CVE-2026-91971: Vikunja before 2.6.0 Denial of Service via Avatar Upload
Summary 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/taskattachment.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.
Fix 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).
Other sources
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.
— NVD
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
go/code.vikunja.io/apito a version that resolves this vulnerability.Fixed in 2.6.0 - Upgrade
Upgrade
Vikunjato a version that resolves this vulnerability.Fixed in 2.6.0 - Configuration
Apply a pixel-dimension decode cap to avatar and project-background image uploads, as on the task-attachment preview path.
Vikunja avatar and project-background upload decode paths maxPixels = 50MP - Configuration
Bound both output dimensions during avatar resizing; do not derive an uncapped width from a fixed height of 1024.
Vikunja avatar resize resize output dimensions = Cap width as well as height
Event History
Frequently Asked Questions
Who can exploit this issue?
An authenticated Vikunja user with access to avatar or project-background uploads can exploit it. No user interaction is required.
Which deployments are affected?
Vikunja versions before 2.6.0 are affected where avatar or project-background upload endpoints are available to authenticated users.
What is required for exploitation?
An attacker needs to upload a crafted image that is small in file size but decodes to an excessive pixel count, such as an image with extreme dimensions. Repeated or concurrent uploads can consume CPU and memory.
How can teams identify possible exploitation?
Investigate unusually frequent or concurrent avatar and project-background uploads, especially uploads associated with CPU or memory exhaustion during image processing.