CVE-2026-27601 in underscore
Summary
by MITRE • 03/04/2026
Underscore.js is a utility-belt library for JavaScript. Prior to 1.13.8, the _.flatten and _.isEqual functions use recursion without a depth limit. Under very specific conditions, detailed below, an attacker could exploit this in a Denial of Service (DoS) attack by triggering a stack overflow. Untrusted input must be used to create a recursive datastructure, for example using JSON.parse, with no enforced depth limit. The datastructure thus created must be passed to _.flatten or _.isEqual. In the case of _.flatten, the vulnerability can only be exploited if it is possible for a remote client to prepare a datastructure that consists of arrays at all levels AND if no finite depth limit is passed as the second argument to _.flatten. In the case of _.isEqual, the vulnerability can only be exploited if there exists a code path in which two distinct datastructures that were submitted by the same remote client are compared using _.isEqual. For example, if a client submits data that are stored in a database, and the same client can later submit another datastructure that is then compared to the data that were saved in the database previously, OR if a client submits a single request, but its data are parsed twice, creating two non-identical but equivalent datastructures that are then compared. Exceptions originating from the call to _.flatten or _.isEqual, as a result of a stack overflow, are not being caught. This vulnerability is fixed in 1.13.8.
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Analysis
by VulDB Data Team • 06/02/2026
The vulnerability identified as CVE-2026-27601 affects the Underscore.js JavaScript utility library, specifically targeting the .flatten and .isEqual functions that lack proper recursion depth limiting mechanisms. This flaw represents a classic stack overflow vulnerability that can be exploited through careful manipulation of recursive data structures, making it particularly dangerous in web applications where user input is processed without adequate validation. The vulnerability exists in versions prior to 1.13.8 and demonstrates how seemingly innocuous library functions can become attack vectors when recursion is not properly bounded. The issue stems from the absence of depth constraints in recursive operations, creating potential for malicious actors to craft input data that will cause excessive stack consumption during processing.
The technical exploitation of this vulnerability requires specific conditions to be met, as outlined in the CVE description. For .flatten function exploitation, attackers must be able to control data input through JSON.parse operations without enforced depth limits, and the resulting data structure must contain arrays at all levels. The second argument to .flatten must also be omitted or set to a value that allows unlimited recursion. For _.isEqual function, exploitation requires a scenario where two data structures submitted by the same remote client are compared, such as when data is stored in a database and later retrieved for comparison, or when the same input is parsed twice creating equivalent but non-identical structures. This dual requirement makes the vulnerability more challenging to exploit but also more targeted in its impact.
The operational impact of CVE-2026-27601 extends beyond simple service disruption, as it can lead to complete application failure and denial of service for legitimate users. When a stack overflow occurs during recursive processing, the JavaScript engine typically crashes or becomes unresponsive, rendering the affected application unusable until the process is restarted. This vulnerability is particularly concerning in server-side applications where Underscore.js is used to process user input, as it can be leveraged to bring down entire web services or APIs. The lack of exception handling for stack overflow conditions means that applications may not recover gracefully from these attacks, potentially leading to extended downtime and service degradation. The vulnerability's classification aligns with CWE-674, which addresses the issue of uncontrolled recursion leading to excessive resource consumption.
Mitigation strategies for this vulnerability involve immediate upgrading to version 1.13.8 or later, which implements proper recursion depth limiting for both affected functions. Organizations should also implement input validation and depth limiting measures at application boundaries to prevent malicious data from reaching the vulnerable functions. Additional defensive measures include monitoring for unusual processing patterns that might indicate attempted exploitation, implementing rate limiting on input processing, and ensuring that all user-provided data is properly sanitized before being passed to library functions. The fix addresses the core issue by introducing maximum depth parameters that prevent infinite recursion, aligning with ATT&CK technique T1499.004 for resource exhaustion attacks. Security teams should also conduct comprehensive testing to ensure that the upgrade does not introduce regressions in existing functionality while verifying that the depth limiting parameters are appropriately configured for their specific use cases.