IO.UNDERTOW:UNDERTOW-CORE · CVE-2026-28369 · Package
FIX → 2.2.40.Final · 2.3.25.Final
Undertow, a Java web server library, strips leading spaces from the first HTTP header line when it should reject them. A remote attacker can use this to smuggle requests past proxies and reach restricted paths or poison caches.
first public Mar 27, 2026 · 2 sources
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Applications that embed undertow-core as their HTTP server, especially those behind a reverse proxy or load balancer that relies on header parsing to make access decisions. Standalone apps with no proxy in front have less exposure but still need the fixed version. Teams that only use Undertow indirectly through a framework should check whether that framework pulls in an affected version.
A flaw was found in Undertow. When Undertow receives an HTTP request where the first header line starts with one or more spaces, it incorrectly processes the request by stripping these leading spaces. This behavior, which violates HTTP standards, can be exploited by a remote attacker to perform request smuggling. Request smuggling allows an attacker to bypass security mechanisms, access restricted information, or manipulate web caches, potentially leading to unauthorized actions or data exposure.
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Not affected if no-undertow-core-dependency, version-below-affected-range
Who cares cwe-444, java, maven, reverse-proxy, undertow
mvn dependency:tree and look for io.undertow:undertow-core and its version2.2.40.Final or in the range 2.3.0.Final to 2.3.25.FinalUpgrade to 2.2.40.Final · 2.3.25.Final
2.2.40.Final on the 2.2 line2.3.25.Final on the 2.3 lineGitHub Advisory Database, CC-BY 4.0 · NVD data is provided by NIST and is public domain. Posts quoted belong to their authors.