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.
Patches
Fixed in 0.26.0 (PR #575). The tag-only gate was removed so that any
pathologically tall childless box collapses regardless of whether it is a
replaced element, closing the missing-src, visibility:hidden,
undecodable-format, and empty-<svg> vectors at once, along with the
trailing-sibling variant.
Workarounds
Upgrade to 0.26.0 or later. If upgrading is not immediately possible, validate
or constrain untrusted CSS (in particular very large height / vh on
elements) before passing HTML to fulgur.
Related
Versions prior to 0.19.0 additionally lacked any page-count cap, allowing an
unbounded (rather than 10,000-page) variant of this amplification and a
non-terminating loop on non-finite heights. That earlier variant is tracked
separately as GHSA-j5cx-ph8g-95v3.
Attack Vector rationale
fulgur performs no network I/O of its own; it renders HTML/CSS handed to it by
the embedding application. This advisory scores the crate independent of any
specific adopting program, so per the CVSS v3.1 User Guide §3.7 (scoring library
vulnerabilities related to incoming data) the Attack Vector is assessed as
Network for the reasonable worst-case deployment — a network-facing service
that renders untrusted, attacker-supplied HTML without user interaction. A
concrete system that receives the HTML in one component and passes it to fulgur
in a separate component may assess a lower environmental Attack Vector (Local,
per §3.10).
References
References
fulgurconverts untrusted HTML/CSS into PDF, commonly on a server thatprocesses 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:
src(the common offline-first case),visibility:hiddenimage,<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 rendererallocates 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.
Patches
Fixed in 0.26.0 (PR #575). The tag-only gate was removed so that any
pathologically tall childless box collapses regardless of whether it is a
replaced element, closing the missing-
src,visibility:hidden,undecodable-format, and empty-
<svg>vectors at once, along with thetrailing-sibling variant.
Workarounds
Upgrade to 0.26.0 or later. If upgrading is not immediately possible, validate
or constrain untrusted CSS (in particular very large
height/vhonelements) before passing HTML to fulgur.
Related
Versions prior to 0.19.0 additionally lacked any page-count cap, allowing an
unbounded (rather than 10,000-page) variant of this amplification and a
non-terminating loop on non-finite heights. That earlier variant is tracked
separately as GHSA-j5cx-ph8g-95v3.
Attack Vector rationale
fulgurperforms no network I/O of its own; it renders HTML/CSS handed to it bythe embedding application. This advisory scores the crate independent of any
specific adopting program, so per the CVSS v3.1 User Guide §3.7 (scoring library
vulnerabilities related to incoming data) the Attack Vector is assessed as
Network for the reasonable worst-case deployment — a network-facing service
that renders untrusted, attacker-supplied HTML without user interaction. A
concrete system that receives the HTML in one component and passes it to fulgur
in a separate component may assess a lower environmental Attack Vector (Local,
per §3.10).
References
References