CVE-2026-107216: Excelize ANCHORARRAY: mutually-referencing array formulas recurse unboundedly via re-entrant CalcCellValue, causing a fatal stack overflow

### Summary `ANCHORARRAY` (calc.go:15137 on current master) evaluates each cell of the referenced spill range by calling the **exported** `CalcCellValue`, which unconditionally constructs a fresh `calcContext` — fresh entry marker, fresh iterations map, full `MaxCalcIterations` budget (calc.go:896-900). The circular-reference control only exists **within one context**: the entry-exclusion marker and per-ref iteration budget are fields of that single context, and completion-based caching (`formulaArgCache`/`calcRawCache`) only ever stores results of evaluations that *finish*. During a pure cycle no nested evaluation ever finishes, so nothing is ever cached to break the recursion, and each hop re-arms the entire budget. Two ordinary, Excel-legal constructs in an attacker-supplied workbook are enough: - `A1` (dynamic array formula, ref `A1:A1`): `_xlfn.ANCHORARRAY($B$1)` - `B1` (dynamic array formula, ref `B1:B1`): `_xlfn.ANCHORARRAY($A$1)` → `CalcCellValue → calcCellValue → evalInfixExp → parseReference → cellResolver → … → ANCHORARRAY → CalcCellValue → …` forever, ending in a **fatal, unrecoverable** Go runtime error: `runtime: goroutine stack exceeds …-byte limit / fatal error: stack overflow`. Go stack overflows cannot be recovered — the whole process aborts. ### Details - The terminating edge the iterations gate is supposed to provide does not exist across contexts: there is no in-flight tracking shared between nested `CalcCellValue` calls. - Both formulas are ordinary Excel-legal constructs; no exotic XML is required. - Excelize's own APIs reach formula evaluation on untrusted cells implicitly (e.g. `pivotTable.go:538`, `picture.go:985/1141`, `col.go:892`), so a service that merely adds a pivot table or a picture over such a workbook dies. - No option value prevents it: `MaxCalcIterations` is irrelevant because every hop gets a fresh budget. - Measured: a 64 MB stack budget is exhausted in ~0.09 s; the ~1 GB default in ~1–2 s. ### PoC A standalone program (public API only) was provided to the maintainer by email (`4-anchorarray-recursion`): `NewFile` + `SetCellFormula` with `FormulaOpts{Type: array, Ref: A1:A1 / B1:B1}`, then `CalcCellValue("Sheet1","A1")`. On master `ecd99d761fe0` (2026-09-08) the process aborts with `fatal error: stack overflow`; with the proposed patch the cycle terminates normally (`CYCLE_TERMINATED`) and existing calc tests pass. ### Impact An attacker ships a workbook containing the two formulas; any service that evaluates a formula over those cells — directly via `CalcCellValue`, or implicitly via `AddPivotTable` / `AddPicture` / auto-fit — aborts. Remote, unauthenticated, process-fatal, no configuration prevents it. ### Proposed fix Evaluate spill-range cells through the **current** calculation context — `fn.f.cellResolver(fn.ctx, …)` instead of the exported `CalcCellValue` — so the entry check and iterations gate of the running calculation apply. `cellResolver` returns the typed value directly (dropping a string round-trip); an `ArgEmpty → ""` shim preserves the existing `ToNumber` behavior for empty spill cells, and a fresh-context fallback covers the legacy nil-ctx test paths. A complete patch has been provided to the maintainer.