ImageSharp is a 2D graphics library. From 2.0.0 until 4.1.2, ExifReader.ReadValues64 trusts the 64-bit BigTIFF IFD entry count and iterates once per declared entry. When fewer than 20 bytes remain, ExifReader.ReadValue64 returns without advancing the stream or terminating the outer loop, so a small malformed BigTIFF can keep one decoder thread executing for an attacker-controlled duration. This report does not claim worker-pool exhaustion. This issue is fixed in version 4.1.2.

Affected: NuGet: SixLabors.ImageSharp

### Summary

`SixLabors.ImageSharp` 4.1.1 can spend an attacker-controlled duration decoding a small malformed BigTIFF. The BigTIFF IFD entry-count field is 64-bit. The reader iterates once per declared entry, but when fewer than 20 bytes remain for an entry, the entry read returns without advancing. A 24-byte input can therefore run billions of iterations without consuming input.

One decoder invocation occupied one executing thread for more than five seconds in the tested environment. This report makes no worker-pool exhaustion claim.

### Affected package and versions

- Package: `SixLabors.ImageSharp` (NuGet) - Affected range: `>= 2.0.0, <= 4.1.1` - Commit `0815358f9202a78bc7f3b83e19282dc3654b500f` corresponds to release **v4.1.1**.

BigTIFF decoding and the unbounded `ReadValues64` loop first appear in v2.0.0. Every release tag from v2.0.0 through v4.1.1 retains that loop without constraining the entry count or terminating when a truncated entry makes no progress. The 24-byte PoC exceeded the five-second timeout on published v2.0.0 and v4.1.1; the one-entry control returned promptly on both. ### Details

[`ReadValues64`](https://github.com/SixLabors/ImageSharp/blob/0815358f9202a78bc7f3b83e19282dc3654b500f/src/ImageSharp/Metadata/Profiles/Exif/ExifReader.cs#L220-L231) trusts the 64-bit IFD count and loops once per declared entry. When fewer than 20 bytes remain, [`ReadValue64`](https://github.com/SixLabors/ImageSharp/blob/0815358f9202a78bc7f3b83e19282dc3654b500f/src/Imag…