GHSA-jxpf-xq2m-q525

### Summary OpenMcdf does not detect cycles in the directory entry red-black tree of a Compound File Binary (CFB) document. A crafted CFB file with a cycle in the `LeftSiblingID` / `RightSiblingID` chain causes `Storage.EnumerateEntries()` and `Storage.OpenStream()` to loop indefinitely, consuming the calling thread with no possibility of recovery via `try/catch`.

### Details CFB directory entries form a red-black tree linked by `LeftSiblingID` and `RightSiblingID` fields. OpenMcdf's `DirectoryTreeEnumerator` and `DirectoryTree.TryGetDirectoryEntry` traverse this tree without tracking visited node IDs, so a crafted cycle (e.g. entry A's `RightSiblingID` points to entry B, and entry B's `LeftSiblingID` points back to entry A) causes traversal to loop indefinitely.

Two distinct code paths are affected:

- **`Storage.EnumerateEntries()`** - `DirectoryTreeEnumerator.MoveNext()` never returns `false`; the same entry is yielded on every iteration and the caller's `foreach` never exits. Heap grows unboundedly as entries accumulate. - **`Storage.OpenStream()`** - `DirectoryTree.TryGetDirectoryEntry` loops indefinitely inside `DirectoryEntries.TryGetSibling` during the name lookup.

### PoC A crafted CFB file with a sibling cycle (see attached) triggers the issue with the following code:

```csharp using OpenMcdf;

using var ms = new MemoryStream(File.ReadAllBytes("crafted.cfb")); using var root = RootStorage.Open(ms);

// Never returns - EnumerateEntries loops indefinitely foreach (var entry in root.EnumerateEntries()) { Console.WriteLine(entry.Name);

if (entry.Type == EntryType.Stream) root.OpenStream(entry.Name); // also hangs depending on the cycle structure } ```

### Impact A denial of service affecting any application that opens untrusted CFB files with OpenMcdf. A small crafted input carrying a valid CFB magic header (`D0 CF 11 E0 A1 B1 1A E1`) is sufficient to pass initial format validation and reach the vulnerable traversal code. No exception is thrown, so `try/catch` cannot protect callers. The affected thread is unrecoverable without killing the process.