CVE-2026-26962

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.

Mitigation

• Update to a patched version of Rack that removes CRLF correctly when unfolding folded multipart header values.
• Avoid copying upload metadata such as filename directly 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.

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.

CVE-2026-26962 has a CVSS score of 4.8 (Medium). The vector is network-reachable, no 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 (3.2.6); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-26962? CVE-2026-26962 is a medium-severity security vulnerability in rack (rubygems), affecting versions >= 3.2.0, < 3.2.6. It is fixed in 3.2.6.
2. How severe is CVE-2026-26962? CVE-2026-26962 has a CVSS score of 4.8 (Medium). 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 rack are affected by CVE-2026-26962? rack (rubygems) versions >= 3.2.0, < 3.2.6 is affected.
4. Is there a fix for CVE-2026-26962? Yes. CVE-2026-26962 is fixed in 3.2.6. Upgrade to this version or later.
5. Is CVE-2026-26962 exploitable, and should I be worried? Whether CVE-2026-26962 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-2026-26962 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-2026-26962? Upgrade rack to 3.2.6 or later.