CVE-2026-54163

Summary secure_headers builds the Content-Security-Policy value by stitching every configured directive together with ; separators. Three directive builders (build_sandbox_list_directive, build_media_type_list_directive, build_report_to_directive) interpolate caller-supplied strings into that value without scrubbing ;, \r, or \n. When an application forwards untrusted input into SecureHeaders.override_content_security_policy_directives (or append_…) for :sandbox, :plugin_types, or :report_to, an attacker can embed a literal ; and inject an arbitrary CSP directive into the header value. Because :sandbox and :plugin_types both sort alphabetically before :script_src in BODY_DIRECTIVES, the injected script-src lands earlier in the header and wins under the CSP first-occurrence rule, defeating the application's real script-src. End result: an 'unsafe-inline' * policy is forced for inline <script> despite the configured strict CSP, giving full XSS reachability anywhere reflected or stored content meets one of these three sinks. An existing ;/\n scrub is already present in the source-list builder (build_source_list_directive), but the three sibling builders here never received the same treatment and still emit caller bytes verbatim into the CSP value. Impact Although piping untrusted input into CSP directives is generally discouraged, applications that do so for one of the three uncovered directives turn that endpoint into an XSS sink with an effective * 'unsafe-inline' script-src, even though the global config says script_src: %w('self'). The same primitive can also be used to point report-to / report-uri at attacker infrastructure to silently siphon CSP violation reports — which include the violated URL, blocked-uri, source-file, line-number and a sample-snippet, useful for fingerprinting and for harvesting victim-internal URLs. The global default CSP set in Configuration.default is supposed to be a backstop: even if a controller appends a single risky value, the s

[![CVE-2026-54163 status](https://www.csirts.com/badge/CVE-2026-54163)](https://www.csirts.com/cve/CVE-2026-54163)