CVE-2026-26961 — rack
LOWCVE-2026-26961 is a low-severity (CVSS 3.7) CWE-436 vulnerability in rack. A fix is available for rack — see the affected versions and patch details below.
Rack: Multipart Boundary Parsing Ambiguity allowing WAF Bypass
Exploitation Status
No confirmed exploitation observed yet
- 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-2026-26961.
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-2026-26961 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
rack💎rack💎rackReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
Summary
Rack::Multipart::Parser extracts the boundary parameter from multipart/form-data using a greedy regular expression. When a Content-Type header contains multiple boundary parameters, Rack selects the last one rather than the first.
In deployments where an upstream proxy, WAF, or intermediary interprets the first boundary parameter, this mismatch can allow an attacker to smuggle multipart content past upstream inspection and have Rack parse a different body structure than the intermediary validated.
Details
Rack identifies the multipart boundary using logic equivalent to:
MULTIPART = %r|\Amultipart/.*boundary=\"?([^\";,]+)\"?|ni
Because the expression is greedy, it matches the last boundary= parameter in a header such as:
Content-Type: multipart/form-data; boundary=safe; boundary=malicious
As a result, Rack parses the request body using malicious, while another component may interpret the same header using safe.
This creates an interpretation conflict. If an upstream WAF or proxy inspects multipart parts using the first boundary and Rack later parses the body using the last boundary, a client may be able to place malicious form fields or uploaded content in parts that Rack accepts but the upstream component did not inspect as intended.
This issue is most relevant in layered deployments where security decisions are made before the request reaches Rack.
Impact
Applications that accept multipart/form-data uploads behind an inspecting proxy or WAF may be affected.
In such deployments, an attacker may be able to bypass upstream filtering of uploaded files or form fields by sending a request with multiple boundary parameters and relying on the intermediary and Rack to parse the request differently.
The practical impact depends on deployment architecture. If no upstream component relies on a different multipart interpretation, this behavior may not provide meaningful additional attacker capability.
Mitigation
- Update to a patched version of Rack that rejects ambiguous multipart
Content-Typeheaders or parses duplicateboundaryparameters consistently. - Reject requests containing multiple
boundaryparameters. - Normalize or regenerate multipart metadata at the trusted edge before forwarding requests to Rack.
- Avoid relying on upstream inspection of malformed multipart requests unless duplicate parameter handling is explicitly consistent across components.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | rack | all versions | 2.2.23bundle update rack --conservative |
| 💎RubyGems | rack | ≥ 3.0.0.beta1&&< 3.1.21 | 3.1.21bundle update rack --conservative |
| 💎RubyGems | rack | ≥ 3.2.0&&< 3.2.6 | 3.2.6bundle update rack --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for rack, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update rack to 2.2.23 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-26961 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-2026-26961 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-26961. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2026-26961 in your dependencies?
O3 Security finds CVE-2026-26961 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.