Excelize is a Go language library for reading and writing Microsoft Excel spreadsheets. From 2.3.1 to 2.11.0, extractPart allocates a byte slice directly from an attacker-controlled CFB directory-entry size before validating the sector chain or size domain. extractPart trusts the CFB directory entry streamSize for EncryptionInfo and EncryptedPackage allocations before validating the stream. When a crafted OLE compound file declares a negative or extremely large EncryptionInfo or EncryptedPackage stream size, the declared size reaches make with a negative length or forces a multi-gigabyte allocation, allowing an attacker to panic or exhaust process memory. No fixed version is available as of this review.

Affected: Go: github.com/xuri/excelize/v2

### Summary

`extractPart` allocates directly from the mscfb directory-entry size with no clamping (`crypt.go:198`, same missing bound at `crypt.go:204`):

```go buf := make([]byte, entry.Size) ```

`entry.Size` is the raw `streamSize` field from the attacker-controlled CFB directory entry (mscfb v1.0.8 `fixFile`, `file.go:109-120`: full uint64 → int64 for major version 4, low uint32 for v3). mscfb only detects short sector chains later, inside `File.Read` (`file.go:485-489` "emergency brake") — i.e. **after** the allocation. The zip path already enforces `UnzipSizeLimit`, and the KDF path bounds work with `maxSpinCount`; this path consults no limit at all.

### Details

Two consequences, both execution-confirmed against pristine master (`ecd99d761fe0`, 2026-09-08):

1. **Hard crash**: a version-4 CFB declaring `streamSize = 0xFFFFFFFFFFFFFFFF` yields `entry.Size == -1` → `panic: runtime error: makeslice: len out of range`, which escapes `Decrypt` (`openReaderAt` converts errors, not panics, `excelize.go:210-214`) and kills the process. 2. **Memory exhaustion**: any CFB (v3 suffices) declaring `streamSize` up to `0xFFFFFFFF` forces up to 4 GiB of zeroed allocation from a ~12 KB file — an amplification factor of ~350,000×, trivially repeated per request in memory-limited deployments.

Call path: `OpenFile`/`OpenReader`/`OpenBytes` → `openReaderAt` (`excelize.go:208-215`, OLE magic sniff) → `Decrypt` (`crypt.go:145-150`) → `extractPart` (`crypt.go:198`).

### PoC

A standalone…