ses's global contour bindings leak into Compartment lexical scope
Impact
Web pages and web extensions using ses and the Compartment API to evaluate third-party code in an isolated execution environment that have also elsewhere used const, let, and class bindings in the top-level scope of a <script> tag will have inadvertently revealed these bindings in the lexical scope of third-party code.
Patches
This compromise is addressed in ses version 1.12.0. The mechanism for confining third-party code involves a with block and a semi-opaque scope Proxy. The proxy previously revealed any named property to the surrounding lexical scope if it were absent on globalThis, so that the third-party code would receive an informative ReferenceError, relying on the invalid assumption that only properties of globalThis are in the top-level lexical scope. The solution makes the scope proxy fully opaque. Consequently, accessing an unbound free lexical name will produce undefined instead of throwing ReferenceError.
Assigning to an unbound free lexical name will continue to throw a ReferenceError.
Workarounds
This problem can be mitigated either by avoiding top-level let, const, or class bindings in <script> tags, which is an existing industry best-practice, or change these to var bindings to be reflected on globalThis, or upgrade ses to version 1.12.0 or greater.
Some bundlers by default transform top-level let, const, and class bindings to var.
Disclosure
This vulnerability was disclosed by @mingijunggrape in the course of their studies at UNIST (Ulsan National Institute of Science and Technology) as a member of the Web Security Lab (https://websec-lab.github.io/).
Affected software
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| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| npm package | ses | < 1.12.0 | 1.12.0 |
References
- CVSS vector
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N- Weakness (CWE)
CWE-497- Platform
- npm
- Package
ses- Disclosed
- 18.04.2025
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-h9w6-f932-gq62 - CC BY 4.0
Vulnerability history: ses
All published vulnerabilities of this package (npm package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 4
- First → latest
- 10.2019 04.2025
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 2
- High 1
- Low 1
Disclosures per year
- 2019: 1
- 2020: 0
- 2021: 0
- 2022: 1
- 2023: 1
- 2024: 0
- 2025: 1
- 2026: 0
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