CVE-2026-25641: @nyariv/sandboxjs vulnerable to sandbox escape via TOCTOU bug on keys in property accesses

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 0.6%, percentile 47: FIRST’s model estimates a 0.6% chance of exploitation activity in the next 30 days, higher than 47% of all scored CVEs (low). 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, there is a sandbox escape vulnerability due to a mismatch between the key on which the validation is performed and the key used for accessing properties. Even though the key used in property accesses is annotated as string, this is never enforced. So, attackers can pass malicious objects that coerce to different string values when used, e.g., one for the time the key is sanitized using hasOwnProperty(key) and a different one for when the key is used for the actual property access. This vulnerability is fixed in 0.8.29.

From the GitHub advisory

Summary A sandbox escape vulnerabilities due to a mismatch between the key on which the validation is performed and the key used for accessing properties.

Details Even though the key used in property accesses ( in the code below) is annotated as , this is never enforced: https://github.com/nyariv/SandboxJS/blob/6103d7147c4666fe48cfda58a4d5f37005b43754/src/executor.ts#L304-L304 So, attackers can pass malicious objects that coerce to different string values when used, e.g., one for the time the key is sanitized using and a different one for when the key is used for the actual property access.

PoC

Impact Remote code execution, if attacker can execute code inside the sandbox.

Weakness type: CWE-367, 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): 0.58% probability of exploitation activity in the next 30 days; percentile 47.

Timeline

• 2026-02-05 — GitHub-reviewed advisory GHSA-7x3h-rm86-3342 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-7x3h-rm86-3342 github.com
2. CVE.org record CVE-2026-25641 cve.org
3. NVD entry CVE-2026-25641 nvd.nist.gov

Other references from the CVE record and advisory

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

Critical npm advisories · Today’s Security Digest