# CVE-2026-55191: FreeRDP buffer overflow in H.264 AVC444 decoding

Source: https://junglewise.ai/threats/cve-2026-55191-freerdp-buffer-overflow-in-h-264-avc444-decoding  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-08-19. Updated: 2026-09-25  
License: CC BY 4.0 with a link back

CVE-2026-55191 · Severity: critical · CVSS 9.8 · Published 2026-08-19

FreeRDP is a widely-used open-source implementation of the Remote Desktop Protocol used by administrators and users to remotely connect to Windows and Linux systems. A malicious RDP server can exploit a buffer overflow in H.264 video decoding to crash connecting clients or execute arbitrary code on their machines, potentially giving an attacker full control of the victim's computer.

Technologies: [FreeRDP](https://junglewise.ai/threats/technologies/freerdp). Vendors: [FreeRDP](https://junglewise.ai/threats/vendors/freerdp).

## Quick answers

**Is CVE-2026-55191 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-55191 being exploited in the wild.

**How severe is CVE-2026-55191?** Critical severity, with a CVSS score of 9.8, published 19 August 2026.

**Which products and versions are affected?** FreeRDP prior to 3.27.0.

**Is there a fix for CVE-2026-55191?** A patch was published on 2026-06-11: Fix merged in FreeRDP 3.27.0 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-55191 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-55191?** Check whether you run FreeRDP at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability is a heap buffer overflow in the avc444_ensure_buffer function within libfreerdp/codec/h264.c. When FreeRDP clients negotiate RDPGFX AVC444 with H.264 decoder backend, the code uses 32-bit arithmetic to calculate YUV444 buffer allocation sizes, causing integer overflow when a malicious server supplies large surface dimensions. The piDstStride multiplied by padDstHeight can wrap to a small value, leading winpr_aligned_recalloc to allocate an undersized buffer. Subsequent writes via YUV420CombineToYUV444 using the actual dimensions overflow the buffer, corrupting the heap. No authentication is required—the attack occurs during the negotiation phase of a normal RDP connection. This can result in denial of service or arbitrary code execution. The issue was fixed in version 3.27.0 by correcting the bounds checking logic.

## Affected products

- FreeRDP FreeRDP prior to 3.27.0

## Timeline

- 2026-08-19: disclosed
- 2026-06-11: patched: Fix merged in FreeRDP 3.27.0

## References

- [https://github.com/FreeRDP/FreeRDP/commit/97f40b9e766af375f4e41ac6a3f4397d708d249c](https://github.com/FreeRDP/FreeRDP/commit/97f40b9e766af375f4e41ac6a3f4397d708d249c)
- [https://github.com/FreeRDP/FreeRDP/pull/12873](https://github.com/FreeRDP/FreeRDP/pull/12873)

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