CriticalCVE-2026-63470

fastapi-code-generator: Call-time RCE via operation summary -> unescaped route-handler docstring (CVE-2026-63470)

Published 03.11.2026Last modified 03.11.2026Sources: Hersteller, CVE.org

What is affected, what to do

Update Koxudaxi Fastapi-code-generator to version 0.8.1.

Affected: Koxudaxi Fastapi-code-generator lt *..0.8.1
Fixed in: Koxudaxi Fastapi-code-generator 0.8.1

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Metrics

9.3

Priority

XIEM Effective CVSS: highest available CVSS rating from NVD and vendor data.

Source: Vendor (CNA), CVSS v4

9.3

CVSS v4

Severity Critical on the scale from 0 to 10.

Source: Hersteller, CVE.org

0.5 %

EPSS score

Low — model estimates < 1% exploitation likelihood.

Source: FIRST EPSS, percentile 40.7

Show all metrics

Severity: Critical
Priority (0–325) The priority score is a consolidated metric that condenses several individual signals into a single, action-guiding ranking: technical severity (CVSS), exploitation likelihood (EPSS), known-exploited status (KEV) and the context of the affected asset. The aim is to answer the only truly important question in vulnerability management: what first? Read in glossary: 76.50 Methodology
Exploit maturity An exploit is code or a technique that specifically uses a concrete vulnerability to force unintended behaviour — such as executing code or escalating privileges. It is the tool that turns a theoretical flaw into a practical attack. Read in glossary: no public PoC known
XIEM Effective CVSS XIEM® is the tiered managed-security operating model from NEOSEC and a registered trademark of NEOSEC GmbH. It extends classic SIEM — the central collection, normalisation and correlation of security-relevant events — with endpoint forensics, network visibility and deception technology, and consolidates them into three building tiers: Sentry, Orchestrate and Command. Read in glossary: 9.3 Methodology
CVSS v4 CVSS is the globally established, open standard for assessing the technical severity of a vulnerability on a scale from 0 to 10. It describes how easily a flaw can be exploited and what damage it can cause — but not how likely its actual exploitation is. CVSS and the complementary EPSS deliberately answer different questions. Read in glossary: 9.3
EPSS score The EPSS score is the absolute model output for an individual vulnerability: the estimated probability of real-world exploitation within the next 30 days, expressed as a value between 0 and 1 (i.e. 0 % to 100 %). A score of 0.42 means „the model estimates a 42 % chance of real-world exploitation within 30 days“. Read in glossary: 0.5 %; Low — model estimates < 1% exploitation likelihood.
EPSS percentile The EPSS percentile is the relative position of a vulnerability within all EPSS-scored CVEs, expressed as a value between 0 and 1. A percentile of 0.90 means „this CVE has a higher EPSS score than 90 % of all other scored CVEs“. Unlike the EPSS score, it is not a probability — but a positioning within a ranking. Read in glossary: 40.7; High — this CVE ranks above the median of all CVEs scored today (rank ≥ 36%).
KEV status The KEV catalogue is a list maintained by the US authority CISA of vulnerabilities for which active exploitation in the wild has been confirmed. It answers not the question „how severe?“ but „is it already being attacked?“ — thereby sharply prioritising patching. Read in glossary: —
Published: 03.11.2026
CISA assessment (SSVC): Exploitation: proof of concept · automatable · Technical impact: total; Source: CISA Vulnrichment, as of 3 November 2026
CWE CWE is a catalogue of typical weakness types in software and hardware — such as flawed input validation, unsafe memory handling or poor authentication. Where a CVE names a concrete vulnerability in a concrete product, a CWE describes the underlying class of defect. Read in glossary: CWE-116, CWE-94

Detection rules

No NEOSEC detection rule is available for this vulnerability yet.

Classification under NIS2 and CRA

Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.

NIS2 NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management. Read in glossary: High technical severity: assess and remediate the vulnerability in vulnerability handling under Art. 21(2)(e) NIS2 and document the decision.
CRA The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates. Read in glossary: From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001 ISO/IEC 27001 is the internationally leading standard for an information security management system (ISMS). It describes a risk-based, process-oriented framework to systematically plan, implement, review and improve information security. Read in glossary: Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.

Description

Summary

fastapi-code-generator (koxudaxi) emits an operation's summary as the generated route handler's docstring, using a triple-quoted string without escaping """. A """ in the summary closes the docstring and, via concatenation, injects a live expression that becomes an expression statement in the handler body -- which runs when the route handler is CALLED (the endpoint is hit). The summary is a pure data field. Verified on fastapi-code-generator 0.8.0 / Python 3 / FastAPI.

Details

def get_thing() -> None:
 (
 """
 x"""
 + (__import__("os").system('…') or "")
 + """y
 """
 )
 pass

The generator post-processes with black/isort, so a syntax-breaking payload aborts generation; a valid-Python breakout ("""+expr+""", kept valid with (… or "")) is written and executes at call time. info.description, tags, parameter defaults are escaped; operation description is not emitted into the function body; the summary in the handler docstring is the unescaped sink. Models come from datamodel-code-generator (repr-escaped, no RCE).

Not a duplicate: distinct product from OpenAPI Generator's python-fastapi; and distinct field/sink from the route path -> @app.get('…') decorator (import-time) in the same product -- this is the operation summary in the handler docstring (call-time).

PoC

reproduce.sh (+ make_spec.py) attached: generates the app and calls the route handler; the injected expression runs and writes a marker. Verified on 0.8.0.

Impact

Code execution on the server when the affected endpoint is invoked, in any app generated with fastapi-code-generator from an attacker-controlled or attacker-influenced spec.

Suggested fix

Escape the summary for the docstring context (escape """/backslashes via repr, or strip triple-quotes) before emitting it as the handler docstring; apply the same to any free-text field rendered into a docstring. maintainer-report.txt make_spec.py reproduce.sh

Source: GitHub Security Advisories, NVD (NIST), cvelistv5

Affected products

The following versions are affected. Older releases of the same product line are also vulnerable unless stated otherwise.

Affected products
		Affected versions	Fixed in
koxudaxi	fastapi-code-generator	`< 0.8.1`	`0.8.1`
pip	fastapi-code-generator	`< 0.8.1`	`0.8.1`

Vendors and products

Koxudaxi Fastapi-code-generator

Weakness classes (CWE)

CWE-116Improper Encoding or Escaping of Output
Class
The product prepares a structured message for communication with another component, but encoding or escaping of the data is either missing or done incorrectly. As a result, the intended structure of the message is not preserved.
Entry at MITRE CWE-116
CWE-94Improper Control of Generation of Code ('Code Injection')
Base
The product constructs all or part of a code segment using externally-influenced input from an upstream component, but it does not neutralize or incorrectly neutralizes special elements that could modify the syntax or behavior of the intended code segment.
Entry at MITRE CWE-94

References & sources

github.comkoxudaxi/fastapi-code-generator/commit/49301dbdb82170d7f5087741…x_refsource_MISC
github.comkoxudaxi/fastapi-code-generator/commit/b3b9bd66e0b0c4d8c686fda7…x_refsource_MISC
github.comkoxudaxi/fastapi-code-generator/releases/tag/0.8.1x_refsource_MISC
github.comkoxudaxi/fastapi-code-generator/security/advisories/GHSA-h667-4…x_refsource_CONFIRM

Changes to this CVE

Who changed this CVE and when: NVD (analysis, affected products, CVSS), the CVE publisher (CNA) and additional providers such as CISA (ADP).

03.11.2026ReceivedGitHub

CVSS V4.0: AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N

Weakness type: CWE-94, CWE-116

Description added or changed

New references:

Show 9 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Description	added	—	fastapi-code-generator creates FastAPI applications from OpenAPI files. Prior to 0.8.1, the generator inserts Operation.summary into a generated route-handler docstring as a triple-double-quoted string without escaping embedded triple quotes. An attacker-controlled or attacker-influenced operation summary can close the docstring and inject a valid Python expression statement into the handler body. Syntax-breaking input only causes generation to fail, but syntactically valid generated code executes with the server process's privileges when the affected route handler is called. This call-time route-handler docstring sink is distinct from the project's separate import-time route-path and additional-response-key sinks. This issue is fixed in version 0.8.1.
CVSS	added	—	AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X
Weakness type (CWE)	added	—	CWE-94
Weakness type (CWE)	added	—	CWE-116
Affected versions (publisher)	added	—	New affected value received. <a href="https://github.com/CVEProject/cvelistV5/blob/main/cves/2026/63xxx/CVE-2026-63470.json">CVE-2026-63470</a>
Reference	added	—	https://github.com/koxudaxi/fastapi-code-generator/commit/49301dbdb82170d7f50877419d994b05cb1fa2bc
Reference	added	—	https://github.com/koxudaxi/fastapi-code-generator/commit/b3b9bd66e0b0c4d8c686fda700258104615a1332
Reference	added	—	https://github.com/koxudaxi/fastapi-code-generator/releases/tag/0.8.1
Reference	added	—	https://github.com/koxudaxi/fastapi-code-generator/security/advisories/GHSA-h667-4jj2-cgv4

Source identifier: security-advisories@github.com

03.11.2026Record modifiedCISA ADP

CISA assessment (SSVC): exploitation: proof of concept, automatable: yes

New references:

https://github.com/koxudaxi/fastapi-code-generator/security/advisories/GHSA-h667-4jj2-cgv4

Show 2 original changes

Individual changes of this event as published by NVD
Type	Action	Before	After
Reference	added	—	https://github.com/koxudaxi/fastapi-code-generator/security/advisories/GHSA-h667-4jj2-cgv4
SSVC assessment	added	—	{"id":"CVE-2026-63470","role":"CISA Coordinator","options":[{"exploitation":"poc"},{"automatable":"yes"},{"technicalImpact":"total"}],"version":"2.0.3","timestamp":"2026-11-03T19:34:23.694611Z"}

Source identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0

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<a href="https://intel.neosec.eu/en/cve/CVE-2026-63470"><img src="https://intel.neosec.eu/badge/cve/CVE-2026-63470.svg?lang=en" alt="CVE-2026-63470 – NEOSEC Intel" height="20"></a>