CVE-2021-3189

Open redirect in Slashify

The package is an Express middleware that normalises routes by stripping any final slash, redirecting, for example, bookings/latest/ to bookings/latest. However, it does not validate the path it redirects to in any way. In particular, if the path starts with two slashes (or two backslashes, or a slash and a backslash, etc.) it may redirect to a different domain.

Consider the example from the docs. Assume we have run it and started a server on localhost:3000, then visiting localhost:3000///github.com/ redirects you to https://github.com.

Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites. Typical impact: phishing and credential harvesting via a trusted domain.

CVE-2021-3189 has a CVSS score of 6.1 (Medium). The vector is network-reachable, no 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. No fixed version is listed yet, so configuration controls and monitoring matter more in the interim.

Kodem intelligence

Already deployed Kodem?

This vulnerability is currently un-patched in the slashify package so there is no known safe version of this package. Discontinuing use of slashify is recommended.

1. What is CVE-2021-3189? CVE-2021-3189 is a medium-severity open redirect vulnerability in slashify (npm), affecting versions <= 1.0.0. No fixed version is listed yet. Untrusted input controls a URL used for redirection, which can forward users to attacker-controlled sites.
2. How severe is CVE-2021-3189? CVE-2021-3189 has a CVSS score of 6.1 (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 slashify are affected by CVE-2021-3189? slashify (npm) versions <= 1.0.0 is affected.
4. Is there a fix for CVE-2021-3189? No fixed version is listed for CVE-2021-3189 yet. Monitor the advisory for updates and apply mitigations in the interim.
5. Is CVE-2021-3189 exploitable, and should I be worried? Whether CVE-2021-3189 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-2021-3189 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-2021-3189? No fixed version is listed yet. In the interim: Validate redirect destinations against an allowlist of permitted URLs or paths. Reject destinations that include an unexpected authority component.