Summary
Nokogiri::XML::Schema trusts input by default, exposing risk of XXE vulnerability
Severity
Nokogiri maintainers have evaluated this as Low Severity (CVSS3 2.6).
Description
In Nokogiri versions <= 1.11.0.rc3, XML Schemas parsed by Nokogiri::XML::Schema are trusted by default, allowing external resources to be accessed over the network, potentially enabling XXE or SSRF attacks.
This behavior is counter to the security policy followed by Nokogiri maintainers, which is to treat all input as untrusted by default whenever possible.
Please note that this security fix was pushed into a new minor version, 1.11.x, rather than a patch release to the 1.10.x branch, because it is a breaking change for some schemas and the risk was assessed to be "Low Severity".
Affected Versions
Nokogiri <= 1.10.10 as well as prereleases 1.11.0.rc1, 1.11.0.rc2, and 1.11.0.rc3
Mitigation
There are no known workarounds for affected versions. Upgrade to Nokogiri 1.11.0.rc4 or later.
If, after upgrading to 1.11.0.rc4 or later, you wish to re-enable network access for resolution of external resources (i.e., return to the previous behavior):
- Ensure the input is trusted. Do not enable this option for untrusted input.
- When invoking the
Nokogiri::XML::Schemaconstructor, pass as the second parameter an instance ofNokogiri::XML::ParseOptionswith theNONETflag turned off.
So if your previous code was:
# in v1.11.0.rc3 and earlier, this call allows resources to be accessed over the network
# but in v1.11.0.rc4 and later, this call will disallow network access for external resources
schema = Nokogiri::XML::Schema.new(schema)
# in v1.11.0.rc4 and later, the following is equivalent to the code above
# (the second parameter is optional, and this demonstrates its default value)
schema = Nokogiri::XML::Schema.new(schema, Nokogiri::XML::ParseOptions::DEFAULT_SCHEMA)
Then you can add the second parameter to indicate that the input is trusted by changing it to:
# in v1.11.0.rc3 and earlier, this would raise an ArgumentError
# but in v1.11.0.rc4 and later, this allows resources to be accessed over the network
schema = Nokogiri::XML::Schema.new(trusted_schema, Nokogiri::XML::ParseOptions.new.nononet)
References
- This issue's public advisory
- Original Hackerone report (private)
- OWASP description of XXE attack
- OWASP description of SSRF attack
Credit
This vulnerability was independently reported by @eric-therond and @gucki.
The Nokogiri maintainers would like to thank HackerOne for providing a secure, responsible mechanism for reporting, and for providing their fantastic service to us.
Impact
An XML parser processes external entity references in untrusted input, causing the server to fetch internal resources or remote URLs. Typical impact: local file disclosure, server-side request forgery, or denial of service.
CVE-2020-26247 has a CVSS score of 4.3 (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 (1.11.0); 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.
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Frequently Asked Questions
- What is CVE-2020-26247? CVE-2020-26247 is a medium-severity XML external entity injection (XXE) vulnerability in nokogiri (rubygems), affecting versions <= 1.10.10. It is fixed in 1.11.0. An XML parser processes external entity references in untrusted input, causing the server to fetch internal resources or remote URLs.
- How severe is CVE-2020-26247? CVE-2020-26247 has a CVSS score of 4.3 (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 nokogiri are affected by CVE-2020-26247? nokogiri (rubygems) versions <= 1.10.10 is affected.
- Is there a fix for CVE-2020-26247? Yes. CVE-2020-26247 is fixed in 1.11.0. Upgrade to this version or later.
- Is CVE-2020-26247 exploitable, and should I be worried? Whether CVE-2020-26247 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-2020-26247 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-2020-26247? Upgrade
nokogirito 1.11.0 or later.