CVE-2026-91971 in Vikunja
Summary
by MITRE • 09/15/2026
Vikunja before 2.6.0 fails to apply pixel decode limits to avatar and project-background upload endpoints, allowing authenticated users to upload crafted images that decode to excessive pixel counts. Attackers can upload small images with extreme aspect ratios that consume significant CPU and memory during processing, causing denial of service through repeated or concurrent uploads.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified in Vikunja versions prior to 2.6.0 represents a critical failure in input validation logic within the image processing subsystem. Specifically, the application fails to enforce pixel decode limits on two primary endpoints: avatar upload and project-background upload. This oversight allows authenticated users to bypass standard security controls designed to prevent resource exhaustion attacks. By submitting crafted images that possess extreme aspect ratios or deceptive file headers, an attacker can trigger a disproportionate consumption of system resources during the decoding phase. The core technical flaw lies in the absence of checks on the decoded dimensions of the image before it is processed for storage or display, leaving the server vulnerable to denial-of-service conditions driven by computational exhaustion rather than network bandwidth saturation.
From a technical perspective, this vulnerability exploits the behavior of common image processing libraries which may allocate memory based on the theoretical maximum pixel count defined in an image header, even if the actual visual data is minimal. An attacker can construct images with very small file sizes but extremely large width and height values encoded in their metadata. When Vikunja attempts to decode these files for validation or thumbnail generation, the underlying library allocates memory proportional to the product of these dimensions. This results in significant CPU usage as the system processes the malformed data and substantial memory allocation that can quickly exhaust available server resources if multiple such requests are processed concurrently. The lack of a hard limit on pixel count means there is no upper bound on resource consumption, making this an efficient vector for causing service degradation or complete unavailability.
The operational impact of this vulnerability is primarily centered around denial-of-service conditions affecting the availability and integrity of the Vikunja instance. Since authentication is required to exploit this flaw, a malicious insider or any compromised user account can trigger these excessive resource allocations. Repeated uploads of such crafted images can lead to server crashes, high latency for legitimate users, or complete service outage due to memory exhaustion. This directly impacts business continuity and operational reliability, as the application becomes unresponsive under load generated by this specific attack vector. The ability to degrade performance through simple file uploads undermines the trust in the platform's stability and security posture, potentially leading to data loss if processes are terminated abruptly during resource-intensive operations.
This vulnerability aligns with CWE-787: Out-of-bounds Read and CWE-400: Uncontrolled Resource Consumption, as it involves improper handling of input that leads to excessive allocation of system resources. In the context of the MITRE ATT&CK framework, this behavior is consistent with T1496: Resource Hijacking, where an attacker uses computing resources for their own purposes or causes denial-of-service by exhausting available capacity. It also reflects aspects of T1505.003: Server Software Component: Web Components, as the exploitation occurs through standard web application endpoints designed to handle file uploads. The failure to validate image dimensions before processing is a classic example of insufficient input validation that leads to resource exhaustion vulnerabilities in modern web applications.
To mitigate this vulnerability, organizations running Vikunja versions prior to 2.6.0 must upgrade immediately to version 2.6.0 or later, where the pixel decode limits have been properly implemented and enforced on all relevant upload endpoints. For environments where immediate upgrading is not feasible, temporary mitigations should include implementing strict rate limiting on avatar and project-background upload endpoints to restrict the frequency of such requests from any single user account. Additionally, deploying a Web Application Firewall with rules capable of detecting anomalous image metadata or enforcing size limits at the network edge can help block malicious uploads before they reach the application server. It is also advisable to configure system-level resource controls, such as memory limits and CPU throttling for web service processes, to contain the impact of any successful exploitation attempts until a permanent patch is applied. Regular security audits focusing on image processing libraries and input validation logic are recommended to prevent similar vulnerabilities in other components of the application stack.