CVE-2021-43840
Path traversal in message_bus
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.
Attack profile
How easy it is to exploit
- Attack vector
- Network
- Complexity
- High
- Privileges needed
- High
- User interaction
- None
What an attacker can affect
- Confidentiality
- High
- Integrity
- None
- Availability
- None
Exploit likelihood
What each source reports
CVE List Score used
Path traversal in message_bus
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.
- Record
CVE-2021-43840- CVSS 3.1
- 4.4 Medium
CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:N/A:N - Weakness
- CWE-22
- Updated
- 2026-11-02
EUVDEUVD-2021-2612
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.
- Record
EUVD-2021-2612- CVSS 3.1
- 4.4 Medium
CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:N/A:N - Updated
- 2026-11-03
GitHub AdvisoriesGHSA-xmgj-5fh3-xjmm
Path traversal when MessageBus::Diagnostics is enabled
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 merge_slashes 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.
- Record
GHSA-xmgj-5fh3-xjmm- CVSS 3.1
- 4.2 Medium
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:N/A:N - Weakness
- CWE-22
- Updated
- 2026-11-02
NVD
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.
- Record
CVE-2021-43840- CVSS 3.1
- 4.4 Medium
CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:N/A:N - CVSS 3.1
- 6.5 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N - CVSS 2.0
- 3.5 Low
AV:N/AC:M/Au:S/C:P/I:N/A:N - Weakness
- CWE-22
- Updated
- 2026-11-02
OSV
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.
- Record
CVE-2021-43840- CVSS 3.1
- 6.5 Medium
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N - Updated
- 2026-11-02
OSVGHSA-xmgj-5fh3-xjmm
Path traversal when MessageBus::Diagnostics is enabled
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 merge_slashes 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.
- Record
GHSA-xmgj-5fh3-xjmm- CVSS 3.1
- 4.2 Medium
CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:N/A:N - Weakness
- CWE-22
- Updated
- 2026-11-02
Affected products
Affected packages
| Package | Affected | Fixed |
|---|---|---|
message_busRubyGems |
before 3.3.7 | 3.3.7 |
message_busRubyGems |
before 3.3.7, plus listed versions 0.0.10.0.20.9.30.9.3.10.9.3.2 Show 45 moreShow less0.9.40.9.50.9.61.0.01.0.11.0.101.0.111.0.121.0.131.0.141.0.151.0.161.0.21.0.31.0.41.0.51.0.61.0.71.0.81.0.91.1.01.1.12.0.02.0.0.beta.12.0.0.beta.102.0.0.beta.112.0.0.beta.22.0.0.beta.32.0.0.beta.42.0.0.beta.52.0.0.beta.62.0.0.beta.72.0.0.beta.82.0.0.beta.92.0.12.0.22.0.32.0.52.0.62.0.72.0.82.0.92.1.02.1.12.1.2 |
3.3.7 |
History
| Recorded | Changed | Sources |
|---|---|---|
| aliases | EUVD | |
| first indexed | CVE List, GitHub Advisories, NVD, OSV |
Dates are when Every CVE recorded each change. Upstream dates may differ.
In short
CVE-2021-43840 is a medium vulnerability (CVSS 4.4) in Discourse Message Bus, published December 17, 2021. No source lists public exploit code. EPSS puts the chance of exploitation in the next 30 days at 1.95%, the 80th percentile. Fixed in message_bus 3.3.7.
References
github.com/discourse/message_bus/security/advisories/GHSA-xmgj-5fh3-xjmm
nvd.nist.gov/vuln/detail/CVE-2021-43840
Left out here: 22 versions. The JSON record has them.