CVE-2026-55827 in FreeRDP
Summary
by MITRE • 07/10/2026
FreeRDP is a free implementation of the Remote Desktop Protocol. Prior to 3.27.1, FreeRDP clients launched with the non-default /cache:codec:rfx option pass desktop stride and height to RemoteFX decoding for Cache Bitmap V3 data while allocating bitmap->data only for the smaller DstWidth and DstHeight in gdi_Bitmap_Decompress, allowing a malicious RDP server to trigger a heap out-of-bounds write with attacker-controlled offset and content. This issue is fixed in version 3.27.1.
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Analysis
by VulDB Data Team • 07/10/2026
The vulnerability in FreeRDP affects versions prior to 3.27.1 where the implementation contains a critical heap-based buffer overflow flaw in the RemoteFX decoding mechanism. This issue manifests when clients are launched with the non-default /cache:codec:rfx option, creating a dangerous mismatch between allocated memory dimensions and actual data processing requirements. The vulnerability stems from improper validation of desktop stride and height parameters passed to the RemoteFX decoding engine during Cache Bitmap V3 data processing.
The technical flaw occurs within the gdi_Bitmap_Decompress function where bitmap->data memory allocation is performed using smaller DstWidth and DstHeight values while the actual desktop stride and height parameters are passed to the RemoteFX decoding process. This dimensional mismatch creates a scenario where attacker-controlled data can be written beyond the bounds of allocated heap memory, resulting in a heap out-of-bounds write condition. The vulnerability specifically targets the Cache Bitmap V3 data handling mechanism within the Remote Desktop Protocol implementation.
Operationally, this vulnerability presents a severe security risk as it allows a malicious RDP server to execute arbitrary code on the victim's system through controlled heap corruption. The attacker-controlled offset and content parameters provide significant flexibility in exploiting the memory corruption, potentially enabling privilege escalation or denial of service conditions. The vulnerability is particularly dangerous because it requires minimal user interaction beyond establishing an RDP connection to a compromised server, making it suitable for automated exploitation scenarios.
Mitigation strategies include upgrading to FreeRDP version 3.27.1 or later where the issue has been resolved through proper parameter validation and memory allocation adjustments. Organizations should also consider implementing network segmentation and access controls to limit exposure to untrusted RDP servers. Additionally, monitoring for unusual RDP connection patterns and implementing intrusion detection systems can help identify potential exploitation attempts. This vulnerability aligns with CWE-121 heap-based buffer overflow and relates to ATT&CK technique T1071.004 for application layer protocol usage, highlighting the importance of proper input validation in remote desktop implementations.
The fix implemented in version 3.27.1 addresses the core issue by ensuring that memory allocation dimensions properly match the parameters passed to the RemoteFX decoding engine. This correction prevents the heap corruption scenario by maintaining consistency between allocated buffer sizes and the actual data processing requirements, thereby eliminating the out-of-bounds write condition that previously enabled remote code execution capabilities through carefully crafted RDP server responses.