Summary
Vikunja: Unbounded image decode on avatar and project-background uploads enables decode/resize amplification
The 50-megapixel decode guard exists only on the task-attachment preview path. Avatar and project-background uploads decode uploaded images with no pixel cap. Worse, the avatar resize fixes the output height at 1024 and derives the width from the aspect ratio, so a tiny extreme-aspect-ratio PNG expands to an enormous output image, an input-side pixel cap would not catch it.
Details
The only maxPixels check (50MP) is in TaskAttachment.GetPreview (pkg/models/task_attachment.go ~lines 332-338). No such check guards:
- avatar upload:
pkg/modules/avatar/upload/upload.go(~lines 82, 137, 141) - project background:
pkg/modules/background/handler/background.go(~lines 174, 269, 289)
imaging.Resize(img, 0, 1024, imaging.Lanczos) (upload.go ~line 141) fixes height=1024 and derives width from the aspect ratio: a 20000x10 input yields a ~2,048,000 x 1024 output (~2.1 billion pixels), so a few-hundred-byte file drives huge CPU and memory.
PoC (verified at runtime against v2.5.0)
PUT /api/v1/user/settings/avatar/upload avatar=8000x8000 PNG (64MP, 192KB) -> 200 (accepted; exceeds the 50MP attachment cap)
PUT /api/v1/user/settings/avatar/upload avatar=20000x10 PNG (681 bytes) -> 200 after ~19.6s of server processing
Contrast (guard present): uploading the 64MP PNG as a task attachment and requesting its preview returns in ~1ms without decoding, GetPreview rejects it via maxPixels and falls back to the raw file.
Impact
A small crafted upload drives disproportionate CPU and memory on the avatar and background paths. Repeated or concurrent requests can exhaust server resources. Amplification comes from both the missing input pixel cap and the height-fixed resize, so an input-side cap alone is insufficient.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Apply a pixel-dimension cap (as on the attachment path) to the avatar and background decode paths, and bound the resize output dimensions (cap width as well as height).
Frequently Asked Questions
- What is CVE-2026-91971? CVE-2026-91971 is a high-severity uncontrolled resource consumption vulnerability in code.vikunja.io/api (go), affecting versions <= 2.5.0. It is fixed in 2.6.0. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- Which versions of code.vikunja.io/api are affected by CVE-2026-91971? code.vikunja.io/api (go) versions <= 2.5.0 is affected.
- Is there a fix for CVE-2026-91971? Yes. CVE-2026-91971 is fixed in 2.6.0. Upgrade to this version or later.
- Is CVE-2026-91971 exploitable, and should I be worried? Whether CVE-2026-91971 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-91971 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-91971? Upgrade
code.vikunja.io/apito 2.6.0 or later.