Prototype pollution in Tampermonkey via the internationalized @antifeature tag

What happened and what to do

The post by the author tmctmt, published on 24 October 2026, describes a prototype pollution flaw in Tampermonkey, a userscript manager. The @antifeature tag, used to flag monetization in scripts, accepts internationalization with a colon and locale, as in @antifeature:polluted. These entries are stored as a nested object keyed by type and locale, which allows values to be assigned to prototype properties. The author notes that the locale passes through a normalization function that strips camelCase and snake_case variables, making an exploitable gadget hard to find. He also tried to use the ask key to install scripts silently but could not overwrite it through the prototype. For that reason, the author himself rates the impact as modest and dependent on user interaction.

For a company, the case highlights two practical points. First, userscript metadata is parsed before installation, so an install page can be driven by untrusted code. An inventory check can show where these managers are installed on workstations and which scripts are in use, backed by an allowlist policy for approved userscripts. Second, any internal tool that reads script metadata can be hardened by validating keys such as __proto__, constructor and prototype, using prototype-free objects and automated tests for these inputs. Monitoring Tampermonkey updates and approved scripts completes the process.

How the consultancy can help

A diagnosis of userscript and manager exposure, such as Tampermonkey, across your organization's devices, covering which scripts are in use and the current installation policy. Then a scoped proposal for an allowlist of scripts, installation controls and, if internal tools read script metadata, a review and validation tests for key handling.

Next step

If your team uses userscript managers or builds tools that read script metadata, send a short description of your case. We will prepare a scoped proposal for the diagnosis and implementation.