability involves heap-buffer-overflow caused by a capabilitySetLength smaller than expected, network intrusion detection systems (NIDS) or packet inspection tools could be configured to flag CB_CLIP_CAPS PDUs with capabilitySetLength values less than the expected size (less than 12 bytes).

However, no specific detection commands or signatures are provided in the available information.

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Mitigation Strategies

The primary mitigation step is to upgrade FreeRDP to version 3.26.0 or later, where this vulnerability has been fixed.

Until the upgrade can be applied, consider restricting or monitoring RDP client connections to the FreeRDP server to prevent potentially malicious clients from sending malformed CB_CLIP_CAPS PDUs.

Additionally, applying network-level protections such as firewall rules or intrusion detection/prevention systems to detect and block suspicious RDP clipboard channel traffic may help reduce exposure.

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Compliance Impact

CVE-2026-44420 allows a malicious RDP client to execute code or cause a denial of service on the FreeRDP server by exploiting a heap-buffer-overflow vulnerability. This can lead to unauthorized access, data corruption, or service disruption.

Such impacts on confidentiality, integrity, and availability can affect compliance with standards and regulations like GDPR and HIPAA, which require protection of sensitive data and system availability.

Specifically, the high CVSS score (8.8) indicates a severe risk that could result in breaches of personal or protected health information, potentially leading to regulatory violations.

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