# CVE-2026-101203: FastStone Image Viewer out-of-bounds write in 1bpp RLE decoder

Source: https://junglewise.ai/threats/cve-2026-101203-a-vulnerability-has-been-found-in-faststone-image-viewer-up-to-8  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-09-28. Updated: 2026-09-28  
License: CC BY 4.0 with a link back

CVE-2026-101203 · Severity: medium · CVSS 6.3 · Published 2026-09-28

FastStone Image Viewer is a popular image viewing and editing application. The application contains an out-of-bounds write vulnerability in its RLE (Run-Length Encoding) decoder component that can be triggered when processing certain image files. An attacker can exploit this remotely by sending a specially crafted image file, potentially leading to application crash or code execution.

Technologies: [Faststone Image Viewer](https://junglewise.ai/threats/technologies/image-viewer). Vendors: [Faststone](https://junglewise.ai/threats/vendors/faststone).

## Quick answers

**Is CVE-2026-101203 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-101203 being exploited in the wild.

**How severe is CVE-2026-101203?** Medium severity, with a CVSS score of 6.3, published 28 September 2026.

**Which products and versions are affected?** Faststone Image Viewer up to 8.3.

**Is there a fix for CVE-2026-101203?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-101203 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-101203?** Check whether you run Faststone Image Viewer at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

The vulnerability exists in the 1bpp (1-bit-per-pixel) RLE decoder component of FastStone Image Viewer, where an unknown function fails to properly validate buffer boundaries during decompression. This out-of-bounds write can be triggered remotely via a malformed image file without requiring authentication or user interaction beyond opening the file. Successful exploitation could result in denial of service or arbitrary code execution with the privileges of the application.

## Affected products

- FastStone Image Viewer up to 8.3

## Timeline

- 2026-09-28: disclosed

## References

- [https://vuldb.com/cve/CVE-2026-101203](https://vuldb.com/cve/CVE-2026-101203)
- [https://vuldb.com/submit/907575](https://vuldb.com/submit/907575)

## Related threats

- [FastStone Image Viewer out-of-bounds read in Box-Average Downsampler](https://junglewise.ai/threats/cve-2026-116463-a-vulnerability-was-identified-in-faststone-image-viewer-up-to-8)
- [FastStone Image Viewer out-of-bounds read in PCX decoder](https://junglewise.ai/threats/cve-2026-101205-a-vulnerability-was-determined-in-faststone-image-viewer-up-to-8)
- [FastStone Image Viewer out-of-bounds read in TGA handler](https://junglewise.ai/threats/cve-2026-101204-a-vulnerability-was-found-in-faststone-image-viewer-up-to-8-3)
- [FastStone Image Viewer out-of-bounds write in TGA handler](https://junglewise.ai/threats/cve-2026-101202-a-flaw-has-been-found-in-faststone-image-viewer-up-to-8-3-the)
- [FastStone Image Viewer integer overflow in PSD parser](https://junglewise.ai/threats/cve-2026-30041-faststone-image-viewer-integer-overflow-in-psd-parser)