lopdf has a stack overflow via deeply nested PDF objects

lopdf::Document::load_mem (and the other load* entry points) parses nested PDF arrays and dictionaries with unbounded recursion. A small crafted PDF whose Catalog contains a deeply nested array (/X [[[ … ]]], on the order of 10,000 levels) exhausts the call stack and aborts the process with SIGABRT.

Because this is a stack-overflow abort rather than a panic!, it cannot be caught with catch_unwind: any service that parses untrusted PDF input with lopdf can be crashed by a ~21 KB file, resulting in a denial of service.

Confirmed on lopdf 0.41.0 and earlier; fixed in 0.42.0. Default configuration, no features changed.

Proof of concept

fn main() {
    let bytes = std::fs::read("poc.pdf").unwrap(); // ~10,380-deep nested array in the Catalog
    let _ = lopdf::Document::load_mem(&bytes);     // stack overflow -> SIGABRT
}

An equivalent PoC is a minimal PDF whose Catalog /X value is "[" * 10380 + "]" * 10380.

Suggested fix

Enforce a maximum object-nesting depth in the parser and return an Err instead of recursing without bound.