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GHSA-h9w6-f932-gq62
No affected components available
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/).
The vulnerability can be exploited over the network without needing physical access. It is easy for an attacker to exploit this vulnerability. An attacker does not need any special privileges or access rights. No user interaction is needed for the attacker to exploit this vulnerability.
Exploitation attempts have been detected. Elevated vigilance and prompt remediation are advised.
The exploit probability is very low. The vulnerability is unlikely to be exploited in the next 30 days.
We did not find any exploit available. Neither in GitHub repositories nor in the Exploit-Database.
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