Description

FreeRDP versions before 3.31.0 contain an out-of-bounds write vulnerability in server-side static virtual channel handling when CHANNEL_OPTION_SHOW_PROTOCOL is enabled. Authenticated clients can queue oversized channel messages that cause buffer underflow and corrupt heap memory including live pointers, potentially enabling code execution.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "7.5",
  "pubDate": "2026-09-15T16:17:48.503Z",
  "pubdate": "2026-09-15T16:17:48.503Z",
  "executiveSummary": "FreeRDP versions prior to 3.31.0 are susceptible to an out-of-bounds write vulnerability within the server-side static virtual channel processing logic. The flaw is triggered when the CHANNEL_OPTION_SHOW_PROTOCOL option is active, allowing an authenticated client to send malformed, oversized channel messages.\nThis vulnerability originates from improper bounds checking during the processing of static virtual channel data, resulting in a buffer underflow that corrupts the heap memory. Because the corrupted memory can encompass live pointers, the vulnerability is highly critical, as it may permit an attacker to hijack control flow or achieve arbitrary code execution on the server host.\nExploitation requires an authenticated session, meaning the attacker must already possess valid credentials to access the remote desktop environment. Once authenticated, the attacker can leverage this flaw to gain elevated privileges or perform post-exploitation actions within the context of the FreeRDP server process. The vulnerability poses a significant risk to the integrity and confidentiality of systems utilizing FreeRDP services.",
  "technicalDetails": "The vulnerability resides in the server-side component of FreeRDP responsible for handling static virtual channels, specifically when the configuration flag CHANNEL_OPTION_SHOW_PROTOCOL is enabled. The root cause is a failure to enforce strict length validation on incoming channel messages before copying them into a fixed-size internal buffer.\nWhen a client initiates a connection and requests a static virtual channel with the specified option, the server allocates heap memory to manage the protocol's state and data buffers. An authenticated client can intentionally craft a malicious packet that exceeds the expected size constraints of the allocated buffer. When the FreeRDP server attempts to write this oversized payload, it triggers an out-of-bounds write operation, specifically manifesting as a buffer underflow.\nThe attack flow proceeds as follows: First, the attacker establishes an authenticated connection to the target FreeRDP server. Second, the attacker interacts with the static virtual channel subsystem, utilizing the vulnerable CHANNEL_OPTION_SHOW_PROTOCOL functionality. Third, the attacker transmits a specially crafted, oversized message designed to overwrite contiguous heap memory regions. Because the heap frequently contains critical data structures, including object pointers, vtables, or function pointers used by the server application, this overwriting process allows the attacker to corrupt memory metadata or redirect execution flow. If the attacker successfully overwrites a function pointer with an address pointing to a controlled payload, they can achieve arbitrary code execution at the privilege level of the FreeRDP service process.\nThis vulnerability is highly dependent on the heap layout at the time of the attack, making it a powerful primitive for memory corruption exploitation. The impact extends beyond simple denial-of-service, as the ability to manipulate heap pointers allows for sophisticated exploitation techniques, potentially bypassing traditional mitigations if not properly managed by the memory allocator. The affected versions include all releases of FreeRDP before 3.31.0. Given that the vulnerability occurs during the processing of virtual channel data, it remains accessible to any client capable of establishing a session, highlighting the necessity of strict access controls and timely software updates."
}