---
id: "CVE-2026-91920"
kind: "vulnerability"
aliases: ["CVE-2026-91920"]
title: "Undertow: crafted chunk size enables HTTP request smuggling on the server"
severity: "medium"
cvss: 6.5
epss: null
exploited: false
status: "open"
category: "runtime"
products: []
ecosystems: []
tags: ["http-smuggling", "java", "jboss-eap", "runtime", "undertow", "wildfly"]
not_affected_if: ["no-untrusted-traffic", "other-web-server"]
published_at: "2026-10-30T02:17:10+00:00"
first_seen_at: "2026-10-30T02:28:06+00:00"
updated_at: "2026-10-30T02:30:08+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-91920", "first_seen_at": "2026-10-30T02:28:06+00:00"}]
---

# Undertow: crafted chunk size enables HTTP request smuggling on the server

A crafted 15-digit chunk size in HTTP chunked transfer encoding bypasses the size check in Undertow's ChunkReader, overflowing into internal state flags. The server can end a request body early and treat the leftover bytes as a separate smuggled request.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check which Undertow version the application uses, for example with `mvn dependency:tree` or by looking for `undertow-core` in the build file.
- Confirm whether the server is exposed to untrusted HTTP traffic, directly or through a proxy that forwards chunked requests.

## Timeline

- 2026-10-30T02:28:06+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-91920

## From the sources

A flaw was found in the ChunkReader component of the Undertow web server, which is used to handle chunked transfer encoding in HTTP requests. The issue occurs because a security check for the chunk size is performed before the value is shifted, allowing a specifically crafted 15-digit chunk size to bypass the limit and overflow into internal state flags. This can cause the server to terminate a request body prematurely and interpret the remaining data as a separate, smuggled request.

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