CVE-2026-26962 — rack
MEDIUMCVE-2026-26962 is a medium-severity (CVSS 4.8) CWE-93 vulnerability in rack. A fix is available for rack — see the affected versions and patch details below.
Rack: Header injection in multipart requests
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-2026-26962.
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-2026-26962 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 379,145 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
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 unfolds folded multipart part headers incorrectly. When a multipart header contains an obs-fold sequence, Rack preserves the embedded CRLF in parsed parameter values such as filename or name instead of removing the folded line break during unfolding.
As a result, applications that later reuse those parsed values in HTTP response headers may be vulnerable to downstream header injection or response splitting.
Details
Rack::Multipart::Parser accepts folded multipart header values and unfolds them during parsing. However, the unfolding behavior does not fully remove the embedded line break sequence from the parsed value.
This means a multipart part header such as:
Content-Disposition: form-data; name="file"; filename="test\r\n foo.txt"
can result in a parsed parameter value that still contains CRLF characters.
The issue is not that Rack creates a second multipart header field. Rather, the problem is that CRLF remains embedded in the parsed metadata value after unfolding. If an application later uses that value in a security-sensitive context, such as constructing an HTTP response header, the preserved CRLF may alter downstream header parsing.
Affected values may include multipart parameters such as filename, name, or similar parsed header attributes.
Impact
Applications that accept multipart form uploads may be affected if they later reuse parsed multipart metadata in HTTP headers or other header-sensitive contexts.
In affected deployments, an attacker may be able to supply a multipart parameter value containing folded line breaks and cause downstream header injection, response splitting, cache poisoning, or related response parsing issues.
The practical impact depends on application behavior. If parsed multipart metadata is not reused in HTTP headers, the issue may be limited to incorrect parsing behavior rather than a direct exploit path.
Mitigation
- Update to a patched version of Rack that removes CRLF correctly when unfolding folded multipart header values.
- Avoid copying upload metadata such as
filenamedirectly into HTTP response headers without sanitization. - Sanitize or reject carriage return and line feed characters in multipart-derived values before reusing them in response headers, logs, or downstream protocol contexts.
- Where feasible, normalize uploaded filenames before storing or reflecting them.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎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 3.2.6 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-26962 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-2026-26962 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-26962. 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-2026-26962 in your dependencies?
O3 Security finds CVE-2026-26962 across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.