CVE-2026-83606 in xmldom
Summary
by MITRE • 09/01/2026
xmldom is a pure JavaScript W3C standard-based (XML DOM Level 2 Core) DOMParser and XMLSerializer module. From 0.9.0-beta.9 until 0.9.11, the processing-instruction production in lib/grammar.js lets the greedy S+ separator and lazy Char*? data group repeatedly repartition a long whitespace tail when the required closing ?> is absent. Both parsePI and parseProcessingInstruction apply the expression to the entire remaining source, causing quadratic backtracking during DOMParser.parseFromString() under default options and allowing a small unauthenticated XML input to stall the Node.js event loop. This issue is fixed in @xmldom/xmldom version 0.9.11.
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Analysis
by VulDB Data Team • 09/01/2026
The vulnerability identified in the xmldom library, specifically affecting versions from 0.9.0-beta.9 through 0.9.11, represents a critical Regular Expression Denial of Service (ReDoS) flaw rooted in the parsing logic for processing instructions within XML documents. As a pure JavaScript implementation adhering to W3C DOM Level 2 Core standards, xmldom is widely utilized in Node.js environments and browser-based applications that require robust XML parsing capabilities. The core technical defect resides in the grammar definition located in lib/grammar.js, where the regular expression used to parse processing instructions contains structural ambiguities involving greedy quantifiers for whitespace separators and lazy optional groups for character data. This combination creates a scenario susceptible to catastrophic backtracking when the input does not conform strictly to expected XML syntax rules.
The operational mechanism of this vulnerability is triggered specifically during the execution of DOMParser.parseFromString() under default configuration options. When an attacker provides a small but carefully crafted unauthenticated XML payload that lacks the required closing sequence for processing instructions, such as ?>, the parser attempts to match the input against the flawed regular expression. The greedy S+ separator and the lazy Char*? data group interact in a way that causes the regex engine to repeatedly repartition long sequences of whitespace at the end of the input string. This interaction forces the underlying JavaScript runtime into quadratic time complexity behavior, leading to exponential backtracking as the engine tries every possible combination of matches before failing or succeeding.
The impact of this flaw is severe in production environments that rely on xmldom for processing user-supplied XML data. Because Node.js operates on a single-threaded event loop model, any operation that blocks execution for an extended period prevents the application from handling other requests or events. A small unauthenticated input can thus stall the entire server process, resulting in a complete denial of service for all users connected to the affected system. This vulnerability is particularly dangerous because it requires no authentication and minimal payload size, making it easy to exploit via standard HTTP POST requests or any interface that accepts XML data without prior validation.
From a classification perspective, this issue aligns with CWE-400, which covers Uncontrolled Resource Consumption, specifically manifesting as a Regular Expression Denial of Service (ReDoS). It also maps to the MITRE ATT&CK technique T1496, Resource Hijacking, where an attacker consumes computational resources to disrupt service availability. The flaw highlights the risks associated with using complex regular expressions for parsing structured data formats like XML without proper safeguards against pathological inputs.
To mitigate this vulnerability and prevent recurrence in similar systems, it is imperative to upgrade immediately to @xmldom/xmldom version 0.9.11 or later, where the grammar has been corrected to eliminate the backtracking issue. For applications that cannot yet update their dependencies, implementing strict input validation before passing XML strings to the parser can serve as a temporary defense-in-depth measure. This includes enforcing maximum length limits on incoming payloads and rejecting any XML content containing malformed processing instructions or excessive whitespace sequences in suspicious contexts. Additionally, integrating static analysis tools capable of detecting ReDoS patterns into the development pipeline can help identify similar regex vulnerabilities early in the software lifecycle.