CVE-2026-54410 Overview
CVE-2026-54410 is an off-by-one buffer overflow in the recv_msg_header() function of the nanoMODBUS library through version v1.23.0. The flaw resides in the Modbus/TCP server code path and is reachable by remote unauthenticated attackers. By sending a crafted Modbus Application Protocol (MBAP) frame whose Length field is set to 255, an attacker writes one attacker-controlled byte past the end of the 260-byte receive buffer. The overflow corrupts the adjacent buffer-index field of the nanoMODBUS state structure, producing denial of service through invalid memory accesses. On bare-metal and RTOS targets without memory protection, the same primitive enables one-byte information disclosure and writes to unintended register addresses on the Write Multiple Registers (Function Code 16) handler path.
Critical Impact
Remote unauthenticated attackers can trigger denial of service in nanoMODBUS Modbus/TCP servers and, on unprotected embedded targets, achieve limited memory disclosure and unintended register writes.
Affected Products
- nanoMODBUS library versions up to and including v1.23.0
- Modbus/TCP server implementations built on nanoMODBUS
- Bare-metal and RTOS deployments without memory protection units enabled
Discovery Timeline
- 2026-06-14 - CVE-2026-54410 published to the National Vulnerability Database (NVD)
- 2026-06-17 - Last updated in the NVD database
Technical Details for CVE-2026-54410
Vulnerability Analysis
The vulnerability is an off-by-one write classified under CWE-193: Off-by-one Error. The nanoMODBUS Modbus/TCP server allocates a fixed 260-byte receive buffer sized for the maximum legal Modbus/TCP Application Data Unit. The recv_msg_header() function parses the 7-byte MBAP header and uses the attacker-controlled Length field to determine how many additional bytes to read into the buffer.
When Length is set to 255, the code computes a read length that places the final byte at index 260, one byte past the buffer boundary. The single overflowing byte falls into the adjacent buffer-index field of the nanoMODBUS state structure, corrupting subsequent indexing operations. On the Write Multiple Registers (FC16) handler path, the corrupted index causes the server to read and write registers at offsets the client never authorized, and to return one byte of adjacent memory in the response.
Root Cause
The defect is an arithmetic boundary error in length validation. The check permits a Length value that, after accounting for the protocol header bytes already consumed, yields a total frame size exceeding the 260-byte fixed buffer by exactly one byte. The handler path also classifies under CWE-787: Out-of-bounds Write because the single byte is written outside the allocated buffer. See the relevant code at nanoMODBUS source line 369.
Attack Vector
The attack vector is network-based and requires no authentication or user interaction. An attacker sends a single crafted Modbus/TCP frame to TCP port 502 (or whichever port the server binds). The crafted frame contains a valid MBAP header with the Length field set to 255, followed by enough payload bytes to fill the over-read. No prior session state is required.
The vulnerability does not contain a verified public proof-of-concept exploit at this time, and there is no evidence of in-the-wild exploitation. Refer to the nanoMODBUS GitHub repository for source-level details of the parser logic.
Detection Methods for CVE-2026-54410
Indicators of Compromise
- Inbound Modbus/TCP frames on port 502 whose MBAP Length field equals 0x00FF (255)
- Unexpected crashes, watchdog resets, or memory faults on Modbus/TCP server devices following inbound traffic
- Write Multiple Registers (FC16) responses referencing register addresses the client did not request
- Anomalous bytes appearing at the tail of Modbus/TCP responses where padding should be deterministic
Detection Strategies
- Deploy network intrusion detection signatures that flag MBAP headers with Length values at or near the 255 boundary on Modbus/TCP sessions
- Perform deep packet inspection on TCP port 502 to validate that Length is consistent with the contained Modbus PDU and within the 253-byte payload limit
- Correlate device reboots and process crashes on operational technology (OT) endpoints with preceding Modbus traffic
Monitoring Recommendations
- Baseline normal MBAP Length distributions on production Modbus/TCP sessions and alert on outliers
- Forward OT firewall and protocol-aware gateway logs to a centralized data lake for retroactive analysis
- Monitor embedded device health telemetry, including watchdog triggers and exception counters, alongside network capture metadata
How to Mitigate CVE-2026-54410
Immediate Actions Required
- Inventory all firmware and applications that statically link or vendor the nanoMODBUS library at v1.23.0 or earlier
- Restrict Modbus/TCP exposure to trusted management networks using firewall rules on TCP port 502
- Apply protocol-aware filtering at the OT/IT boundary to drop frames with malformed or out-of-spec MBAP Length values
- Validate that memory protection unit (MPU) features are enabled on RTOS and bare-metal targets where available
Patch Information
No fixed release version is identified in the NVD record at the time of publication. Monitor the nanoMODBUS GitHub repository for an upstream patch and rebuild affected firmware once a release supersedes v1.23.0. Vendors that ship products embedding nanoMODBUS should issue firmware updates that incorporate the upstream fix.
Workarounds
- Place Modbus/TCP servers behind a protocol-aware gateway that rejects frames whose Length field equals 255 or otherwise exceeds the 253-byte PDU maximum
- Disable network exposure of the Modbus/TCP service on devices that do not require remote control
- Apply rate limiting and source allow-listing on TCP port 502 to reduce attack surface from untrusted networks
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
