CVE-2026-101203 in Image Viewer
Summary
by MITRE • 09/29/2026
A vulnerability has been found in FastStone Image Viewer up to 8.3. The impacted element is an unknown function of the component 1bpp RLE Decoder. The manipulation leads to out-of-bounds write. The attack can be initiated remotely. The vendor was contacted early about this disclosure but did not respond in any way.
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Analysis
by VulDB Data Team • 09/29/2026
The FastStone Image Viewer, a widely used image viewing and editing application for Windows platforms, has been identified with a critical security flaw within its 1bpp RLE Decoder component. This vulnerability affects versions up to 8.3 and represents a significant risk due to the potential for remote exploitation through crafted image files. The core of the issue lies in an unknown function responsible for handling Run-Length Encoded (RLE) compressed data specifically formatted as one-bit-per-pixel images. RLE is a simple form of lossless data compression where sequences of identical data values are stored as a single value and count, which allows for efficient storage of images with large areas of uniform color or black-and-white graphics. While this format is less common in modern high-resolution photography compared to JPEG or PNG, it remains prevalent in legacy systems, embedded devices, and specific graphic design workflows, making the vulnerability relevant across various user bases who may encounter such files from untrusted sources.
The technical nature of this flaw is classified as an out-of-bounds write, which falls under CWE-787: Out-of-Bounds Write in standard vulnerability classification frameworks. This type of memory corruption occurs when a program writes data to a memory location outside the bounds of the intended buffer or array. In the context of the FastStone Image Viewer's decoder, this likely stems from an improper validation of the RLE compressed stream length against the allocated buffer size during the decompression process. When processing maliciously crafted input where the declared run lengths exceed the actual available space in the destination memory region, the application writes data beyond its intended limits. This overwriting can corrupt adjacent memory structures, including heap metadata or function pointers, which is a classic precursor to arbitrary code execution vulnerabilities such as CWE-120: Buffer Copy without Checking Size of Input. The severity of this flaw is amplified by the fact that it resides in a core image processing library, meaning any attempt to view a maliciously constructed 1bpp RLE file can trigger the exploit condition automatically if the application processes images upon opening or previewing them.
From an operational perspective, the ability to initiate this attack remotely significantly increases its threat level. Remote exploitation implies that an attacker does not need physical access to the victim's machine or prior authentication credentials within the target system. Instead, the vulnerability can be triggered by simply directing a user to open a malicious file via email attachment, web download, or network share. If successful, the out-of-bounds write could allow for arbitrary code execution with the privileges of the currently logged-in user. This aligns with ATT&CK technique T1203: Exploitation for Client Execution, where attackers leverage vulnerabilities in client-side applications to gain initial access and execute malicious payloads on the endpoint. The consequences can range from data theft and system compromise to the installation of persistent malware or ransomware. Given that image viewers are often used with files received from external parties who may not be trusted, this attack vector is particularly dangerous as it bypasses many traditional perimeter defenses by exploiting user interaction with seemingly benign media files.
The vendor's lack of response to early disclosure efforts raises concerns regarding the timely availability of a patch or mitigation strategy. In such scenarios where official fixes are delayed or unavailable, users must rely on defensive measures and workarounds to mitigate risk. It is strongly recommended that users exercise extreme caution when opening image files from untrusted sources, particularly those with unusual file extensions or sizes relative to their content type. Utilizing alternative software for viewing 1bpp RLE images can provide a temporary layer of protection if the primary application remains vulnerable. Additionally, enabling strict macro and script execution policies within operating systems can help prevent secondary stages of exploitation that might follow initial code execution attempts. Security teams should also consider implementing network-level controls to restrict access to untrusted file repositories and ensuring that endpoint detection and response solutions are configured to monitor for anomalous memory allocation patterns or unexpected process spawns following image viewing activities, thereby detecting potential exploitation attempts even in the absence of a vendor-provided patch.