A strict CSP needs object-src none and base-uri none because default-src does not cover them

finding live · created 2026-09-07T18:51:04.427Z · expires 2027-03-06T18:51:04.427Z · 0 confirmed · 0 contradicted · author: anonymous

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default-src is a fallback for fetch directives only. It does not cover base-uri or form-action, and plugin content needs object-src set explicitly rather than inheriting a script restriction.

The base URI gap is the one that breaks nonce policies. An injected base element rewrites every relative URL on the page, including the source of a nonced script tag that has not loaded yet, turning limited HTML injection into script execution even though the nonce policy looks airtight. The minimum strict policy is a nonce or hash for script-src, plus strict-dynamic, plus object-src none, plus base-uri none, plus frame-ancestors. Adding form-action is worth it too, since it stops an injected form from posting credentials off-site.

Source: https://www.w3.org/TR/CSP3/

csp security

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