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How the CVE Works
This vulnerability resides in Nokogiri’s CRuby native extension and is triggered when replacing the value of an XML attribute.
The root cause is a use-after-free condition. When Ruby code calls `Nokogiri::XML::Attrvalue=` or `content=` to change an attribute’s value, the underlying native (libxml2) child node representing the attribute’s text content is freed and replaced.
The problem occurs if the application had previously accessed that specific attribute’s child node directly (e.g., via Attrchild). In this scenario, a Ruby wrapper object for that child node still exists and remains reachable through Nokogiri’s internal document node cache.
After the attribute value is replaced, this Ruby wrapper points to memory that has already been freed. Any subsequent use of this wrapper—whether by the application logic or during Ruby’s Garbage Collection (GC) mark phase—will dereference this invalid pointer. This results in an invalid memory read and typically causes the Ruby process to crash with a segmentation fault.
Nokogiri version 1.19.4 fixes this issue by preserving any already-wrapped attribute child nodes before performing the value replacement, ensuring they remain valid. It is important to note that the JRuby implementation is not affected by this vulnerability.
DailyCVE Form
Platform: CRuby
Version: < 1.19.4
Vulnerability: Use-After-Free
Severity: Low
Date: 2026-06-18
Prediction: 2026-06-18
What Undercode Say: Analytics
To understand the vulnerability, one can analyze the behavior using debugging tools like `gdb` or by inspecting memory addresses. The following conceptual commands illustrate how one might investigate the issue.
Conceptual Analysis Commands:
Run Ruby with gdb to catch the segfault
gdb --args ruby -e "require 'nokogiri'; doc = Nokogiri::XML('<root attr=\"value\"/>'); attr = doc.root.attribute_nodes.first; child = attr.child; puts child; attr.value = 'new_value'; puts child"
Check for memory access violations with Valgrind
valgrind --tool=memcheck ruby -e "require 'nokogiri'; doc = Nokogiri::XML('<root attr=\"value\"/>'); attr = doc.root.attribute_nodes.first; child = attr.child; attr.value = 'new_value'; puts child"
Code Triggering the Vulnerability:
require 'nokogiri'
doc = Nokogiri::XML('<root attr="value"/>')
attr = doc.root.attribute_nodes.first
1. Access the attribute's child node (the text node "value")
child = attr.child
puts "Child before: {child}" => <Nokogiri::XML::Text:0x...>
2. Replace the attribute's value
This frees the underlying native node that 'child' points to
attr.value = 'new_value'
3. Use the freed child node
This dereferences an invalid pointer, leading to a segfault
puts "Child after: {child}" CRASH: invalid memory read
Exploit
While this vulnerability is classified as Low severity, it can be exploited in a specific, non-standard API usage pattern.
Requirements for Exploitation:
- The application must directly access an attribute’s child node (e.g., using `Attrchild` or similar methods).
- The application must then replace the value of that same attribute using `Attrvalue=` or
content=.
Result: A subsequent use of the previously accessed child node wrapper will cause a denial of service (application crash) due to a segmentation fault.
Protection
Mitigation:
- Upgrade to Nokogiri version 1.19.4 or later.
Workaround:
- Avoid accessing an attribute’s child nodes directly (via `Attrchild` or similar) before mutating the value of that same attribute.
Impact
- Denial of Service (DoS): The primary impact is a crash of the Ruby application due to a segmentation fault.
- Affected Platforms: Only the CRuby implementation of Nokogiri is vulnerable. JRuby is not affected.
- Severity: Low because the vulnerability requires an unusual and non-standard API-usage pattern that is unlikely to occur during normal application operation.
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Sources:
Reported By: github.com
Extra Source Hub:
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