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Fencer Intel
medium 4.4

CVE-2021-43840 json

Path traversal in message_bus

published 4y ago modified 4mo ago also GHSA-xmgj-5fh3-xjmm , EUVD-2021-2612

cvss
4.4
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
1.9%
EPSS 1.9% chance of exploitation in 30 days, percentile 80%
public exploits
0
none known

Summary

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.

Patch / fix

  • rubygems message_bus 3.3.7
  • discourse message_bus 3.3.7

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Exploit prediction

1.95% chance of exploitation in the next 30 days, higher than 80% of all scored CVEs (FIRST EPSS, 2026-11-09).

CVSS scores

assigned by version score vector
CNA 3.1 4.4 CVSS:3.1/AV:N/AC:H/PR:H/UI:N/S:U/C:H/I:N/A:N
NVD 3.1 6.5 CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N
NVD 2.0 3.5 AV:N/AC:M/Au:S/C:P/I:N/A:N
GitHub 3.1 4.2 CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:H/I:N/A:N

Affected packages

ecosystem package affected fixed in
rubygems message_bus
<3.3.7
3.3.7

Affected products

  • discourse message_bus

Classification

Weakness
  • CWE-22 Improper Limitation of a Pathname to a Restricted Directory

References

Sources

Fencer Intel keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.