Excelize is a Go language library for reading and writing Microsoft Excel spreadsheets. From 2.3.1 to 2.11.0, agile decryption accepts an attacker-controlled spinCount and performs that many password-key derivation iterations before verifier validation. OpenFile reaches agileDecrypt, which passes the unbounded spinCount to convertPasswdToKey before password verification. When a crafted OLE encrypted-workbook header supplies an excessive spinCount and the file is opened, the key-derivation loop performs unbounded attacker-selected work and cannot be cancelled, allowing an attacker to consume a CPU core for an attacker-controlled duration. No fixed version is available as of this review.

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

Opening a file whose first eight bytes are the OLE magic number sends excelize down the decryption path whether or not the caller set a password, or supports encrypted workbooks at all. `openReaderAt` (excelize.go:198-215) branches on the header alone, and `agileDecrypt` calls `convertPasswdToKey` before the verifier hash is checked, so the key-derivation loop runs `spinCount` times regardless.

`spinCount` (crypt.go:98) is a plain `int` filled by a bare `xml.Unmarshal` of the file's own EncryptionInfo stream. Nothing bounds it.

A 3072-byte file with spinCount 100000000 makes `OpenFile` take 58.65s on v2.11.0 with default options, then return `zip: not a valid zip file`. It is linear at about 0.6 microseconds per iteration and the attacker picks the number, so 1e9 is roughly ten minutes. Nothing on the path takes a `context.Context`, so the caller cannot cancel it; in an HTTP handler the write timeout returns a response while the goroutine keeps spinning. Memory stays flat at 24 MB, so nothing reclaims it either.

``` v2.5.0 spinCount=10000000 5.307s v2.9.1 spinCount=10000000 5.444s v2.11.0 spinCount=10000000 7.529s v2.11.0 spinCount=100000000 58.654s ```

The loop arrived with `crypt.go` in v2.3.1 and is unchanged through v2.11.0.

Excel and LibreOffice write spinCount 100000, which costs 61ms here, so a ceiling well above the legitimate value would cost real files nothing.

This is availability only, and it is not a vulnerability for a program that only open…