CVE-2026-18691: Software Security Vulnerability Vulnerability

CRITICAL
Advisory Identifier: GHSA-jx4v-jf9c-3rh7
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CVSS 8.8
CWE-757
Published
Advisory Summary

CVE-2026-18691 is a CRITICAL (CVSS 8.8) security vulnerability vulnerability (CWE-757: Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade')) affecting Software libraries in the Maven ecosystem. The weakness occurs when a protocol or its implementation supports interaction between multiple actors and allows those actors to negotiate which algorithm should be used as a protection mechanism such as encryption or authentication, but it does not select the strongest algorithm that is available to both parties.. Remediation is available in the latest vendor maintenance releases.

CVE-2026-18691 at a Glance

CVE Identifier CVE-2026-18691
GHSA Identifier GHSA-jx4v-jf9c-3rh7
Severity CRITICAL
CVSS v3.1 Base Score 8.8
CWE Classification CWE-757: Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade')
Affected Product Software
Package Ecosystem Maven
Published Date
Last Updated

Security Impact

CVSS: 8.8

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

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

How to Fix CVE-2026-18691

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-2026-18691, 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-757: Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade'), where a protocol or its implementation supports interaction between multiple actors and allows those actors to negotiate which algorithm should be used as a protection mechanism such as encryption or authentication, but it does not select the strongest algorithm that is available to both parties. 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

An issue in MongoDB Server's intra-cluster connection setup could allow a party with suitable network access to influence which authentication mechanism is used when one replica set member connects to another. Under certain conditions, this could cause the cluster's shared internal credential to be transmitted in a less-protected form, potentially allowing that credential to be recovered. If recovered, the credential could be used to authenticate as the internal superuser to nodes in the deployment.

References

NVD — CVE-2026-18691 NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-18691
jira.mongodb.org/browse/server-13026... WEB
https://jira.mongodb.org/browse/SERVER-130264

Frequently Asked Questions

What is CVE-2026-18691?
CVE-2026-18691 is a security vulnerability in Software classified as a security vulnerability vulnerability under CWE-757: Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade').
Which Software versions are affected?
Affected versions include releases prior to current release patches.
What version fixes CVE-2026-18691?
Upgrading to current release patches (or the latest patched release for your respective branch) resolves the vulnerability.
What is the CVSS score of CVE-2026-18691?
CVE-2026-18691 has a CVSS v3.1 base score of 8.8 (CRITICAL) with vector string CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H.
What CWE is associated with CVE-2026-18691?
CVE-2026-18691 is classified under CWE-757: Selection of Less-Secure Algorithm During Negotiation ('Algorithm Downgrade') (A protocol or its implementation supports interaction between multiple actors and allows those actors to negotiate which algorithm should be used as a protection mechanism such as encryption or authentication, but it does not select the strongest algorithm that is available to both parties.).
How can I check Maven dependencies for CVE-2026-18691?
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-jx4v-jf9c-3rh7",
  "modified" : "2026-08-11T21:33:06Z",
  "published" : "2026-08-11T21:33:06Z",
  "aliases" : [ "CVE-2026-18691" ],
  "details" : "An issue in MongoDB Server's intra-cluster connection setup could allow a party with suitable network access to influence which authentication mechanism is used when one replica set member connects to another. Under certain conditions, this could cause the cluster's shared internal credential to be transmitted in a less-protected form, potentially allowing that credential to be recovered. If recovered, the credential could be used to authenticate as the internal superuser to nodes in the deployment.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H"
  }, {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:A/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:H/SI:H/SA:H/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X"
  } ],
  "affected" : [ ],
  "references" : [ {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-18691"
  }, {
    "type" : "WEB",
    "url" : "https://jira.mongodb.org/browse/SERVER-130264"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-757" ],
    "severity" : "CRITICAL",
    "github_reviewed" : false,
    "github_reviewed_at" : null,
    "nvd_published_at" : "2026-08-11T19:17:22Z"
  }
}