Path traversal when MessageBus::Diagnostics is enabled

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.2 · CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:N/A:N

EPSS

• CVE-2021-43840: 1.95% estimated exploitation activity in the next 30 days; percentile 80.22 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact Users who deployed message bus with diagnostics features enabled (default off) were 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 mergeslashes enabled, the number of steps up the directories that can be read is limited to 3 levels. Patches Patched in 3.3.7. Workarounds Disable MessageBus::Diagnostics in production like environments.

Published by the source

17 Dec 2021, 19:59 UTC

Last modified by the source

4 May 2023, 19:54 UTC

Source last checked

26 Oct 2026, 15:47 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-xmgj-5fh3-xjmm

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# Patch Next advisory brief: CVE-2021-43840

Advisory: Path traversal when MessageBus::Diagnostics is enabled
IDs: CVE-2021-43840, GHSA-xmgj-5fh3-xjmm
Record: https://patchnext.com/advisories/cve-2021-43840 (revision 1, source checked 26 Oct 2026, 15:47 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: message_bus 3.3.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- RubyGems message_bus: affected < 3.3.7; fixed in 3.3.7

## Suggested inspection (read-only)
- RubyGems message_bus: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on message_bus.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-xmgj-5fh3-xjmm (GitHub-reviewed advisory GHSA-xmgj-5fh3-xjmm lists affected ranges: RubyGems message_bus >=0 <3.3.7)

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