CVE-2026-73851 — Microsoft Kiota: Path traversal in...

### Impact Kiota generates AI plugin manifests from an OpenAPI description. When the description contains an `x-ai-capabilities` response semantics `static_template` (or the adaptive-card extension `x-ai-adaptive-card`), the `file` reference is written into the generated manifest's `response_semantics.static_template.file` and is later resolved by the AI host **relative to the plugin package**. An attacker who controls or tampers with the OpenAPI description consumed by Kiota can supply a `file` reference that resolves **outside** the manifest package (e.g. `../../../../etc/passwd`, an absolute path, or a `file://` / `http(s)://` URI). When the generated manifest is deployed and consumed by an AI host, this can lead to inclusion or disclosure of files outside the intended package boundary (CWE-22 Path Traversal, CWE-829 Inclusion of Functionality from an Untrusted Control Sphere). A mitigation shipped in v1.32.5 (`ExtensionResponseSemanticsStaticTemplate.IsSafeFileReference`) rejected literal traversal, rooted paths, drive-qualified paths, and absolute URIs. However, that check inspected the **raw** reference string, so **percent-encoded** payloads bypassed every check and were still emitted verbatim. Examples that were incorrectly accepted as safe: | Input | Decodes to | |-------|-----------| | `%2e%2e/card.json` | `../card.json` | | `..%2f..%2f..%2f..%2f..%2f..%2fetc%2fpasswd` | `../../../../../../etc/passwd` | | `file%3A%2F%2F%2Fetc%2Fpasswd` | `file:///etc/passwd` | | `%2e%2e%2f%2e%2e%2f%2e%2e%2fetc%2fpasswd` | `../../../etc/passwd` | Multi-level (double) encoding such as `%252e%252e%252fcard.json` was also affected. A follow-up review found additional residual bypasses of the same validator: an embedded NUL byte (`%00`) that truncated the path and defeated the parent-directory segment check, encoding nested deeper than the decode budget (which failed open), and Unicode full-width homoglyphs (e.g. `%EF%BC%8E%EF%BC%8E` → `．．`). ### Patches - The percent-encoding bypass is fixed by decoding the reference (bounded multi-pass) before validation: https://github.com/microsoft/kiota/pull/7910 - Residual bypasses (NUL / control characters, decode-budget fail-open, Unicode homoglyphs) are fixed by failing closed on residual encoding, rejecting control characters, and NFKC-folding before validation: https://github.com/microsoft/kiota/pull/7913 (tracking issue https://github.com/microsoft/kiota/issues/7912) Users should upgrade to the first released `Microsoft.OpenApi.Kiota` version that includes these fixes (the release following 1.33.0). ### Workarounds - Only generate clients/plugins from trusted OpenAPI descriptions. - Review generated plugin manifests before deployment and reject any `response_semantics.static_template.file` value that is not a simple relative path within the `adaptiveCards/` package folder (no `..`, no rooted/absolute paths, no URIs, no percent-encoded separators). ### References - CWE-22: Improper Limitation of a Pathname to a Restricted Directory ('Path Traversal') - CWE-829: Inclusion of Functionality from an Untrusted Control Sphere - Affected code: `src/Kiota.Builder/OpenApiExtensions/OpenApiAiCapabilitiesExtension.cs` (`IsSafeFileReference`) and enforcement in `src/Kiota.Builder/Plugins/PluginsGenerationService.cs`.