GHSA-gpv5-7x3g-ghjv fast-xml-parser regex vulnerability patch could be improved from a safety perspective

Not scored Published 15 Jun 2023

An attacker can exploit insufficient entity name validation in fast-xml-parser to potentially trigger XML External Entity attacks, leading to data exposure or service disruption.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is GHSA-gpv5-7x3g-ghjv?

GHSA-gpv5-7x3g-ghjv is a vulnerability in fast-xml-parser, published on 15 Jun 2023. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

Summary

This is a comment on https://github.com/NaturalIntelligence/fast-xml-parser/security/advisories/GHSA-6w63-h3fj-q4vw and the patches fixing it.

Details

The code which validates a name calls the validator: https://github.com/NaturalIntelligence/fast-xml-parser/blob/ecf6016f9b48aec1a921e673158be0773d07283e/src/xmlparser/DocTypeReader.js#L145-L153 This checks for the presence of an invalid character. Such an approach is always risky, as it is so easy to forget to include an invalid character in the list. A safer approach is to validate entity names against the XML specification: https://www.w3.org/TR/xml11/#sec-common-syn - an ENTITY name is a Name:

[4]   NameStartChar ::= &#34;:&#34; | [A-Z] | &#34;_&#34; | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] |
                        [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] |
                        [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]
[4a]  NameChar ::= NameStartChar | &#34;-&#34; | &#34;.&#34; | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
[5]   Name ::= NameStartChar (NameChar)*

so the safest way to validate an entity name is to build a regex to represent this expression and check whether the name given matches the regex. (Something along the lines of /^[name start char class][name char class]*$/.) There's probably a nice way to simplify the explicit list rather than typing it out verbatim using Unicode character properties, but I don't know enough to do so.

Which versions are affected by GHSA-gpv5-7x3g-ghjv?

2 open source packages ship an affected version, including fast-xml-parser.

Package	Affected	Fixed in
`fast-xml-parser` npm	From `4.2.4`	`4.2.5`
`fast-xml-parser` npm	From `4.2.4`	`4.2.5`

How do I fix GHSA-gpv5-7x3g-ghjv?

Upgrade fast-xml-parser to 4.2.5 or later.

Is GHSA-gpv5-7x3g-ghjv being exploited?

There is no known exploitation of GHSA-gpv5-7x3g-ghjv in the wild and no public exploit code.