CVE-2026-33864: Convict has Prototype Pollution via startsWith() function

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• No confirmed exploitation. It is not in CISA’s KEV catalog. That can change quickly after a public write-up; re-check if you are affected.
• Needs local access needed. Lower urgency for most teams unless an attacker could already run code on the machine.
• Only applies if your dependency tree includes convict (npm) <= 6.2.4, fixed in 6.2.5. Check your lockfile, including transitive dependencies.

What it is

From the GitHub advisory

Summary A prototype pollution vulnerability exists in the latest version of the convict npm package (6.2.4). Despite a previous fix that attempted to mitigate prototype pollution by checking whether user input started with a forbidden key, it is still possible to pollute Object.prototype via a crafted input using String.prototype.

Details The vulnerability resides in line 564 of https://github.com/mozilla/node-convict/blob/master/packages/convict/src/main.js where startsWith() function is used to check whether user provided input contain forbidden strings.

PoC Steps to reproduce 1. Install latest version of convict using npm install or cloning from git 2. Run the following code snippet:

Expected behavior Prototype pollution should be prevented and {} should not gain new properties. This should be printed on the console:

Actual behavior Object.prototype is polluted This is printed on the console:

Weakness type: CWE-1321.

Product: convict (npm) · Published: 2026-03-26

Affected and fixed versions

Severity scores

• Attack vector: local access needed
• Attack complexity: low
• Attack requirements: none
• Privileges required: none (no login)
• User interaction: none

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).

Timeline

• 2026-03-26 — GitHub-reviewed advisory GHSA-44fc-8fm5-q62h published (critical)

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-44fc-8fm5-q62h github.com
2. CVE.org record CVE-2026-33864 cve.org
3. NVD entry CVE-2026-33864 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com

Critical npm advisories · Today’s Security Digest