CVE-2026-55497 Overview
Cloudreve is a self-hosted file management and sharing system. CVE-2026-55497 affects the built-in thumbnail and avatar image decoders in versions prior to 4.17.0. The decoders limit the compressed file size but do not constrain decoded pixel dimensions. An authenticated user can upload a small crafted PNG, JPEG, or GIF that expands into enormous pixel buffers during decoding. This triggers unbounded memory allocation and terminates the Cloudreve process through a fatal out-of-memory (OOM) condition. The vulnerability is categorized as uncontrolled resource consumption [CWE-400]. Cloudreve released version 4.17.0 to address the flaw.
Critical Impact
An authenticated attacker can crash the Cloudreve service on demand by submitting a small image file, denying access to file management and sharing functionality for all users.
Affected Products
- Cloudreve versions prior to 4.17.0
- Built-in thumbnail image decoder in pkg/thumb/builtin.go
- Avatar image processing pipeline (PNG, JPEG, GIF formats)
Discovery Timeline
- 2026-07-31 - CVE-2026-55497 published to NVD
- 2026-07-31 - Last updated in NVD database
Technical Details for CVE-2026-55497
Vulnerability Analysis
Cloudreve's built-in image decoder processes user-supplied thumbnails and avatars using standard Go image libraries. The application enforces a maximum compressed input size but never validates the decoded pixel dimensions. Attackers exploit this asymmetry between compressed and decompressed representations of image formats. A well-known example is the pixel flood attack, where a small compressed PNG declares dimensions such as 60000x60000 pixels. When Go's image decoder allocates the pixel buffer, memory consumption balloons into the gigabyte range. The runtime terminates the process with a fatal OOM error, denying service to every connected user.
Root Cause
The root cause is uncontrolled resource consumption [CWE-400] in the thumbnail generation logic. The builtin thumbnail generator in pkg/thumb/builtin.go did not enforce a maximum pixel dimension constraint before decoding. Size limits applied only to the on-disk compressed payload, leaving the in-memory decoded representation unbounded.
Attack Vector
Exploitation requires an authenticated account on the target Cloudreve instance. The attacker uploads a specially crafted PNG, JPEG, or GIF file with large declared pixel dimensions but small compressed size. When Cloudreve generates a thumbnail or processes the avatar, the decoder attempts to allocate memory proportional to width times height times bytes-per-pixel. The Go runtime then terminates the entire Cloudreve process, affecting all tenants of the shared service.
package thumb
import (
+ "bytes"
"context"
"fmt"
"github.com/cloudreve/Cloudreve/v4/pkg/filemanager/manager/entitysource"
Source: GitHub Commit 3607f79 — the patch introduces a maximum pixel dimensions constraint in the builtin thumbnail generator, decoding image headers first to reject oversized dimensions before allocating pixel buffers.
Detection Methods for CVE-2026-55497
Indicators of Compromise
- Sudden termination of the Cloudreve process with a Go runtime fatal error: runtime: out of memory message in service logs.
- Repeated container or systemd restart events for the Cloudreve service without corresponding legitimate load spikes.
- Upload activity from authenticated users involving small image files immediately preceding a service crash.
- Kernel oom-killer log entries targeting the Cloudreve binary in dmesg or /var/log/messages.
Detection Strategies
- Monitor Cloudreve service uptime and correlate crash events with recent thumbnail or avatar upload requests.
- Inspect uploaded image files server-side for declared pixel dimensions that exceed reasonable thresholds prior to decoding.
- Alert on Go runtime panics or OOM signatures in application logs and container orchestrator events.
Monitoring Recommendations
- Track memory usage of the Cloudreve process and generate alerts when resident set size approaches configured limits.
- Log the source user account, IP address, and file metadata for every thumbnail generation request to support post-incident attribution.
- Enable host-level auditing of OOM kill events and forward them to a centralized log platform for correlation.
How to Mitigate CVE-2026-55497
Immediate Actions Required
- Upgrade Cloudreve to version 4.17.0 or later, which enforces maximum pixel dimensions in the built-in thumbnail generator.
- Audit user accounts and disable or restrict registration to trusted users until the patch is deployed.
- Review recent service crash logs to determine whether exploitation has already occurred.
Patch Information
The fix is delivered in Cloudreve 4.17.0. See the GitHub Release 4.17.0 notes and the corresponding GitHub Security Advisory GHSA-g9j2-8w95-3vwv. The code change in GitHub Commit 3607f79 adds a dimension check to pkg/thumb/builtin.go before decoding image content into memory.
Workarounds
- Deploy Cloudreve behind a reverse proxy that inspects uploaded images and rejects files whose decoded dimensions exceed a safe threshold.
- Run Cloudreve inside a container or systemd unit with strict memory limits and automatic restart policies to contain impact.
- Configure an external image processing service, such as ImageMagick with resource limits, in place of the built-in decoder where feasible.
# Configuration example: constrain Cloudreve memory and restart on OOM (systemd)
[Service]
MemoryMax=1G
MemoryHigh=768M
Restart=on-failure
RestartSec=5s
# Or with Docker:
# docker run --memory=1g --memory-swap=1g --restart=on-failure cloudreve/cloudreve:4.17.0
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
