CVE-2026-27601

Impact

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.

Patches

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.

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

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Underscore.js is a utility-belt library for JavaScript that provides support for the usual functional suspects (each, map, reduce, filter...) without extending any core JavaScript objects.

Security Fix(es):

Underscore.js is a utility-belt library for JavaScript. Prior to version 1.13.8, the _.flatten and _.isEqual functions use recursion without a depth limit. Under very specific conditions, an attacker could exploit this to cause a Denial of Service (DoS) attack by triggering a stack overflow. Exploitation requires all of the following: untrusted input must be used to create a deeply recursive data structure (e.g., via JSON.parse with no enforced depth limit), and this structure must be passed to _.flatten or _.isEqual. For _.flatten, the attacker must be able to prepare a data structure consisting solely of arrays at all levels, and no finite depth limit must be passed as the second argument to _.flatten. For _.isEqual, there must exist a code path where two distinct but structurally equivalent data structures, submitted by the same remote client, are compared using _.isEqual. Additionally, exceptions resulting from the stack overflow must not be caught. This vulnerability is fixed in version 1.13.8.(CVE-2026-27601)

Underscore.js is a utility-belt library for JavaScript that provides support for the usual functional suspects (each, map, reduce, filter...) without extending any core JavaScript objects.

Security Fix(es):

Underscore.js is a utility-belt library for JavaScript. Prior to version 1.13.8, the _.flatten and _.isEqual functions use recursion without a depth limit. Under very specific conditions, an attacker could exploit this to cause a Denial of Service (DoS) attack by triggering a stack overflow. Exploitation requires all of the following: untrusted input must be used to create a deeply recursive data structure (e.g., via JSON.parse with no enforced depth limit), and this structure must be passed to _.flatten or _.isEqual. For _.flatten, the attacker must be able to prepare a data structure consisting solely of arrays at all levels, and no finite depth limit must be passed as the second argument to _.flatten. For _.isEqual, there must exist a code path where two distinct but structurally equivalent data structures, submitted by the same remote client, are compared using _.isEqual. Additionally, exceptions resulting from the stack overflow must not be caught. This vulnerability is fixed in version 1.13.8.(CVE-2026-27601)

Underscore.js is a utility-belt library for JavaScript that provides support for the usual functional suspects (each, map, reduce, filter...) without extending any core JavaScript objects.

Security Fix(es):

Underscore.js is a utility-belt library for JavaScript. Prior to version 1.13.8, the _.flatten and _.isEqual functions use recursion without a depth limit. Under very specific conditions, an attacker could exploit this to cause a Denial of Service (DoS) attack by triggering a stack overflow. Exploitation requires all of the following: untrusted input must be used to create a deeply recursive data structure (e.g., via JSON.parse with no enforced depth limit), and this structure must be passed to _.flatten or _.isEqual. For _.flatten, the attacker must be able to prepare a data structure consisting solely of arrays at all levels, and no finite depth limit must be passed as the second argument to _.flatten. For _.isEqual, there must exist a code path where two distinct but structurally equivalent data structures, submitted by the same remote client, are compared using _.isEqual. Additionally, exceptions resulting from the stack overflow must not be caught. This vulnerability is fixed in version 1.13.8.(CVE-2026-27601)

Underscore.js is a utility-belt library for JavaScript that provides support for the usual functional suspects (each, map, reduce, filter...) without extending any core JavaScript objects.

Security Fix(es):

Underscore.js is a utility-belt library for JavaScript. Prior to version 1.13.8, the _.flatten and _.isEqual functions use recursion without a depth limit. Under very specific conditions, an attacker could exploit this to cause a Denial of Service (DoS) attack by triggering a stack overflow. Exploitation requires all of the following: untrusted input must be used to create a deeply recursive data structure (e.g., via JSON.parse with no enforced depth limit), and this structure must be passed to _.flatten or _.isEqual. For _.flatten, the attacker must be able to prepare a data structure consisting solely of arrays at all levels, and no finite depth limit must be passed as the second argument to _.flatten. For _.isEqual, there must exist a code path where two distinct but structurally equivalent data structures, submitted by the same remote client, are compared using _.isEqual. Additionally, exceptions resulting from the stack overflow must not be caught. This vulnerability is fixed in version 1.13.8.(CVE-2026-27601)

Root has patched CVE-2026-27601 in the underscore package for Root:npm. Multiple fixed versions available.

Root has patched CVE-2026-27601 in the underscore package for Root:Debian:12. Multiple fixed versions available.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

TuxCare rebuilt underscore to address 2 CVEs. Fixed in 1.7.0-tuxcare.1.

TuxCare rebuilt underscore to address 2 CVEs. Fixed in 1.9.1-tuxcare.1.

TuxCare rebuilt underscore to address 1 CVE. Fixed in 1.13.6-tuxcare.1.

These are all security issues fixed in the jupyter-bqplot-jupyterlab-0.5.46-14.1 package on the GA media of openSUSE Tumbleweed.

These are all security issues fixed in the jupyter-matplotlib-0.11.7-17.1 package on the GA media of openSUSE Tumbleweed.

These are all security issues fixed in the jupyter-nbclassic-1.3.3-1.1 package on the GA media of openSUSE Tumbleweed.

These are all security issues fixed in the velociraptor-0.76.7.0.git145.b60ef1146-1.1 package on the GA media of openSUSE Tumbleweed.

This update for agama-web-ui fixes the following issues:

• CVE-2025-7783: form-data: unsafe Math.random() function is used to select a boundary value for multipart form-encoded data (bsc#1246822).
• CVE-2026-12143: form-data: CRLF injection via unescaped multipart field names and filenames (bsc#1272310).
• CVE-2026-13149: brace-expansion: expand() function exhibits exponential-time complexity when processing non-expanding {} brace groups (bsc#1269927).
• CVE-2026-13311: shell-quote: quadratic complexity in parse() function when processing specially crafted strings (bsc#1269359).
• CVE-2026-13676: fast-uri: host-based policy bypass due to failure to canonicalize Unicode/IDN hostnames for HTTP-family URLs (bsc#1269595).
• CVE-2026-27601: underscore: DoS via stack overflow due to missing depth limits in _.flatten and _.isEqual functions (bsc#1259169).
• CVE-2026-40181: react-router: open redirect to an external domain due to path values starting with // being reinterpreted as protocol-relative URLs (bsc#1272311).
• CVE-2026-49356: @babel/core: arbitrary file read via sourceMappingURL comment (bsc#1272317).
• CVE-2026-53550: js-yaml: quadratic complexity in merge-key processing when processing a crafted YAML document (bsc#1268851).
• CVE-2026-53632: launch-editor: NTLMv2 hash disclosure via UNC path handling on Windows (bsc#1272319).
• CVE-2026-54466: websocket-driver: message corruption via abuse of protocol length headers (bsc#1272312).
• CVE-2026-54490: websocket-driver: resource limit bypass via message compression (bsc#1272313).
• CVE-2026-55602: http-proxy-middleware: Host-header-driven backend routing bypass via router host+path substring matching (bsc#1272318).