joi: quadratic-time split in `string().email({ multiple: true })` allows denial of service

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:

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:

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 affected.

Workarounds for users who cannot upgrade:

• Add .max() before .email(), for example Joi.string().max(1000).email({ multiple: true }). The max rule then rejects long values before the email rule runs.
• Do not use the abortEarly: false option with this schema. With abortEarly: false, the email rule runs even when the max rule fails.
• Do not use .trim() as a workaround. It removes only the whitespace at the start and at the end of the value.

Proof of concept

const Joi = require('joi');

const schema = Joi.string().email({ multiple: true });
for (const n of [25000, 50000, 100000, 200000]) {
    const start = Date.now();
    schema.validate(' '.repeat(n));
    console.log(`${n} spaces: ${Date.now() - start} ms`);
}