CVE-2026-55827 Overview
CVE-2026-55827 is a heap out-of-bounds write vulnerability in FreeRDP, an open-source implementation of the Remote Desktop Protocol (RDP). The flaw affects FreeRDP clients launched with the non-default /cache:codec:rfx option. When processing Cache Bitmap V3 data, the client passes the desktop stride and height to the RemoteFX decoder while only allocating bitmap->data for the smaller DstWidth and DstHeight inside gdi_Bitmap_Decompress. A malicious RDP server can exploit this size mismatch to trigger a heap out-of-bounds write with an attacker-controlled offset and content. The issue is fixed in FreeRDP version 3.27.1.
Critical Impact
A malicious RDP server can achieve heap memory corruption on a connecting FreeRDP client, potentially leading to remote code execution in the client process context.
Affected Products
- FreeRDP versions prior to 3.27.1
- FreeRDP clients invoked with the /cache:codec:rfx option enabled
- Downstream applications and distributions bundling vulnerable FreeRDP libraries
Discovery Timeline
- 2026-07-10 - CVE-2026-55827 published to NVD
- 2026-07-15 - Last updated in NVD database
Technical Details for CVE-2026-55827
Vulnerability Analysis
The vulnerability resides in the gdi_Bitmap_Decompress function inside the FreeRDP GDI subsystem. When a client negotiates the RemoteFX codec through the /cache:codec:rfx option, incoming Cache Bitmap V3 data is routed to the RemoteFX decoder. The decoder receives the full desktop stride and height as its output surface parameters. However, the underlying bitmap->data buffer is allocated using the much smaller DstWidth and DstHeight values supplied for the individual bitmap tile. This size mismatch is classified under [CWE-131] Incorrect Calculation of Buffer Size.
Because the RemoteFX decoder writes pixels according to the desktop-sized stride and height, it walks past the end of the allocated tile buffer. A server controlling the protocol stream chooses the geometry values, giving an adversary control over both the write offset and the data written into adjacent heap regions.
Root Cause
The root cause is inconsistent buffer sizing between allocation and consumption. Allocation uses tile dimensions (DstWidth × DstHeight), while the decoder writes using desktop dimensions (stride × height). Any time the desktop exceeds the tile dimensions, decoding overruns the destination heap chunk.
Attack Vector
Exploitation requires a user to initiate an RDP connection to an attacker-controlled server with the non-default RemoteFX cache codec option enabled. The attack vector is network-based with user interaction, matching the CVSS UI:R metric. Once connected, the malicious server sends crafted Cache Bitmap V3 orders whose declared dimensions trigger the overflow during decoding. Successful exploitation corrupts adjacent heap metadata or objects, enabling code execution in the client process. The FreeRDP maintainers document the fix in the GitHub Security Advisory GHSA-c495-h83v-3prp and the corresponding patch commit.
Detection Methods for CVE-2026-55827
Indicators of Compromise
- FreeRDP client processes crashing shortly after connecting to an untrusted RDP endpoint, particularly with RemoteFX enabled.
- Outbound RDP sessions on TCP/3389 initiated with command lines containing /cache:codec:rfx to unfamiliar or externally hosted servers.
- Heap corruption signatures in crash dumps referencing gdi_Bitmap_Decompress or RemoteFX decoding routines.
Detection Strategies
- Inventory endpoints for FreeRDP binaries below version 3.27.1 using package managers or software asset management tooling.
- Inspect scripts, shortcuts, and configuration files launching xfreerdp or wlfreerdp for the /cache:codec:rfx argument.
- Enable Address Sanitizer or heap protection in test environments to surface out-of-bounds writes during RDP client testing.
Monitoring Recommendations
- Log outbound RDP connections from workstations and jump hosts, alerting on sessions to non-corporate destinations.
- Monitor FreeRDP process telemetry for abnormal exits, segmentation faults, or child process creation following RDP session initiation.
- Correlate DNS and NetFlow data to identify RDP connections to newly registered or low-reputation domains.
How to Mitigate CVE-2026-55827
Immediate Actions Required
- Upgrade all FreeRDP installations to version 3.27.1 or later across servers, workstations, and container images.
- Avoid launching FreeRDP with /cache:codec:rfx until patched builds are deployed everywhere the client is used.
- Restrict outbound RDP connectivity from managed endpoints to a defined list of trusted internal RDP servers.
Patch Information
The issue is fixed in FreeRDP 3.27.1. The upstream fix is committed as e58adf922ea4c0d5495e59a1fe488d70092e0e3e and merged via pull request #12899. Release artifacts are available at the FreeRDP 3.27.1 release tag. Distributions that vendor FreeRDP as a shared library should rebuild dependent packages against the patched version.
Workarounds
- Do not pass the /cache:codec:rfx option when launching FreeRDP; the vulnerable code path is not reached under the default configuration.
- Only connect to trusted RDP servers within controlled network segments, since exploitation requires a malicious server.
- Apply egress firewall rules that block RDP traffic from user endpoints to untrusted or external destinations.
# Verify the installed FreeRDP version is patched
xfreerdp --version
# Example: launch without the vulnerable RemoteFX cache codec option
xfreerdp /v:trusted-host.internal /u:user
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
