# CVE-2026-27607: rustfs XSS — Fix Available 🦀 🦀 crates.io Not in CISA KEV HIGH severity # CVE-2026-27607 — rustfs HIGH CVE-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 Also known as[GHSA-w5fh-f8xh-5x3p](/vulnerability/GHSA-w5fh-f8xh-5x3p) Published Feb 25, 2026 Updated Oct 28, 2026 Affected 1 pkg Patched 1 / 1 Exploits None indexed Exploitation data as of Nov 6, 2026 · OSV.dev, NVD, FIRST.org (EPSS) ## 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 [via FIRST.org ↗](https://www.first.org/epss/) 0.4%probability of exploitation in next 30 days Lower Risk0.00% Lower risk than most CVEs31th percentile — riskier than 31% of all scored CVEsHighest risk 0.00%0.30%0.60%0.91%0.1%0.4%May 26Sep 26Nov 26 Probability of exploitation in the next 30 days, from [FIRST.org EPSS](https://www.first.org/epss/). ## How urgent is this, really ## Where this sits among everything scored ## Real-World Exposure 1 pkg affected 🦀`rustfs` Real-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: 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. #### 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: 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. ## Affected Packages 1 total 1 fixed Ecosystem Package Vulnerable range Fix 🦀crates.io `rustfs` ≥ 1.0.0-alpha.56&&< 1.0.0-alpha.83 1.0.0-alpha.83`cargo update -p rustfs --precise 1.0.0-alpha.83` ## Affected Products 1 product · 27 configurations Application `rustfs`rustfs 1 version 1.0.0 ## Detection & mitigation playbook Open-source dependency 1. ### Detect 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. 2. ### 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. 3. ### Workarounds Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page. ## Frequently Asked Questions \### 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 O3 Security · Impact-Aware SCA ### Is CVE-2026-27607 in your dependencies? Find it across crates.io, including transitive dependencies. [Scan my dependencies](/book-demo) [How O3 SCA works](/impact-aware-sca)