CVE-2026-25586: @nyariv/sandboxjs has Sandbox Escape via Prototype Whitelist Bypass and Host Prototype Pollution

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 0.7%, percentile 54: FIRST’s model estimates a 0.7% chance of exploitation activity in the next 30 days, higher than 54% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes @nyariv/sandboxjs (npm) <= 0.8.28, fixed in 0.8.29. Check your lockfile, including transitive dependencies.

What it is

SandboxJS is a JavaScript sandboxing library. Prior to 0.8.29, a sandbox escape is possible by shadowing hasOwnProperty on a sandbox object, which disables prototype whitelist enforcement in the property-access path. This permits direct access to proto and other blocked prototype properties, enabling host Object.prototype pollution and persistent cross-sandbox impact. This vulnerability is fixed in 0.8.29.

From the GitHub advisory

Summary A sandbox escape is possible by shadowing hasOwnProperty on a sandbox object, which disables prototype whitelist enforcement in the property-access path. This permits direct access to proto and other blocked prototype properties, enabling host Object.prototype pollution and persistent cross-sandbox impact.

The issue was reproducible on Node v23.9.0 using the project’s current build output. The bypass works with default Sandbox configuration and does not require custom globals or whitelists.

Root Cause prototypeAccess uses a.hasOwnProperty(b) directly, which can be attacker‑controlled if the sandboxed object shadows hasOwnProperty. When this returns true, the whitelist checks are skipped.

- src/executor.ts:348 const prototypeAccess = isFunction || !(a.hasOwnProperty(b) || typeof b === 'number');

- src/executor.ts:367-399 prototype whitelist enforcement only happens when prototypeAccess is true.

Weakness type: CWE-74.

Product: @nyariv/sandboxjs (npm) · Assigned by: GitHub_M · Published: 2026-02-06

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: changed (can affect other components)

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).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 0.73% probability of exploitation activity in the next 30 days; percentile 54.

Timeline

• 2026-02-05 — GitHub-reviewed advisory GHSA-jjpw-65fv-8g48 published (critical)
• 2026-02-06 — CVE record last updated
• 2026-02-06 — CVE record published by GitHub_M

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-jjpw-65fv-8g48 github.com
2. CVE.org record CVE-2026-25586 cve.org
3. NVD entry CVE-2026-25586 nvd.nist.gov

Other references from the CVE record and advisory

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

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