Report privately, not through the public issue tracker:

• Preferred: GitHub's private vulnerability reporting, from the repository's Security tab.
• Alternative: email libraz@libraz.net.

Include the formula or workbook that triggers it, what happened, the affected version and binding, and a minimal reproduction if you have one. Expect an acknowledgement within a few days.

Pre-1.0. Fixes land on the latest release only; older versions are not patched.

Formulon evaluates formulas it did not write — that is the whole job — so the parser and the evaluator are the interesting surface. In scope:

• A formula or workbook that causes a crash, a hang, unbounded recursion or unbounded memory growth, in the C++ core or through the WebAssembly, Python or CLI packaging.
• Anything reached through FILTERXML, which parses a string the caller did not write: entity expansion, external entity resolution, or resource exhaustion.
• Any path by which evaluating a formula touches the filesystem or the network. Evaluation is meant to be pure; a reachable side effect is a vulnerability even if it looks benign.
• Escapes from the WebAssembly sandbox, or a formula that reads memory belonging to another workbook in the same instance.

• Divergence from Excel. A cell that computes a different value than Excel does is a correctness bug, tracked against the oracle data; report it as a normal issue.
• Documented ceilings behaving as documented — iteration, depth and cell-count limits exist so a hostile workbook cannot exhaust the host. A ceiling that can be bypassed is in scope.
• Findings that require an attacker to already control the process embedding the engine.