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

> 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.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 15 Jun 2023

## 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 ::= ":" | [A-Z] | "_" | [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 | "-" | "." | [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 | Ecosystem | 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.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-gpv5-7x3g-ghjv and fix it if it is affected.

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

Affected packages and fixed versions:
- fast-xml-parser (npm): affected from 4.2.4; fixed in 4.2.5
- fast-xml-parser (npm): affected from 4.2.4; fixed in 4.2.5

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-gpv5-7x3g-ghjv/ (Markdown: https://insights.sbomify.com/GHSA-gpv5-7x3g-ghjv/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-gpv5-7x3g-ghjv

---
From sbomify Insights: https://insights.sbomify.com/GHSA-gpv5-7x3g-ghjv/
Summaries are generated and may contain mistakes; the official description above is the record.