CVE-2026-102761 in NetX Duo
Summary
by MITRE • 09/29/2026
NetX Duo's WebSocket client resets the unmasking cursor to the first `NX_PACKET` each time it advances through a chained packet, while the loop's upper bound belongs to the current packet. With the standard contiguous packet-pool layout, a masked server frame split across two packets therefore drives the XOR loop through the first packet's unused payload area and on through the second packet's `NX_PACKET` control block.
The four-byte WebSocket masking key controls the bytes written, so the corruption is attacker-chosen rather than incidental.
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Analysis
by VulDB Data Team • 09/29/2026
NetX Duo contains a critical memory corruption vulnerability within its WebSocket client implementation that arises from an incorrect handling of chained network packets during frame unmasking operations. The underlying technical flaw stems from a logic error in the loop control mechanism used to process masked data frames. Specifically, when the WebSocket protocol requires processing a payload that spans multiple contiguous buffers or packet chains, the software incorrectly resets the internal cursor pointing to the current position within the packet structure back to the beginning of the first NX_PACKET object every time it advances to a new chained packet segment. However, the upper bound for the XOR unmasking loop is calculated based on the size and limits of only the currently active packet in the chain rather than accounting for the cumulative offset or maintaining state across the entire payload span. This discrepancy creates a scenario where the iteration logic fails to respect the actual boundaries of the data being processed relative to the memory layout.
In standard network stack implementations using NetX Duo, packets are allocated from contiguous pools with specific control blocks preceding the user-accessible payload area. When a masked server frame is split across two such packets due to size constraints or fragmentation, the flawed logic causes the unmasking routine to operate outside the intended data boundaries. Instead of stopping at the end of the first packet's valid payload and correctly transitioning to the next segment with proper offset adjustments, the loop continues executing through the unused padding area of the first NX_PACKET structure. More critically, because the cursor is reset improperly while the bound remains tied to local packet metrics without global context, the XOR operation proceeds into memory regions that are not part of the message payload. This includes traversing past the end of the data buffer and entering adjacent memory structures within the same allocation block or neighboring objects in the pool.
The operational impact of this vulnerability is severe, as it leads to arbitrary write operations via memory corruption. The WebSocket masking key consists of four bytes that are XORed against each byte of the payload during transmission to ensure basic obfuscation. Because the attacker controls the content of the masked frame and consequently determines which parts of the corrupted memory region are written to using these known mask values, they can exert precise control over the data being overwritten. This is not a case of incidental corruption with unpredictable outcomes; rather, it allows for deterministic exploitation where specific bytes in adjacent memory structures can be modified to arbitrary values chosen by the attacker. Such writes can overwrite function pointers, object headers, or other critical control data within the NX_PACKET structure itself or neighboring objects in the packet pool, potentially leading to remote code execution, denial of service through system crashes, or privilege escalation depending on the surrounding application context and memory layout.
This vulnerability aligns with CWE-787 Out-of-bounds Write, as it involves writing data beyond the allocated buffer boundaries due to improper boundary checks. Furthermore, from a tactical perspective related to MITRE ATT&CK, this flaw facilitates techniques associated with Memory Corruption (T1204) and potentially Command and Control via WebSocket if an attacker can manipulate execution flow or inject malicious payloads into adjacent memory spaces that are later interpreted as code or configuration data. The ability to write arbitrary values using a known mask makes exploitation significantly more reliable compared to vulnerabilities where corruption is random or dependent on timing conditions.
Mitigation strategies must address the root cause in the packet processing logic. Developers should ensure that when iterating through chained packets, the loop bounds and cursor positions are calculated based on the total remaining payload length rather than resetting state incorrectly for each segment. It is essential to maintain a continuous view of the data stream across packet boundaries so that memory accesses remain strictly within the allocated payload regions of all involved NX_PACKET objects. Additionally, implementing strict boundary checks before every write operation during unmasking can prevent out-of-bounds access even if logic errors persist in higher-level flow control. Updating NetX Duo to a patched version that corrects this cursor management and bound calculation is the primary remediation step for affected systems relying on WebSocket communication over TCP/IP stacks utilizing this library.