CVE-2021-43840

What it is

message_bus is a messaging bus for Ruby processes and web clients. In versions prior to 3.3.7 users who deployed message bus with diagnostics features enabled (default off) are vulnerable to a path traversal bug, which could lead to disclosure of secret information on a machine if an unintended user were to gain access to the diagnostic route. The impact is also greater if there is no proxy for your web application as the number of steps up the directories is not bounded. For deployments which uses a proxy, the impact varies. For example, If a request goes through a proxy like Nginx with `merge_slashes` enabled, the number of steps up the directories that can be read is limited to 3 levels. This issue has been patched in version 3.3.7. Users unable to upgrade should ensure that MessageBus::Diagnostics is disabled.

At a glance

Exploitability

Can be exploited over the network with an admin-level account. High risk of data exposure.

Exploit odds

1.9% chance of exploitation in the next 30 days (EPSS).

Fix

A patch, advisory or fixed version is referenced. Upgrade or apply it.

Timeline

1. 2021-12-17 19:15 UTC PublishedDisclosed publicly on CVE.org and NVD
2. 2026-06-17 04:11 UTC Last updatedMost recent change to the record

Why it ranks here

• Exploited in the wild: no, CISA does not list it.
• Exploit odds: 1.9% in the next 30 days (EPSS).
• Severity: 4.4 MEDIUM (CVSS).
• Fix: a patch or advisory is linked.

Details

Published

2021-12-17 19:15 UTC

Last modified

2026-06-17 04:11 UTC

Exploit probability

1.9% · EPSS, next 30 days

Weakness

CWE-22 · Path traversal

Record status

Modified

Affected

• Discourse Message Bus < 3.3.7

Attack profile CVSS 3.1 · 4.4

Attack vector

Network

Complexity

High

Privileges needed

High

User interaction

None

Scope

Unchanged

Confidentiality

High

Integrity

None

Availability

None

Is it in your code?

Hand this to your coding agent. It searches your project for the affected products and checks versions. Nothing leaves your machine.

Read https://cve-list.com/agent/cve/CVE-2021-43840.md and follow it. Check whether this project is affected by CVE-2021-43840. Read only. Don't change anything and don't upload my code. Tell me yes, no or not sure, with the file as proof.

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