Hey API: `buildClientParams` template: prototype chain substitution via unknown `$<slot>_proto` key

published 2026-07-01 4 mo ago modified 2026-10-24 15 d ago also GHSA-hhx9-57xq-r5rw , EUVD-2026-45267

cvss: 4.8; version 3.1, CNA
cisa kev: Not listed; not in CISA's catalog
epss: 0.3%; EPSS 0.3% chance of exploitation in 30 days, percentile 19%
public exploits: 0; none known

Hey API is an ecosystem for turning API specifications into production-ready code. Prior to 0.97.3, dist/clients/core/params.ts ships a runtime template copied into generated SDKs as params.gen.ts, and buildClientParams writes unknown slot-prefixed keys such as $body_, $headers_, $path_, and $query_ directly to the corresponding slot, allowing $query___proto__ alongside a legitimate q field to set params.query through params["query"]["__proto__"] = value, call Object.setPrototypeOf(params.query, value), and expose inherited attacker-controlled keys during for..in iteration. This issue is fixed in version 0.97.3.

Patch / fix

fixed in 1 links 3

npm @hey-api/openapi-ts 0.97.3

CISA SSVC

Exploitation: Proof of concept is public
Automatable: no
Technical impact: partial
Assessed: 3 mo ago (2026-07-20)

Exploit prediction

0.27% chance of exploitation in the next 30 days, higher than 19% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

github.com https://github.com/hey-api/hey-api/security/advisories/GHSA-hhx9-57xq-r5rw referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by	version	score	vector
CNA	3.1	4.8	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N
GitHub	3.1	4.8	CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:N

Affected packages

ecosystem	package	affected	fixed in
npm	@hey-api/openapi-ts	<0.97.3	0.97.3

Affected products

hey-api openapi-ts

Classification

Weakness

CWE-1321 Improperly Controlled Modification of Object Prototype Attributes

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

Summary