fastapi-code-generator: Import-time RCE via route path -> unescaped @app.get('...') decorator

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

9.3 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N · score source

EPSS

• CVE-2026-63471: 0.31% estimated exploitation activity in the next 30 days; percentile 24.36 (FIRST EPSS, 2026-11-10). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary fastapi-code-generator (koxudaxi) emits the OpenAPI path into the generated main.py route decorator as a single-quoted string literal (`@app.get('/u/ /x')`) without escaping the `'`. A `'` in the path closes the decorator string and, via concatenation, injects a live expression that the decorator argument evaluates when main.py is imported (the ASGI server imports it at startup) -- code execution, no request needed. The path is a pure data field. Verified on fastapi-code-generator 0.8.0 / Python 3 / FastAPI.

Details ```python @app.get('/u/'+(import("os").system('…') or "")+'/x', response_model=None) def get_thing() -> None: pass ``` The generator post-processes output with black/isort, so a syntax-breaking payload aborts generation (DoS); a valid-Python concat (`'+expr+'`, kept valid with `(… or "")`) is written and runs at import. info.description (`description='…'`), tags, and parameter defaults are `'`-escaped; the path in @app.get('…') is the unescaped sink.…

Published by the source

9 Nov 2026, 19:48 UTC

Last modified by the source

9 Nov 2026, 20:00 UTC

Source last checked

10 Nov 2026, 06:35 UTC

1. Revision 2 · amended · 10 Nov 2026

   Amended from osv record GHSA-p2mm-64f6-gmpp

2. Revision 1 · created · 9 Nov 2026

   Published from ghsa record GHSA-p2mm-64f6-gmpp

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# Patch Next advisory brief: CVE-2026-63471

Advisory: fastapi-code-generator: Import-time RCE via route path -> unescaped @app.get('...') decorator
IDs: CVE-2026-63471, GHSA-p2mm-64f6-gmpp
Record: https://patchnext.com/advisories/cve-2026-63471 (revision 2, source checked 10 Nov 2026, 06:35 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Critical (CVSS 9.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: fastapi-code-generator 0.8.1. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI fastapi-code-generator: affected < 0.8.1; fixed in 0.8.1

## Suggested inspection (read-only)
- `python -m pip show fastapi-code-generator`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on fastapi-code-generator.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-p2mm-64f6-gmpp (GitHub-reviewed advisory GHSA-p2mm-64f6-gmpp lists affected ranges: PyPI fastapi-code-generator >=0 <0.8.1)
- Verified: https://osv.dev/vulnerability/GHSA-p2mm-64f6-gmpp (OSV record GHSA-p2mm-64f6-gmpp lists affected ranges: PyPI fastapi-code-generator >=0 <0.8.1 (fixed))

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