CVE-2026-59921 (CVSS 5.7)

Description

Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, HttpPostRequestEncoder constructs multipart HTTP request bodies by directly concatenating user-supplied filenames and field names into Content-Disposition MIME headers without validating or sanitizing CRLF characters (\r\n). Since MIME headers are delimited by CRLF, an attacker who controls the filename can inject arbitrary MIME headers into the multipart body part. The root cause is that neither the encoder nor the FileUpload implementations' setFilename() methods, which only check for null, neutralize CRLF characters before the filename is embedded into the header. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final.

Is your site exposed to CVE-2026-59921?

Run a free security scan — no signup, results in seconds.

CVSS v3.1 Score

5.7

MEDIUM

CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

EPSS — Exploit Prediction

0.0028

Probability of exploitation

0.20%

Percentile rank

EPSS estimates the probability that this vulnerability will be exploited in the wild within the next 30 days. A higher score means more likely to be exploited.

Weakness Type (CWE)

CWE-93 CWE-93

References

Other References

https://github.com/netty/netty/security/advisories/GHSA-gcjf-9mgh-3p7g https://github.com/netty/netty/security/advisories/GHSA-gcjf-9mgh-3p7g

Frequently Asked Questions

What is CVE-2026-59921? +

Netty is an asynchronous, event-driven network application framework. Prior to versions 4.1.136.Final and 4.2.16.Final, HttpPostRequestEncoder constructs multipart HTTP request bodies by directly concatenating user-supplied filenames and field names into Content-Disposition MIME headers without validating or sanitizing CRLF characters (\r\n). Since MIME headers are delimited by CRLF, an attacker who controls the filename can inject arbitrary MIME headers into the multipart body part. The root cause is that neither the encoder nor the FileUpload implementations' setFilename() methods, which only check for null, neutralize CRLF characters before the filename is embedded into the header. This issue has been fixed in versions 4.1.136.Final and 4.2.16.Final. It has a CVSS v3.1 base score of 5.7 (MEDIUM).

How severe is CVE-2026-59921? +

CVE-2026-59921 has a CVSS v3.1 score of 5.7 out of 10, rated MEDIUM. This is a medium-severity vulnerability that should be remediated as part of regular maintenance. The EPSS score is 0.0028, placing it in the 0th percentile for exploitation probability.

How do I check if I'm vulnerable to CVE-2026-59921? +

You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

Related Vulnerabilities

CVE-2026-15429

A privilege escalation vulnerability exists in the HTTP authentication component in Archer VX1800v v1. Improper handling of user-controlled input may …

CVE-2026-43969

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in ninenines cowlib allows HTTP request splitting and cookie smuggling via unvalidated …

CVE-2024-51501

Refit is an automatic type-safe REST library for .NET Core, Xamarin and .NET The various header-related Refit attributes (Header, HeaderCollection …

CVE-2026-48861

Improper Neutralization of CRLF Sequences ('CRLF Injection') vulnerability in elixir-mint Mint allows HTTP Request Splitting and HTTP Request Smuggling. In …

CVE-2025-59419

Netty is an asynchronous, event-driven network application framework. In versions prior to 4.1.128.Final and 4.2.7.Final, the SMTP codec in Netty …

CVE-2026-42258

Net::IMAP implements Internet Message Access Protocol (IMAP) client functionality in Ruby. Prior to versions 0.4.24, 0.5.14, and 0.6.4, symbol arguments …

CVE-2026-59921