Microsoft Kiota: Path traversal in generated plugin manifest static_template.file reference (percent-encoding bypass)

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.