Impact

• using string-to-date parsing in moment (more specifically rfc2822 parsing, which is tried by default) has quadratic (N^2) complexity on specific inputs
• noticeable slowdown is observed with inputs above 10k characters
• users who pass user-provided strings without sanity length checks to moment constructor are vulnerable to (Re)DoS attacks

Patches

The problem is patched in 2.29.4, the patch can be applied to all affected versions with minimal tweaking.

Workarounds

In general, given the proliferation of ReDoS attacks, it makes sense to limit the length of the user input to something sane, like 200 characters or less. I haven't seen legitimate cases of date-time strings longer than that, so all moment users who do pass a user-originating string to constructor are encouraged to apply such a rudimentary filter, that would help with this but also most future ReDoS vulnerabilities.

References

There is an excellent writeup of the issue here: #6015 (comment)

Details

The issue is rooted in the code that removes legacy comments (stuff inside parenthesis) from strings during rfc2822 parsing. moment("(".repeat(500000)) will take a few minutes to process, which is unacceptable.