CVE-2026-71263: FreeModbus Buffer Overflow Vulnerability

CVE-2026-71263 Overview

CVE-2026-71263 is an off-by-one buffer overflow in the LINUXTCP port of FreeModbus, located in the xMBPortTCPPool() function inside demo/LINUXTCP/port/porttcp.c. The bounds check if (usTCPFrameBytesLeft > MB_TCP_BUF_SIZE) uses a strict greater-than comparison instead of greater-than-or-equal against the 263-byte MB_TCP_BUF_SIZE limit. A crafted Modbus Application Protocol (MBAP) frame with a Length field of 264 bypasses the check and overflows the static buffer aucTCPBuf by 7 bytes into the adjacent usTCPBufPos variable. The flaw is classified under [CWE-787] Out-of-bounds Write.

Critical Impact
A single unauthenticated Modbus TCP packet triggers a stack-adjacent static memory overflow, compromising integrity and availability of any device running the affected FreeModbus LINUXTCP port.

Affected Products

FreeModbus LINUXTCP demo port (demo/LINUXTCP/port/porttcp.c)
Downstream forks and embedded projects reusing the LINUXTCP port implementation
Modbus TCP server applications built on the cwalter-at/freemodbus codebase

Discovery Timeline

2026-08-05 - CVE-2026-71263 published to the National Vulnerability Database (NVD)
2026-08-05 - Last updated in NVD database

Technical Details for CVE-2026-71263

Vulnerability Analysis

The defect resides in the Modbus TCP frame reception loop of xMBPortTCPPool(). The function reads the MBAP header, extracts the Length field, and uses it to compute usTCPFrameBytesLeft, which represents the number of payload bytes to receive after the 7-byte MBAP header. The subsequent recv() call writes those bytes into aucTCPBuf starting at offset 7. The buffer aucTCPBuf is a static array sized to MB_TCP_BUF_SIZE, which is 263 bytes. Because aucTCPBuf is a static variable, the overflow does not corrupt a stack frame but instead corrupts an adjacent static variable, usTCPBufPos, which tracks the current receive position for the next call.

Root Cause

The root cause is a classic off-by-one comparison error. The code path evaluates if (usTCPFrameBytesLeft > MB_TCP_BUF_SIZE) to reject oversized frames. The correct predicate must be >= when the value being compared represents a length that will be written starting at a nonzero offset, or the comparison must include the header offset in the calculation. When an attacker sets the MBAP Length field to 264, usTCPFrameBytesLeft becomes 263, which passes the flawed check. The subsequent write starts at offset 7 and extends 7 bytes past the end of the 263-byte buffer.

Attack Vector

Modbus TCP has no built-in authentication, so any host able to reach the target on TCP port 502 can send the malicious frame. An attacker constructs an MBAP header with the Length field set to 264 and sends the corresponding payload in a single TCP segment. The overflow corrupts usTCPBufPos, which controls subsequent buffer positioning. Manipulating that variable enables further out-of-bounds writes on later frames, corruption of parser state, and denial of service. The unauthenticated network reachability and low attack complexity make exploitation trivial on exposed operational technology (OT) networks.

Code examples are not published for this issue. Technical details are available in the GitHub Freemodbus TCP Port Example.

Detection Methods for CVE-2026-71263

Indicators of Compromise

Inbound Modbus TCP packets on port 502 with an MBAP Length field greater than the protocol maximum of 260 bytes
Modbus TCP servers exhibiting unexpected restarts, parser desynchronization, or dropped connections following a single malformed frame
Anomalous static-memory write patterns in embedded devices running FreeModbus-derived stacks

Detection Strategies

Deploy deep packet inspection rules on OT-aware intrusion detection systems (IDS) such as Snort, Suricata, or Zeek that validate the MBAP Length field against protocol limits
Monitor for Modbus TCP frames with Length values above 260 and alert on any single-packet frames of 271 bytes or more targeting port 502
Correlate Modbus TCP protocol violations with subsequent process anomalies on the same host to identify successful exploitation

Monitoring Recommendations

Log all Modbus TCP sessions at network chokepoints between IT and OT zones and retain MBAP header fields for forensic review
Baseline expected Modbus TCP peers and alert on new or unauthorized sources initiating sessions to affected devices
Instrument affected servers with runtime memory protections such as stack canaries or address sanitizer (ASan) in test builds to catch overflow attempts during validation

How to Mitigate CVE-2026-71263

Immediate Actions Required

Block or restrict TCP port 502 at network boundaries to trusted management hosts only
Audit deployed firmware and applications for use of the FreeModbus LINUXTCP port and inventory affected assets
Patch the comparison in xMBPortTCPPool() to use >= or to correctly account for the 7-byte MBAP header offset

Patch Information

No official upstream patch release date is recorded in the NVD entry. Operators must apply a source-level fix to demo/LINUXTCP/port/porttcp.c by changing the bounds check to reject frames where usTCPFrameBytesLeft is greater than or equal to the payload capacity, or by re-deriving the check as (usTCPFrameBytesLeft + MB_TCP_UID) > MB_TCP_BUF_SIZE. Rebuild and redeploy all downstream binaries. Reference the upstream source at the GitHub Freemodbus Repository.

Workarounds

Place affected Modbus TCP endpoints behind an OT firewall that enforces MBAP Length field validation and drops malformed frames
Terminate Modbus TCP inside a protocol-aware gateway or VPN that authenticates peers before forwarding to legacy devices
Disable exposure of Modbus TCP to any interface reachable from untrusted networks until a patched build is deployed

bash

# Configuration example: restrict Modbus TCP to a management subnet with iptables
iptables -A INPUT -p tcp --dport 502 -s 10.20.30.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 502 -j DROP