Seroval: Script injection through RegExp sources in serialized output

Summary

seroval writes a RegExp as a regular expression literal. In affected versions, the < character in a RegExp source is written without escaping. A source built from untrusted input can then end the HTML <script> element that the serialized output is placed in, and run script in the page.

Details

seroval escapes < in strings, so string values cannot end a <script> element. RegExp sources did not get the same escaping:

• Versions before 1.0.0 wrote the source as is.
• Version 1.5.5 stopped escaping < in RegExp sources, so that named groups and lookbehinds keep working. This removed the escaping for every < in a source.

These functions are affected:

• serialize, serializeAsync
• crossSerialize, crossSerializeAsync, crossSerializeStream
• the Serializer class
• compileJSON

toJSON, fromJSON, toCrossJSON and fromCrossJSON are not affected, because they never produce JavaScript.

Impact

An application is affected when all of these are true:

1. It builds a RegExp from untrusted input that is used as regular expression syntax. Input passed through an escaping helper (such as an escapeRegExp function) is not enough to exploit this.
2. It serializes that RegExp with one of the functions above.
3. It places the output inside an HTML <script> element, as server rendering usually does.

In that case an attacker can run script in the page of any user who receives that output.

Patches

Fixed in 1.6.10. A RegExp whose source contains < is now written with the RegExp constructor and an escaped string source. Named groups and lookbehinds keep working.

Workarounds

If you cannot upgrade:

• Do not serialize a RegExp built from untrusted pattern syntax. Send its source and flags as strings, and build the RegExp on the client.
• Or reject patterns that contain < before they reach the serializer.