CVE-2026-27641 is a critical-severity (CVSS 9.8) CWE-1336 vulnerability in flask-reuploaded. A fix is available for flask-reuploaded — see the affected versions and patch details below.
Flask-Reuploaded vulnerable to Remote Code Execution via Server-Side Template Injection
Exploitation Status
No confirmed exploitation observed yet
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-27641.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-27641 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
flask-reuploadedReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Impact
A critical path traversal and extension bypass vulnerability in Flask-Reuploaded allows remote attackers to achieve arbitrary file write and remote code execution through Server-Side Template Injection (SSTI).
Patches
Flask-Reuploaded has been patched in version 1.5.0
Workarounds
- Do not pass user input to the
nameparameter - Use auto-generated filenames only
- Implement strict input validation if
namemust be used
from werkzeug.utils import secure_filename
import os
# Sanitize user input before passing to save()
safe_name = secure_filename(request.form.get('custom_name'))
# Remove path separators
safe_name = os.path.basename(safe_name)
# Validate extension matches policy
if not photos.extension_allowed(photos.get_extension(safe_name)):
abort(400)
filename = photos.save(file, name=safe_name)
Resources
The fix is documented in the pull request, see https://github.com/jugmac00/flask-reuploaded/pull/180.
A proper write-up was created by the reporter of the vulnerability, Jaron Cabral (https://www.linkedin.com/in/jaron-cabral-751994357/), but is not yet available as of time of this publication.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | flask-reuploaded | all versions | 1.5.0pip install --upgrade 'flask-reuploaded==1.5.0' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for flask-reuploaded, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update flask-reuploaded to 1.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27641 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-27641 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-27641. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-27641 in your dependencies?
O3 Security finds CVE-2026-27641 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.