Microsoft Kiota: Generation-time SSRF + remote/local file inclusion via unrestricted $ref

Summary

Microsoft Kiota resolved OpenAPI $refs by fetching remote http(s) URLs and reading local files (including absolute / out-of-tree paths), inlining the referenced schema into the generated client. Running kiota generate on a spec whose $ref pointed at an attacker/internal URL or an arbitrary local file yielded SSRF, remote file inclusion, and local file inclusion. Verified on 1.32.3 / 1.32.4.

Details

• $ref: http://attacker/internal-evil.json#/... → build host fetches the URL (SSRF) and inlines the remote schema (RFI); confirmed property REMOTE_KIOTA_PROP in the generated client.
• $ref: /abs/path.json#/... or ../../secret.json#/... → Kiota reads the out-of-tree local file and inlines its schema (LFI); confirmed Leaked schema in the generated client. Resolution is transitive across nesting levels.

Kiota escapes its output sinks (comments/strings/identifiers), so attacker-controlled remote/local content cannot break out into code — no RCE. The chain stops at SSRF + RFI + LFI.

Impact

Build-time SSRF (CWE-918) from the developer or CI host, disclosure of arbitrary local files (CWE-22), and inclusion of untrusted remote content (CWE-829), from running the generator on an attacker-controlled or attacker-influenced OpenAPI description. No code execution. Notable because Kiota is otherwise the hardened generator (it resists the code-injection class).

The relevant threat is not "change the generated output" (an attacker who fully controls the description can already do that) but the side effects on the build host: outbound requests from inside the CI network (cloud metadata, internal-only services) and reads of local files the attacker never possessed, whose contents are then inlined into the generated — and typically committed/published — client. It also bypasses controls that review the description document but not externally-referenced content.

Patches

Fixed in 1.32.5 (https://github.com/microsoft/kiota/pull/7888). External reference resolution is now default-deny: a new AllowedExternalOriginsStreamLoader refuses to load any external $ref — remote http(s) URLs and local file paths alike — unless its origin/path is explicitly allow-listed. A new --allowed-external-origins parameter (added to the commands that load OpenAPI descriptions) opts specific origins back in, accepting *, full URIs, URI patterns, full paths, relative paths, or path patterns (wildcards supported). With no allow-list entries, external references are not loaded at all.

Remediation

Upgrade to Kiota 1.32.5 or later. External references now require explicit opt-in via --allowed-external-origins; add only trusted origins/paths.