CVE-2026-18691: mongodb Algorithm Downgrade

Vulnerability summary

CVE-2026-18691 is a critical (CVSS 9.0) Algorithm Downgrade vulnerability in mongodb. EPSS estimates a 0.36% chance of exploitation in the next 30 days, higher than 27.9% of scored vulnerabilities.

Timeline

1. 11-08-2026
   PublishedVulnerability first disclosed.
2. 11-08-2026
   Last modifiedVulnerability information updated.

Description

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.

• CWE-757Selection 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.