CVE-2021-43840

Path traversal when MessageBus::Diagnostics is enabled

Workarounds

Disable MessageBus::Diagnostics in production like environments.

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.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2021-43840 has a CVSS score of 4.2 (Medium). The vector is network-reachable, high privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (3.3.7); upgrading removes the vulnerable code path.

Kodem intelligence

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Patched in 3.3.7.

1. What is CVE-2021-43840? CVE-2021-43840 is a medium-severity path traversal vulnerability in message_bus (rubygems), affecting versions < 3.3.7. It is fixed in 3.3.7. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2021-43840? CVE-2021-43840 has a CVSS score of 4.2 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of message_bus are affected by CVE-2021-43840? message_bus (rubygems) versions < 3.3.7 is affected.
4. Is there a fix for CVE-2021-43840? Yes. CVE-2021-43840 is fixed in 3.3.7. Upgrade to this version or later.
5. Is CVE-2021-43840 exploitable, and should I be worried? Whether CVE-2021-43840 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-43840 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-43840? Upgrade message_bus to 3.3.7 or later.