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.

Recommendation

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.

References

• https://github.com/divshot/slashify
• https://www.npmjs.com/package/slashify
• https://nvd.nist.gov/vuln/detail/CVE-2021-3189
• https://securitylab.github.com/advisories/GHSL-2020-199-open-redirect-slashify
• https://security.netapp.com/advisory/ntap-20210401-0004/