For input validation on a website, are there any security concerns with disclosing to the user exactly what characters are valid or invalid for a given field?
CWE-200: Information Exposure says one should try not to disclose information "that could be useful in an attack but is normally not available to the attacker". A specific example would be preventing a system from exposing stack traces (addressed by CWE-209: Information Exposure Through an Error Message).
Should one ensure that error messages are vague, like "The text you entered contains invalid characters"?
Or is this a form of "security through obscurity" as the user/attacker can deduce what characters are valid by repeatedly submitting different characters to see if they produce errors?
Is it worth the hits to user experience to potentially slow down an attack?
There have been several questions (including answers that I went to the effort of writing comments on that have since been deleted) as to why one should be doing any input validation.
First off, thank you to @Loek for the comment pointing to OWASP's Application Security Verification Standard that provides guidance on passwords on page 23 and 24: "Verify that there are no password composition rules limiting the type of characters permitted. There should be no requirement for upper or lower case or numbers or special characters.".
I think we can all agree that limiting characters in a password is generally a bad idea (see @Merchako's answer). As @emory points out it probably can't be a hard and fast rule (e.g. I have seen many mobile apps that use an easier to use secondary "PIN" to secure the app even someone else has access to log into the device.) I didn't really have passwords in mind when I asked this question but that's one direction the comments and answers went. So for the purposes of this question let's consider it to be for non-password fields.
Input validation is part of "defense in depth" for websites, web services, and apps to prevent injection attacks. Injection attacks, as stated by OWASP, "can result in data loss, corruption, or disclosure to unauthorized parties, loss of accountability, or denial of access. Injection can sometimes lead to complete host takeover. The business impact depends on the needs of the application and data."
See OWASP's #1 vulnerability, A1-Injection, and CWE-20: Improper Input Validation for more detailed information. Note they both say input validation is not a complete defense but rather one layer of that "defense in depth" for software products.