Nokogiri (鋸) makes it easy and painless to work with XML and HTML from Ruby. It provides a sensible, easy-to-understand API for reading, writing, modifying, and querying documents. It is fast and standards-compliant by relying on native parsers like libxml2, libgumbo, or xerces.
Stack-buffer overflow is possible when reporting DTD validation errors if the input contains a long (~3kb) QName prefix.
CVE-2024-56171
Use-after-free is possible during validation against untrusted XML Schemas (.xsd) and, potentially, validation of untrusted documents against trusted Schemas if they make use of xsd:keyref in combination with recursively defined types that have additional identity constraints.
Nokogiri's CSS selector tokenizer contains regular expressions whose construction may result in exponential regex backtracking on adversarial selectors. Three ReDoS vectors are addressed in this release:
String-literal tokenization on certain unterminated quoted-string input.
String-literal tokenization on a separate class of hex-escape-rich input.
Identifier tokenization on hex-escape-rich input.
The public CSS selector methods that funnel through the affected tokenizer are Nokogiri::CSS.xpath_for, Node#css, Node#at_css, Searchable#search, and CSS::Parser#parse.
Mitigation
Upgrade to Nokogiri >= 1.19.3.
If users are unable to upgrade, two options are available:
Avoid the use of attacker-controlled text in CSS selectors. Applications that only pass developer-authored selectors to Nokogiri are not directly exposed.
Set global Regexp.timeout (Ruby 3.2+, JRuby 9.4+) to bound parse time.
Severity
The Nokogiri maintainers have evaluated this as High Severity (CVSS 7.5, AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).
An attacker able to inject user-supplied text into a CSS selector parse method can cause exponential backtracking, resulting in a potential denial of service.
Nokogiri's Nokogiri::XSLT::Stylesheet#transform leaks a small heap allocation when passed a Ruby string parameter containing a null byte.
For applications that pass attacker-controlled input through XSLT.transform parameters, this may be a vector for a denial of service attack against long-running processes.
Mitigation
Upgrade to Nokogiri >= 1.19.3.
Users may also be able to mitigate this issue without upgrading by validating untrusted transform parameters before passing them to Nokogiri::XSLT::Stylesheet#transform.
Severity
The Nokogiri maintainers have evaluated this as Moderate Severity, CVSS 5.3.
Each leaked allocation is approximately 24–32 bytes, so meaningful memory growth requires sustained attacker-controlled traffic at high call rates. The bug does not cause memory corruption, information disclosure, or any change in the behavior of the transform itself, and the string-handling exception is raised as expected.
Applications that do not pass raw attacker-controlled bytes to XSLT parameters are unlikely to be affected in practice.
Please note that this advisory only applies to the CRuby implementation of Nokogiri, and only if the packaged libraries are being used. If you've overridden defaults at installation time to use system libraries instead of packaged libraries, you should instead pay attention to your distro's libxml2 release announcements.
JRuby users are not affected.
Severity
The Nokogiri maintainers have evaluated this as Moderate.
Impact
From the CVE description, this issue applies to the xmlTextReader module (which underlies Nokogiri::XML::Reader):
When using the XML Reader interface with DTD validation and XInclude expansion enabled, processing crafted XML documents can lead to an xmlValidatePopElement use-after-free.
Mitigation
Upgrade to Nokogiri ~> 1.15.6 or >= 1.16.2.
Users who are unable to upgrade Nokogiri may also choose a more complicated mitigation: compile and link Nokogiri against patched external libxml2 libraries which will also address these same issues.
Nokogiri's CRuby extension fails to check the return value from xmlC14NExecute in the method Nokogiri::XML::Document#canonicalize and Nokogiri::XML::Node#canonicalize. When canonicalization fails, an empty string is returned instead of raising an exception. This incorrect return value may allow downstream libraries to accept invalid or incomplete canonicalized XML, which has been demonstrated to enable signature validation bypass in SAML libraries.
JRuby is not affected, as the Java implementation correctly raises RuntimeError on canonicalization failure.
Mitigation
Upgrade to Nokogiri >= 1.19.1.
Severity
The maintainers have assessed this as Medium severity. Nokogiri itself is a parsing library without a clear security boundary related to canonicalization, so the direct impact is that a method returns incorrect data on invalid input. However, this behavior was exploited in practice to bypass SAML signature validation in downstream libraries (see References).
Credit
This vulnerability was responsibly reported by HackerOne researcher d4d.
Nokogiri v1.18.9 patches the vendored libxml2 to address CVE-2025-6021, CVE-2025-6170, CVE-2025-49794, CVE-2025-49795, and CVE-2025-49796.
Impact and severity
CVE-2025-6021
A flaw was found in libxml2's xmlBuildQName function, where integer overflows in buffer size calculations can lead to a stack-based buffer overflow. This issue can result in memory corruption or a denial of service when processing crafted input.
NVD claims a severity of 7.5 High (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
A flaw was found in the interactive shell of the xmllint command-line tool, used for parsing XML files. When a user inputs an overly long command, the program does not check the input size properly, which can cause it to crash. This issue might allow attackers to run harmful code in rare configurations without modern protections.
NVD claims a severity of 2.5 Low (CVSS:3.1/AV:L/AC:H/PR:N/UI:R/S:U/C:N/I:N/A:L)
A use-after-free vulnerability was found in libxml2. This issue occurs when parsing XPath elements under certain circumstances when the XML schematron has the schema elements. This flaw allows a malicious actor to craft a malicious XML document used as input for libxml, resulting in the program's crash using libxml or other possible undefined behaviors.
NVD claims a severity of 9.1 Critical (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H)
A NULL pointer dereference vulnerability was found in libxml2 when processing XPath XML expressions. This flaw allows an attacker to craft a malicious XML input to libxml2, leading to a denial of service.
NVD claims a severity of 7.5 High (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
A vulnerability was found in libxml2. Processing certain sch:name elements from the input XML file can trigger a memory corruption issue. This flaw allows an attacker to craft a malicious XML input file that can lead libxml to crash, resulting in a denial of service or other possible undefined behavior due to sensitive data being corrupted in memory.
NVD claims a severity of 9.1 Critical (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H)
Nokogiri < 1.18.9 when using CRuby (MRI) with vendored libxml2
Patched Versions
Nokogiri >= 1.18.9
Mitigation
Upgrade to Nokogiri v1.18.9 or later.
Users who are unable to upgrade Nokogiri may also choose a more complicated mitigation: compile and link Nokogiri against patched external libxml2 libraries which will also address these same issues.
In libxml2 before 2.13.8 and 2.14.x before 2.14.2, out-of-bounds memory access can occur in the Python API (Python bindings) because of an incorrect return value. This occurs in xmlPythonFileRead and xmlPythonFileReadRaw because of a difference between bytes and characters.
There is no impact from this CVE for Nokogiri users.
CVE-2025-32415: Low impact
In libxml2 before 2.13.8 and 2.14.x before 2.14.2, xmlSchemaIDCFillNodeTables in xmlschemas.c has a heap-based buffer under-read. To exploit this, a crafted XML document must be validated against an XML schema with certain identity constraints, or a crafted XML schema must be used.
In the upstream issue, further context is provided by the maintainer:
The bug affects validation against untrusted XML Schemas (.xsd) and validation of untrusted documents against trusted Schemas if they make use of xsd:keyref in combination with recursively defined types that have additional identity constraints.
MITRE has published a severity score of 2.9 LOW (CVSS:3.1/AV:L/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L) for this CVE.
Stack-buffer overflow is possible when reporting DTD validation errors if the input contains a long (~3kb) QName prefix.
CVE-2024-56171
Use-after-free is possible during validation against untrusted XML Schemas (.xsd) and, potentially, validation of untrusted documents against trusted Schemas if they make use of xsd:keyref in combination with recursively defined types that have additional identity constraints.
Please note that this advisory only applies to the CRuby implementation of Nokogiri, and only if the packaged libraries are being used. If you've overridden defaults at installation time to use system libraries instead of packaged libraries, you should instead pay attention to your distro's libxml2 release announcements.
JRuby users are not affected.
Severity
The Nokogiri maintainers have evaluated this as Moderate.
Impact
From the CVE description, this issue applies to the xmlTextReader module (which underlies Nokogiri::XML::Reader):
When using the XML Reader interface with DTD validation and XInclude expansion enabled, processing crafted XML documents can lead to an xmlValidatePopElement use-after-free.
Mitigation
Upgrade to Nokogiri ~> 1.15.6 or >= 1.16.2.
Users who are unable to upgrade Nokogiri may also choose a more complicated mitigation: compile and link Nokogiri against patched external libxml2 libraries which will also address these same issues.
