Summary
Use of Insufficiently Random Values in Railties Allows Remote Code Execution
Possible Remote Code Execution Exploit in Rails Development Mode
Impact
With some knowledge of a target application it is possible for an attacker to guess the automatically generated development mode secret token. This secret token can be used in combination with other Rails internals to escalate to a remote code execution exploit.
All users running an affected release should either upgrade or use one of the workarounds immediately.
Releases
The 6.0.0.beta3 and 5.2.2.1 releases are available at the normal locations.
Workarounds
This issue can be mitigated by specifying a secret key in development mode.
In "config/environments/development.rb" add this:
config.secret_key_base = SecureRandom.hex(64)
Please note that only the 5.2.x, 5.1.x, 5.0.x, and 4.2.x series are supported at present. Users of earlier unsupported releases are advised to upgrade as soon as possible as we cannot guarantee the continued availability of security fixes for unsupported releases.
Credits
Thanks to ooooooo_q
Impact
Untrusted input is inserted into a command that is later executed by the application, allowing the attacker to alter the intent of that command. Typical impact: arbitrary command execution in the application's environment.
CVE-2019-5420 has a CVSS score of 9.8 (Critical). The vector is network-reachable, no privileges required, and no user interaction. 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 (5.2.2.1); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.
Frequently Asked Questions
- What is CVE-2019-5420? CVE-2019-5420 is a critical-severity command injection vulnerability in railties (rubygems), affecting versions >= 5.2.0, <= 5.2.2.0. It is fixed in 5.2.2.1. Untrusted input is inserted into a command that is later executed by the application, allowing the attacker to alter the intent of that command.
- How severe is CVE-2019-5420? CVE-2019-5420 has a CVSS score of 9.8 (Critical). 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.
- Which versions of railties are affected by CVE-2019-5420? railties (rubygems) versions >= 5.2.0, <= 5.2.2.0 is affected.
- Is there a fix for CVE-2019-5420? Yes. CVE-2019-5420 is fixed in 5.2.2.1. Upgrade to this version or later.
- Is CVE-2019-5420 exploitable, and should I be worried? Whether CVE-2019-5420 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
- What actually determines whether CVE-2019-5420 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.
- How do I fix CVE-2019-5420? Upgrade
railtiesto 5.2.2.1 or later.