Summary
Workarounds
User can check for "proto" in the XML string before parsing it to the parser.
References
https://gist.github.com/Sudistark/a5a45bd0804d522a1392cb5023aa7ef7
Impact
As a part of this vulnerability, user was able to se code using __proto__ as a tag or attribute name.
const { XMLParser, XMLBuilder, XMLValidator} = require("fast-xml-parser");
let XMLdata = "<__proto__><polluted>hacked</polluted></__proto__>"
const parser = new XMLParser();
let jObj = parser.parse(XMLdata);
console.log(jObj.polluted) // should return hacked
CVE-2023-26920 has a CVSS score of 6.5 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.1.2); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
The problem has been patched in v4.1.2
Frequently Asked Questions
- What is CVE-2023-26920? CVE-2023-26920 is a medium-severity security vulnerability in fast-xml-parser (npm), affecting versions < 4.1.2. It is fixed in 4.1.2.
- How severe is CVE-2023-26920? CVE-2023-26920 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of fast-xml-parser are affected by CVE-2023-26920? fast-xml-parser (npm) versions < 4.1.2 is affected.
- Is there a fix for CVE-2023-26920? Yes. CVE-2023-26920 is fixed in 4.1.2. Upgrade to this version or later.
- Is CVE-2023-26920 exploitable, and should I be worried? Whether CVE-2023-26920 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2023-26920 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2023-26920? Upgrade
fast-xml-parserto 4.1.2 or later.