CVE-2026-71263: Software Security Vulnerability Vulnerability

CRITICAL
Advisory Identifier: GHSA-cjvm-2pqq-6v6h
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CVSS 9.1
CWE-787
Published
Advisory Summary

CVE-2026-71263 is a CRITICAL (CVSS 9.1) security vulnerability vulnerability (CWE-787: Out-of-bounds Write) affecting Software libraries in the Maven ecosystem. The weakness occurs when the product writes data past the end, or before the beginning, of the intended buffer.. Remediation is available in the latest vendor maintenance releases.

CVE-2026-71263 at a Glance

CVE Identifier CVE-2026-71263
GHSA Identifier GHSA-cjvm-2pqq-6v6h
Severity CRITICAL
CVSS v3.1 Base Score 9.1
CWE Classification CWE-787: Out-of-bounds Write
Affected Product Software
Package Ecosystem Maven
Published Date
Last Updated

Security Impact

CVSS: 9.1

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity High (I:H)
Availability High (A:H)

How to Fix CVE-2026-71263

To remediate this vulnerability, upgrade all affected Software dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

mvn dependency:tree

How to Check if You Are Affected

To determine whether your Java application is affected by CVE-2026-71263, inspect your build dependencies using the Maven Dependency Plugin:

mvn dependency:tree

Analyze the terminal output to locate any version of Software in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than the patched release), your application is affected.

Note: The mvn dependency:tree command inspects the resolved dependency graph declared in your project's POM files; it does not perform automated security scanning or inspect runtime dynamic classpaths. Always verify runtime container classpaths and shaded JARs.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-787: Out-of-bounds Write, where the product writes data past the end, or before the beginning, of the intended buffer. In Software, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

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.

References

NVD — CVE-2026-71263 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-71263
Software Source Repository REPOSITORY
https://github.com/cwalter-at/freemodbus
Software Source Repository REPOSITORY
https://github.com/cwalter-at/freemodbus/blob/master/demo/LINUXTCP/port/porttcp.c

Frequently Asked Questions

What is CVE-2026-71263?
CVE-2026-71263 is a security vulnerability in Software classified as a security vulnerability vulnerability under CWE-787: Out-of-bounds Write.
Which Software versions are affected?
Affected versions include releases prior to current release patches.
What version fixes CVE-2026-71263?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
What is the CVSS score of CVE-2026-71263?
CVE-2026-71263 has a CVSS v3.1 base score of 9.1 (CRITICAL) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H.
What CWE is associated with CVE-2026-71263?
CVE-2026-71263 is classified under CWE-787: Out-of-bounds Write (The product writes data past the end, or before the beginning, of the intended buffer.).
How can I check Maven dependencies for CVE-2026-71263?
Run the dependency tree command: 'mvn dependency:tree'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-cjvm-2pqq-6v6h",
  "modified" : "2026-08-05T15:32:19Z",
  "published" : "2026-08-05T15:32:19Z",
  "aliases" : [ "CVE-2026-71263" ],
  "details" : "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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H"
  } ],
  "affected" : [ ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-71263"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/cwalter-at/freemodbus"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/cwalter-at/freemodbus/blob/master/demo/LINUXTCP/port/porttcp.c"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-787" ],
    "severity" : "CRITICAL",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2026-08-05T13:24:50Z"
  }
}