CVE-2026-59866: Microsoft Kiota Arbitrary File Write + Code-injection Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-4vv7-jj25-4gh6",
  "modified" : "2026-08-17T14:55:49Z",
  "published" : "2026-07-24T16:12:14Z",
  "aliases" : [ "CVE-2026-59866" ],
  "summary" : "Microsoft Kiota: Arbitrary file write + code-injection via x-ms-kiota-info clientClassName and clientNamespaceName",
  "details" : "### Summary\n\nMicrosoft Kiota emitted the `x-ms-kiota-info` extension's `clientClassName` or `clientNamespaceName` value\n**raw**, with no identifier or path sanitization, as **both** the generated client's class/namespace name\n**and** part of the generated output path. When `kiota generate` is run **without `-c/--class-name`** — the\nzero-config workflow that `x-ms-kiota-info` is explicitly designed for (the API provider supplies the names in\nthe description so consumers don't have to) — an attacker who controls or tampers with the OpenAPI description\ncould therefore:\n\n- **(CWE-22) write the generated source file to a path outside the `-o` output directory** — e.g.\n  `clientClassName: \"/var/www/html/shell\"`; and\n- **(CWE-94) inject arbitrary text into the generated class/namespace declaration**, corrupting the generated\n  client.\n\nConfirmed on Kiota **1.32.4** (the self-contained `linux-x64` release binary), i.e. **after** the earlier\nwriter-sink hardening — that fix escaped property/enum/default/serialization sinks but never sanitized the\nprovider-supplied `clientClassName` / `clientNamespaceName`.\n\n### Details\n\n`clientClassName` reached two unsanitized sinks (observed in the generated C#; the same raw emission occurred\nfor Java, Go, TypeScript, Python, and PHP):\n\n```\n# output FILENAME (CWE-22): clientClassName flows into the file path\nclientClassName: \"/abs/path/PWNED\"   ->   /abs/path/PWNED.cs   (written outside -o)\n\n# class declaration (CWE-94): clientClassName flows verbatim into the type declaration\nclientClassName: 'Pwn { } public class INJECTED { } public partial class RealClient'\n   ->  public partial class Pwn { } public class INJECTED { } public partial class RealClient : ... { }\n```\n\n`clientNamespaceName` reached the analogous namespace/path sinks.\n\n### Impact\n\nA developer or CI host generating a client from an attacker-controlled or compromised OpenAPI description\n(without `-c`) could create/overwrite a generated source file at an attacker-influenced path and emit\nattacker-controlled text into the generated client.\n\nThis does **not** reach clean remote code execution: because `clientClassName` is reused verbatim at multiple\nsites (the class name **and** the constructor name), injected code cannot be made to compile — it breaks the\nbuild. So the code-injection vector is a generation/build-corruption (integrity/DoS), and the high-severity\nprimitive is the file write. CWE-22 / CWE-94.\n\n### Patches\n\nFixed in **1.29.1 and 1.32.5** (https://github.com/microsoft/kiota/pull/7884). `clientClassName` and\n`clientNamespaceName` sourced from `x-ms-kiota-info` are now sanitized before use:\n`GenerationConfiguration.SanitizeClientClassName` strips any character outside `[A-Za-z0-9_]` and any invalid\nleading character (falling back to `ApiClient`), and `SanitizeClientNamespaceName` restricts to\n`[A-Za-z0-9._-]`, collapses consecutive dots, strips invalid leading characters (falling back to `ApiSdk`).\nThis removes path separators, drive/colon, `..`, quotes, and braces, so the values can no longer influence\nthe output path or inject into the declaration.\n\n### Remediation\n\nUpgrade to Kiota **1.29.1, 1.32.5,** or later and regenerate affected clients.",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:H/VA:L/SC:N/SI:N/SA:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Microsoft.OpenApi.Kiota"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.30.0"
      }, {
        "fixed" : "1.32.5"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Microsoft.OpenApi.Kiota.Builder"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "1.30.0"
      }, {
        "fixed" : "1.32.5"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Microsoft.OpenApi.Kiota"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.29.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "NuGet",
      "name" : "Microsoft.OpenApi.Kiota.Builder"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.29.1"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/microsoft/kiota/security/advisories/GHSA-4vv7-jj25-4gh6"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-59866"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/microsoft/kiota/pull/7884"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/microsoft/kiota/commit/dc812dbbf88ef7edf53a890d36b2f9d1460e947d"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/microsoft/kiota"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/microsoft/kiota/releases/tag/v1.32.5"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-22", "CWE-94" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-07-24T16:12:14Z",
    "nvd_published_at" : "2026-07-16T16:19:15Z"
  }
}