CVE-2026-73649 in Velocity.js
Summary
by MITRE • 08/13/2026
Velocity.js is a JavaScript implementation of the Apache Velocity template engine. Prior to 2.1.7, the earlier fix for CVE-2026-44966 filtered constructor, __proto__, and prototype only in the #set assignment handler in src/compile/set.ts, while property-read expressions in src/compile/references.ts remained unfiltered. The getReferences() flow called getAttributes(), whose property access allowed an attacker-controlled template to traverse constructor.constructor to the JavaScript Function constructor. The #set handler validated only the assignment target and did not inspect the right-hand property-read expression, allowing arbitrary shell commands, environment-variable access, cloud-credential access, and internal-network access in the server process. This issue is fixed in version 2.1.7.
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Analysis
by VulDB Data Team • 08/13/2026
Velocity.js represents a JavaScript implementation of the Apache Velocity template engine that enables server-side template processing for web applications. The vulnerability described in CVE-2026-44966 demonstrates a critical security flaw in the template engine's handling of property access and assignment operations. This issue affects versions prior to 2.1.7 where an incomplete fix was implemented that only addressed constructor, prototype, and _proto_ filtering within the #set assignment handler located in src/compile/set.ts while leaving other code paths unaddressed.
The technical flaw stems from the getReferences() function flow which calls getAttributes() and allows attackers to traverse property access paths through constructor.constructor to reach the JavaScript Function constructor. This represents a classic prototype pollution vulnerability pattern that enables attackers to manipulate the prototype chain of objects within the template execution environment. The incomplete filtering approach created a security gap where the #set handler only validated assignment targets but failed to inspect right-hand side property-read expressions, leaving the system vulnerable to arbitrary code execution and privilege escalation attacks.
The operational impact of this vulnerability is severe as it allows attackers to execute arbitrary shell commands directly within the server process context. This capability enables unauthorized access to environment variables, cloud credentials stored in the server's memory, and internal network resources that the application process can reach. The vulnerability essentially provides attackers with a backdoor to the underlying system through template processing, potentially leading to complete compromise of the server and any associated data or services. The attack vector leverages JavaScript's prototype chain traversal mechanisms to bypass traditional input validation measures.
The fix implemented in version 2.1.7 addresses this by ensuring comprehensive filtering across all code paths including both assignment handlers and property-read expressions throughout the template compilation process. This complete remediation prevents traversal through constructor.constructor paths and eliminates the ability for attackers to gain elevated privileges or execute arbitrary code. Organizations should immediately upgrade to version 2.1.7 or later to mitigate this vulnerability, as the flaw represents a critical security risk that could lead to complete system compromise.
This vulnerability aligns with CWE-471 which describes the weakness of "Modification of Assumed-Immutable Data" and demonstrates characteristics similar to those found in ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript'. The issue represents a prototype pollution attack that leverages JavaScript's dynamic nature to manipulate object prototypes and gain unauthorized access to system resources. Security practitioners should monitor for related vulnerabilities in template engines and ensure comprehensive input validation across all code paths that handle user-supplied content, particularly in server-side template processing frameworks that may be susceptible to similar prototype chain traversal attacks.