{"id":"CVE-2026-59866","aliases":["GHSA-4vv7-jj25-4gh6"],"url":"https://o3.security/vulnerability/CVE-2026-59866","summary":"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.","published":"2026-07-16T14:46:50.738Z","modified":"2026-10-07T07:05:30.732193827Z","cvss":null,"epss":{"score":0.01355,"percentile":0.70213,"asOf":"2026-09-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"NuGet","name":"Microsoft.OpenApi.Kiota","fixedVersion":"1.32.5"},{"ecosystem":"NuGet","name":"Microsoft.OpenApi.Kiota.Builder","fixedVersion":"1.32.5"},{"ecosystem":"NuGet","name":"Microsoft.OpenApi.Kiota","fixedVersion":"1.29.1"},{"ecosystem":"NuGet","name":"Microsoft.OpenApi.Kiota.Builder","fixedVersion":"1.29.1"}],"fix":{"url":"https://github.com/microsoft/kiota/commit/dc812dbbf88ef7edf53a890d36b2f9d1460e947d","label":"microsoft/kiota@dc812db"},"references":[{"type":"WEB","url":"https://github.com/microsoft/kiota/releases/tag/v1.32.5"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/59xxx/CVE-2026-59866.json"},{"type":"ADVISORY","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":"FIX","url":"https://github.com/microsoft/kiota/commit/dc812dbbf88ef7edf53a890d36b2f9d1460e947d"},{"type":"FIX","url":"https://github.com/microsoft/kiota/pull/7884"},{"type":"PACKAGE","url":"https://github.com/microsoft/kiota"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-07T07:05:30.732193827Z"}}