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Underscore.js has unlimited recursion in _.flatten and _.isEqual, potential for DoS attack

Type: advisorySources: OSV

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.

Products
Not specified
Affected versions
Not specified
Fixed versions
Not specified
CVEs
CVE-2026-27601
Confirmed fixed CVEs
Not specified
Updated in this feed
4 Nov 2026, 04:43 UTC · Revision 1

Source evidence

Underscore.js has unlimited recursion in _.flatten and _.isEqual, potential for DoS attack

core-osv · Original source

Published 3 Mar 2026, 22:38 UTC

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.

osv:CVE-2026-27601
Complete source record
{
  "id": "78b1478a4deba3d5f59c8e61a03b86cc43c46411273b0601928e70cf68a081c9",
  "url": "https://osv.dev/vulnerability/CVE-2026-27601",
  "method": "structured",
  "payload": {
    "kind": "advisory",
    "title": "Underscore.js has unlimited recursion in _.flatten and _.isEqual, potential for DoS attack",
    "summary": "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.",
    "evidence": "osv:CVE-2026-27601",
    "products": [],
    "severity": "high",
    "structured": {
      "id": "CVE-2026-27601",
      "aliases": [
        "CVE-2026-27601",
        "GHSA-qpx9-hpmf-5gmw"
      ],
      "metrics": [
        {
          "type": "CVSS_V4",
          "score": "CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N"
        }
      ],
      "affected": [
        {
          "ranges": [
            {
              "repo": "https://github.com/jashkenas/underscore",
              "type": "GIT",
              "events": [
                {
                  "introduced": "0"
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                {
                  "fixed": "9374840c22e348083d0d072f30dc980622523259"
                },
                {
                  "fixed": "411e222eb0ca5d570cc4f6315c02c05b830ed2b4"
                },
                {
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                    "range": [
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                      {
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                    "source": "CPE_RANGE"
                  },
                  {
                    "range": [
                      {
                        "fixed": "411e222eb0ca5d570cc4f6315c02c05b830ed2b4"
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                    "source": "REFERENCES_COMMIT"
                  },
                  {
                    "range": [
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                    "source": "REFERENCES_COMMIT"
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          "versions": [
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            "1.3.1",
            "1.3.0",
            "1.2.4",
            "1.2.3",
            "1.2.2",
            "1.2.1",
            "1.1.2",
            "1.2.0",
            "1.1.6",
            "1.1.5",
            "1.1.4",
            "1.1.3",
            "1.1.1",
            "1.1.0",
            "1.0.4",
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          ],
          "database_specific": {
            "source": "https://storage.googleapis.com/cve-osv-conversion/osv-output/CVE-2026-27601.json"
          }
        }
      ],
      "modified": "2026-10-28T02:30:33.518101644Z",
      "provider": "osv",
      "references": [
        "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27601.json",
        "https://github.com/jashkenas/underscore/security/advisories/GHSA-qpx9-hpmf-5gmw",
        "https://nvd.nist.gov/vuln/detail/CVE-2026-27601",
        "https://github.com/jashkenas/underscore/commit/411e222eb0ca5d570cc4f6315c02c05b830ed2b4",
        "https://github.com/jashkenas/underscore/commit/a6e23ae9647461ec33ad9f92a2ecfc220eea0a84"
      ],
      "provider_metadata": {
        "cwe_ids": [
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        ],
        "cna_assigner": "GitHub_M",
        "osv_generated_from": "https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27601.json"
      }
    },
    "related_cves": [
      {
        "value": "CVE-2026-27601",
        "evidence": "/id"
      }
    ],
    "security_relevant": true,
    "severity_evidence": "/severity/0/score"
  },
  "source_id": "core-osv",
  "document_id": 500359,
  "published_at": "2026-03-03T22:38:38.955+00:00"
}