CVE-2026-27607 — rustfs
HIGHCVE-2026-27607 is a high-severity (CVSS 8.1) Improper Input Validation vulnerability in rustfs. A fix is available for rustfs — see the affected versions and patch details below.
RustFS's Missing Post Policy Validation leads to Arbitrary Object Write
Exploitation Status
No confirmed exploitation observed yet
- 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-2026-27607.
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-2026-27607 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
rustfsReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects crates.io packages — download data is not available via public APIs for these ecosystems.
Description
Summary
RustFS does not validate policy conditions in presigned POST uploads (PostObject), allowing attackers to bypass content-length-range, starts-with, and Content-Type constraints. This enables unauthorized file uploads exceeding size limits, uploads to arbitrary object keys, and content-type spoofing, potentially leading to storage exhaustion, unauthorized data access, and security bypasses.
Details
When generating presigned POST URLs via the AWS SDK, applications can specify policy conditions to restrict uploads. RustFS accepts these presigned requests but fails to validate the following conditions server-side:
content-length-rangenot enforced: The server does not verify that the uploaded file size falls within the specified minimum and maximum bounds. An attacker can upload arbitrarily large files despite restrictions.starts-withnot enforced: The server does not validate that the object key matches the required prefix. An attacker can modify the key field to upload files to any path in the bucket.Content-Type(exact match) not enforced: The server does not verify that the uploaded file's content type matches the policy constraint. An attacker can upload files with any content type.
The vulnerability exists in the PostObject endpoint implementation, where the signed policy conditions are not parsed and validated against the actual upload request.
Impact
Vulnerability Type: Improper Input Validation / Authorization Bypass
Who is affected:
Any application using RustFS as an S3-compatible backend that relies on presigned POST policy conditions for access control or upload restrictions.
Potential attack scenarios:
- Storage Exhaustion / Denial of Service: Attackers can upload arbitrarily large files, bypassing size limits, potentially filling up disk space and causing service outages.
- Unauthorized Data Access/Modification: By bypassing starts-with conditions, attackers can upload files to restricted paths (e.g., overwriting configuration files, accessing other users' directories in multi-tenant systems).
- Content-Type Spoofing: Bypassing content-type restrictions could enable serving malicious content (e.g., HTML/JavaScript files in contexts expecting only images), potentially leading to XSS attacks if files are served to browsers.
Severity: The vulnerability allows complete bypass of server-enforced upload policies, undermining the security model that applications rely upon.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🦀crates.io | rustfs | ≥ 1.0.0-alpha.56&&< 1.0.0-alpha.83 | 1.0.0-alpha.83cargo update -p rustfs --precise 1.0.0-alpha.83 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rustfs, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update rustfs to 1.0.0-alpha.83 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-27607 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-2026-27607 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-27607. 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-2026-27607 in your dependencies?
O3 Security finds CVE-2026-27607 across crates.io dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.