cwalter-at FreeModbus TCP Port porttcp.c xMBPortTCPPool Length off-by-one

Summaryinfo

A vulnerability was found in cwalter-at FreeModbus. It has been classified as critical. The impacted element is the function xMBPortTCPPool of the file demo/LINUXTCP/port/porttcp.c of the component TCP Port. The manipulation of the argument Length leads to off-by-one. This vulnerability is referenced as CVE-2026-71263. Remote exploitation of the attack is possible. No exploit is available.

Detailsinfo

A vulnerability was found in cwalter-at FreeModbus (unknown version) and classified as critical. This issue affects the function xMBPortTCPPool of the file demo/LINUXTCP/port/porttcp.c of the component TCP Port. The manipulation of the argument Length with an unknown input leads to a off-by-one vulnerability. Using CWE to declare the problem leads to CWE-193. A product calculates or uses an incorrect maximum or minimum value that is 1 more, or 1 less, than the correct value. Impacted is confidentiality, integrity, and availability. The summary by CVE is:

The LINUXTCP port of FreeModbus contains an off-by-one bounds check in xMBPortTCPPool() (demo/LINUXTCP/port/porttcp.c). The 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. An MBAP frame with a Length field of 264 makes usTCPFrameBytesLeft equal to 263, which passes the flawed check, and the subsequent recv() call writes up to 263 bytes starting at buffer offset 7 into the 263-byte static buffer aucTCPBuf, overflowing it by 7 bytes into the adjacent static variable usTCPBufPos. A single crafted, unauthenticated Modbus TCP packet triggers the overflow, since Modbus has no built-in authentication.

It is possible to read the advisory at github.com. The identification of this vulnerability is CVE-2026-71263 since 08/05/2026. The exploitation is known to be easy. The attack may be initiated remotely. No form of authentication is needed for a successful exploitation. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 08/05/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Vendor

• cwalter-at

Name

• FreeModbus

CPE 2.3info

• 🔒

CPE 2.2info

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