CVE-2026-40345 in deepmerge-ts
Summary
by MITRE • 08/20/2026
deepmerge-ts is a typescript library providing functionality to deep merging of javascript objects. Prior to 8.0.0, the deepmerge, deepmergeCustom, deepmergeInto, and deepmergeIntoCustom APIs do not track visited objects or object pairs when recursively merging records. When two input values contain self-references at the same property path, the merge logic repeatedly revisits the same pair until Node.js raises RangeError: Maximum call stack size exceeded. Applications that merge attacker-controlled recursive object graphs can synchronously crash the affected process or cause repeated worker restarts. Plain JSON input alone cannot create the recursive graph required to trigger the issue. This issue is fixed in version 8.0.0.
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Analysis
by VulDB Data Team • 08/20/2026
The deepmerge-ts library, a widely utilized TypeScript utility for performing deep merges of JavaScript objects, contains a critical vulnerability affecting versions prior to 8.0.0. The core functionality relies on recursive algorithms to traverse and merge nested object structures. However, the implementation lacks proper cycle detection mechanisms when handling complex object graphs. Specifically, the APIs including deepmerge, deepmergeCustom, deepmergeInto, and deepmergeIntoCustom fail to track visited objects or pairs of objects during recursion. This architectural oversight means that if two input values contain self-references at the same property path, the merge logic enters an infinite loop by repeatedly revisiting the same object pair.
From a technical perspective, this flaw represents a classic failure in handling recursive data structures without maintaining state about previously processed nodes. When the algorithm encounters a circular reference where an object references itself or forms a cycle with another merged object, it does not recognize that this path has already been explored. Consequently, the call stack grows indefinitely until Node.js enforces its maximum call stack size limit, resulting in a RangeError: Maximum call stack size exceeded exception. This is categorized under CWE-674 as Uncontrolled Recursion and aligns with ATT&CK technique T1053 Scheduled Task/Job which can be abused for denial of service through resource exhaustion if the application attempts to restart workers repeatedly due to crashes.
The operational impact of this vulnerability is significant, particularly in applications that process user-supplied or attacker-controlled data structures. An adversary who can inject recursive object graphs into a system utilizing vulnerable versions of deepmerge-ts can trigger synchronous crashes of the affected Node.js process. In environments employing worker processes or cluster modes common in modern web frameworks like Express or NestJS, this crash often leads to repeated worker restarts. This behavior effectively results in a Denial of Service condition where the application becomes unstable and unresponsive as it continuously attempts to recover from the fatal exception without addressing the underlying recursive input causing the loop.
It is important to note that plain JSON input cannot trigger this vulnerability because standard JSON serialization does not support circular references; such structures are typically converted into null values or throw errors during stringification before reaching the merge logic. Therefore, exploitation requires an attacker to have control over JavaScript object literals passed directly to the library functions, often through API endpoints accepting complex nested payloads rather than simple flat data.
Mitigation strategies primarily involve upgrading to version 8.0.0 of deepmerge-ts where this issue has been resolved by implementing proper visited set tracking during recursion. For applications unable to upgrade immediately due to dependency constraints or compatibility issues, defensive coding practices should be adopted. This includes validating and sanitizing input objects before passing them to the merge function, ensuring that circular references are detected and handled gracefully rather than passed directly into the recursive algorithm. Additionally, implementing timeout mechanisms for long-running operations can provide a secondary layer of defense against potential resource exhaustion attacks derived from similar vulnerabilities in other libraries.