CVE-2026-91948 in FreeRDP
Summary
by MITRE • 09/15/2026
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.
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Analysis
by VulDB Data Team • 09/15/2026
The identified security flaw resides within the FreeRDP implementation prior to version 3.31.0, specifically affecting the server-side handling of static virtual channels when the CHANNEL_OPTION_SHOW_PROTOCOL flag is active. This vulnerability represents a critical failure in input validation and memory management logic during the processing of remote desktop protocol messages. The core technical issue stems from an out-of-bounds write condition triggered by authenticated clients submitting channel messages that exceed expected size limits. When such oversized payloads are queued for processing, the application fails to correctly calculate buffer boundaries or enforce strict length constraints before writing data into allocated memory structures. This oversight leads directly to a heap-based buffer overflow scenario where writes extend beyond the intended allocation space, resulting in corruption of adjacent heap metadata and potentially overwriting live pointers within the memory structure.
The operational impact of this vulnerability is severe due to its potential for remote code execution on affected systems. Because the flaw allows an authenticated attacker to corrupt heap memory including function pointers or object vtables, it creates a reliable pathway for arbitrary code execution with the privileges of the FreeRDP server process. This capability undermines the fundamental security guarantees provided by the Remote Desktop Protocol infrastructure. An adversary leveraging this weakness could bypass standard authentication mechanisms if they possess valid credentials and subsequently gain full control over the host system. The presence of live pointer corruption significantly increases the exploitability rate, as it allows for precise memory layout manipulation techniques such as heap spraying or arbitrary read-write primitives to be employed with greater stability compared to simpler buffer overflow scenarios that might only cause denial of service through application crashes.
From a classification perspective, this vulnerability aligns closely with CWE-787: Out-of-bounds Write and CWE-120: Buffer Copy without Checking Size of Input in C/C++. These categories highlight the fundamental lack of bounds checking during memory operations involving user-controlled input data. Furthermore, within the context of offensive security frameworks like MITRE ATT&CK, this flaw facilitates techniques associated with Initial Access via valid credentials followed by Execution through exploitation of application vulnerabilities. The specific mechanism of heap corruption and pointer manipulation also relates to post-exploitation activities where attackers seek to establish persistence or escalate privileges by modifying critical system structures.
Mitigation strategies must prioritize immediate software updates as the primary defense vector. Organizations running FreeRDP servers should upgrade to version 3.31.0 or later, which includes patches for this specific heap corruption issue in static virtual channel handling. In environments where upgrading is not immediately feasible, network-level controls such as strict firewall rules restricting access to RDP ports from untrusted networks can reduce the attack surface by limiting exposure to authenticated attackers who might attempt to exploit this flaw. Additionally, implementing rigorous input validation at the application gateway level and monitoring for anomalous memory usage patterns or unexpected process terminations can aid in early detection of exploitation attempts. Regular security audits focusing on buffer management practices within custom virtual channel implementations are also recommended to prevent similar out-of-bounds write conditions from being introduced during future development cycles.