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.
The record classifies CVE-2026-59921 under weakness class CWE-93. Its CVSS v3 base vector states that the flaw is reachable from an adjacent network, needs low-privilege credentials, needs no user interaction, and has high impact on integrity. 1 affected-product entry is recorded, across 1 vendor, listed below. The identifier was first published 69 days ago.
Severity and exploitation probability
- CVSS v3.1 base score
- 5.7 — MEDIUM
CVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N - EPSS (FIRST)
- 0.2% probability of exploitation in the next 30 days, higher than 20.3% of all scored CVEs
- CISA KEV
- Not listed in the CISA Known Exploited Vulnerabilities catalog
- Threadlinqs priority
- 3.7/10 — a Threadlinqs composite of the CVSS base score, the EPSS percentile and public exploit availability
- Published
- 2026-07-28, last modified 2026-07-29
Is CVE-2026-59921 being exploited?
It currently carries a trending score of 35 in the Threadlinqs vulnerability feed.
Affected products and versions
- netty: netty
How to fix CVE-2026-59921
No vendor patch reference has been recorded for CVE-2026-59921 in the tracked sources. Follow the references below for a fix, and treat the products listed above as exposed until the vendor states otherwise.
Threat activity tracking CVE-2026-59921
No threat campaign in the Threadlinqs corpus currently references CVE-2026-59921, in its CVE list or as an indicator. The linkage is recomputed from the live corpus on every daily ingest, so this section fills in as soon as a tracked campaign cites the identifier.
Sources
Enriched from CVE.org, NVD, FIRST EPSS. Last verified by Threadlinqs on . This product uses the NVD API but is not endorsed or certified by the NVD.