joi is a schema description language and data validator for JavaScript. From 17.1.1 until 17.13.9 and 18.3.1, the Joi.string().email({ multiple: true }) rule uses a separator regular expression whose leading whitespace quantifier is retried at every position in a long whitespace run that contains no separator, causing quadratic work during String.prototype.split. The behavior applies to the default separator and every custom separator. Applications that validate attacker-controlled values with the multiple option and no effective prior length limit can have the Node.js event loop blocked for seconds by one request, preventing other work. A .max() rule is not sufficient when abortEarly: false allows the email rule to run after the maximum-length check fails. This vulnerability is fixed in versions 17.13.9 and 18.3.1.

Affected: npm: joi, npm: @hapi/joi

### Summary

`Joi.string().email({ multiple: true })` takes quadratic time to split a value that contains a long run of whitespace. One request with 100 KB of spaces blocks the Node.js event loop for several seconds, which causes a denial of service.

### Details

When the `multiple` option is set, the `email` rule splits the value into addresses with a regular expression that removes whitespace on each side of the separator:

```js const regex = new RegExp(`\\s*[${options.separator ? escapeRegex(options.separator) : ','}]\\s*`); // ... const emails = options.multiple ? value.split(regex) : [value]; ```

`String.prototype.split` tries a match at each position of the value. At each position inside a whitespace run, the leading `\s*` reads the rest of the run, does not find a separator, and backtracks through all of it. A run of `n` whitespace characters without a separator therefore costs `n²` steps. This applies to the default separator and to all custom `separator` values.

The fix lets the leading whitespace match start only at the beginning of a whitespace run, so the split is linear:

```js const regex = new RegExp(`(?:(?<!\\s)\\s+)?[${options.separator ? escapeRegex(options.separator) : ','}]\\s*`); ```

The split results do not change. The regular expression is now built only when `multiple` is set.

Every `joi` release from `17.1.1` up to and including `17.13.8`, and from `18.0.0` up to and including `18.3.0`, is affected. Every `@hapi/joi` release from `16.0.0` is af…