CVE-2026-59876: protobufjs Text Format String Map Parsing Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-jfj6-75fj-8934",
  "modified" : "2026-07-20T22:04:07Z",
  "published" : "2026-07-20T22:04:07Z",
  "aliases" : [ "CVE-2026-59876" ],
  "summary" : "protobufjs: Text Format string map parsing can mutate returned map object prototype",
  "details" : "## Summary\n\nThe protobuf.js text format extension parsed string-keyed map entries using ordinary property assignment. A text-format map entry with key `__proto__` could therefore change the prototype of the returned map object instead of creating an own map entry.\n\nThis affected the optional Text Format extension. Other protobufjs input paths, including binary decode, `fromObject`, and ProtoJSON conversion, are not affected.\n\n## Impact\n\nAn attacker who can provide protobuf text-format input parsed by an application using `protobufjs/ext/textformat` may be able to create message objects whose string-keyed map fields have attacker-controlled prototypes.\n\nThis is per-object prototype mutation, not global `Object.prototype` pollution. Impact depends on downstream application logic treating inherited properties as meaningful, for example by using `in`, truthiness checks, or direct property access on parsed map objects instead of own-property checks.\n\nApplications that do not parse untrusted Text Format input, or that do not use inherited properties from parsed map objects in security-relevant logic, are not directly affected.\n\n## Preconditions\n\n* The application must parse attacker-controlled protobuf Text Format input with `protobufjs/ext/textformat`.\n* The target schema must contain a string-keyed map field.\n* The crafted input must provide a map entry with key `__proto__`.\n* Downstream application logic must treat inherited properties on the returned map object as meaningful for impact beyond malformed output.\n\n## Workarounds\n\nUpgrade to protobufjs 8.6.5 or newer.\n\nIf immediate upgrade is not possible, do not parse untrusted protobuf Text Format input with affected versions. Applications can also reject string map keys named `__proto__` before or during Text Format parsing, and should use own-property checks such as `Object.hasOwnProperty.call(map, key)` when consuming parsed map objects.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "npm",
      "name" : "protobufjs"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "8.2.0"
      }, {
        "fixed" : "8.6.5"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 8.6.4"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/protobufjs/protobuf.js/security/advisories/GHSA-jfj6-75fj-8934"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-59876"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/protobufjs/protobuf.js/pull/2335"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/protobufjs/protobuf.js/commit/9f97fe413072d3beb52c74e62d88ea8adc9444d8"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/protobufjs/protobuf.js"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/protobufjs/protobuf.js/releases/tag/protobufjs-v8.6.5"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-1321" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-20T22:04:07Z",
    "nvd_published_at" : "2026-07-08T16:16:34Z"
  }
}