CVE-2026-68537: Fulgur: Non-painting replaced elements amplify to thousands of blank PDF pages (denial of service)

fulgur converts untrusted HTML/CSS into PDF, commonly on a server that processes input supplied by many tenants. In versions prior to 0.26.0, a childless box that resolves to a pathologically tall height was amplified into thousands of blank PDF pages, even when it produces no visible output.

The childless-collapse defense that would normally collapse such a box was gated by a tag-only “replaced content” check, so any non-painting replaced element bypassed it, including:

• an unresolved src (the common offline-first case),
• a visibility:hidden image,
• an undecodable image format, and
• an empty <svg>.

A trailing-sibling variant of the same gap was also open. A few bytes of HTML therefore amplified into roughly MAX_PAGES (10,000) blank pages; the renderer allocates and runs a per-page loop over them, producing CPU and memory exhaustion. An attacker able to submit HTML to a fulgur-based conversion service can trigger this with a trivially small payload, denying service to the host and any co-tenants.