CVE-2026-27607

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:

1. content-length-range not 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.
2. starts-with not 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.
3. 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.

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:

1. Storage Exhaustion / Denial of Service: Attackers can upload arbitrarily large files, bypassing size limits, potentially filling up disk space and causing service outages.
2. 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).
3. 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.

Vulnerability Type: Improper Input Validation / Authorization Bypass

The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions. Typical impact: unauthorized data access or execution of privileged operations.

CVE-2026-27607 has a CVSS score of 8.1 (High). 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 (1.0.0-alpha.83); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-27607? CVE-2026-27607 is a high-severity incorrect authorization vulnerability in rustfs (rust), affecting versions >= 1.0.0-alpha.56, < 1.0.0-alpha.82. It is fixed in 1.0.0-alpha.83. The application does not correctly enforce access controls, allowing a principal to access resources or operations beyond their granted permissions.
2. How severe is CVE-2026-27607? CVE-2026-27607 has a CVSS score of 8.1 (High). 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 rustfs are affected by CVE-2026-27607? rustfs (rust) versions >= 1.0.0-alpha.56, < 1.0.0-alpha.82 is affected.
4. Is there a fix for CVE-2026-27607? Yes. CVE-2026-27607 is fixed in 1.0.0-alpha.83. Upgrade to this version or later.
5. Is CVE-2026-27607 exploitable, and should I be worried? Whether CVE-2026-27607 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-2026-27607 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-2026-27607? Upgrade rustfs to 1.0.0-alpha.83 or later.