CVE-2026-74866

@fastify/busboy vulnerable to CRLF injection via multipart Content-Disposition filename and name

Workarounds

Validate or strip carriage return and line feed characters from the filename and field name before using them in any filesystem, logging, or outbound-header context.

@fastify/busboy 3.2.1 and earlier retains a bare carriage return or line feed inside the parsed Content-Disposition parameters returned to the application. The multipart header parser ends a header line only on the two-byte \r\n sequence, so a lone CR or LF embedded in a part header is carried verbatim into the filename and field name delivered by the file and field events. An application that forwards the supplied filename or name into a CR/LF-sensitive sink cannot anticipate the embedded control character.

An attacker who uploads a file whose filename or field name contains a bare CR or LF can pollute filenames on disk when the upload is saved under its original name, inject forged lines into logs, or inject headers on a downstream hop that does not re-validate CR/LF, such as object-storage metadata or a proxied backend. Applications using @fastify/busboy directly, or through @fastify/multipart and its consumers, are affected.

CVE-2026-74866 has a CVSS score of 5.8 (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 (3.2.2); upgrading removes the vulnerable code path.

Kodem intelligence

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Upgrade to @fastify/busboy 3.2.2 or later.

1. What is CVE-2026-74866? CVE-2026-74866 is a medium-severity security vulnerability in @fastify/busboy (npm), affecting versions < 3.2.2. It is fixed in 3.2.2.
2. How severe is CVE-2026-74866? CVE-2026-74866 has a CVSS score of 5.8 (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 @fastify/busboy are affected by CVE-2026-74866? @fastify/busboy (npm) versions < 3.2.2 is affected.
4. Is there a fix for CVE-2026-74866? Yes. CVE-2026-74866 is fixed in 3.2.2. Upgrade to this version or later.
5. Is CVE-2026-74866 exploitable, and should I be worried? Whether CVE-2026-74866 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-2026-74866 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-2026-74866? Upgrade @fastify/busboy to 3.2.2 or later.

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