CVE-2026-27601

Workarounds

A workaround that works for both functions is to enforce a depth limit on the datastructure that is created from untrusted input. A limit of 1000 levels should prevent attacks from being successful on most systems. In systems with highly constrained hardware, we recommend lower limits, for example 100 levels.

Another possible workaround that only works for _.flatten, is to pass a second argument that limits the flattening depth to 1000 or less.

References

• https://github.com/jashkenas/underscore/issues/3011
• https://underscorejs.org/#1.13.8
• https://underscorejs.org/#flatten
• https://underscorejs.org/#isEqual

In simple words, some programs that use _.flatten or _.isEqual could be made to crash. Someone who wants to do harm may be able to do this on purpose. This can only be done if the program has special properties. It only works in Underscore versions up to 1.13.7. A more detailed explanation follows.

In affected versions of Underscore, 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.

A proof of concept (PoC) for this type of attack with _.isEqual:

const _ = require('underscore');

// build JSON string for nested object ~4500 levels deep
// (for this to be an attack, the JSON would have to come from
// a request or other untrusted input)
let json = '';
for (let i = 0; i < 4500; i++) json += '{"n":';
json += '"x"';
for (let i = 0; i < 4500; i++) json += '}';

// construct two distinct objects with equal shape from the above JSON
const a = JSON.parse(json);
const b = JSON.parse(json);

_.isEqual(a, b); // RangeError: Maximum call stack size exceeded

A proof of concept (PoC) for this type of attack with _.flatten:

const _ = require('underscore');

// build nested array ~4500 levels deep
// (like with _.isEqual, this nested array would have to be sourced
// from an untrusted external source for it to be an attack)
let nested = [];
for (let i = 0; i < 4500; i++) nested = [nested];

_.flatten(nested); // RangeError: Maximum call stack size exceeded

An application that crashes because of this can be restarted, so the bug is most relevant to applications for which continued operation is important, such as server applications. Furthermore, an application is only vulnerable to this type of attack if ALL of the following conditions are met:

• 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.

All versions of Underscore up to and including 1.13.7 are affected by this weakness.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-27601 has a CVSS score of 5.9 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.13.8); upgrading removes the vulnerable code path.

Kodem intelligence

The problem has been patched in version 1.13.8. Upgrading to 1.13.8 or later completely prevents exploitation.

Note: historically, there have been breaking changes in minor releases of Underscore, especially between versions 1.6 and 1.9. However, upgrading from version 1.9 or later to any later 1.x version should be feasible with little or no effort for all users.

1. What is CVE-2026-27601? CVE-2026-27601 is a high-severity allocation of resources without limits or throttling vulnerability in underscore (npm), affecting versions <= 1.13.7. It is fixed in 1.13.8. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-27601? CVE-2026-27601 has a CVSS score of 5.9 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of underscore are affected by CVE-2026-27601? underscore (npm) versions <= 1.13.7 is affected.
4. Is there a fix for CVE-2026-27601? Yes. CVE-2026-27601 is fixed in 1.13.8. Upgrade to this version or later.
5. Is CVE-2026-27601 exploitable, and should I be worried? Whether CVE-2026-27601 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-27601 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-27601? Upgrade underscore to 1.13.8 or later.