CVE-2026-91920
Undertow: undertow: chunkreader message length permits chunk-size overflow
What happened?
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.
Affected software and versions
| Software | Version information | Fixed version |
|---|---|---|
| undertow-servletRed Hat | Default status: affected | See vendor advisory |
| pki-core:10.6/resteasyRed Hat | Default status: unknown | See vendor advisory |
| pki-deps:10.6/resteasyRed Hat | Default status: unknown | See vendor advisory |
| resteasyRed Hat | Default status: unknown | See vendor advisory |
| undertow-servletRed Hat | Default status: unaffected | See vendor advisory |
| jboss-eap-7/eap74-els-openjdk11-openshift-rhel8Red Hat | Default status: affected | See vendor advisory |
| jboss-eap-7/eap74-els-openjdk17-openshift-rhel8Red Hat | Default status: affected | See vendor advisory |
| jboss-eap-7/eap74-els-openjdk8-openshift-rhel8Red Hat | Default status: affected | See vendor advisory |
| undertow-servletRed Hat | Default status: affected | See vendor advisory |
| undertow-servletRed Hat | Default status: affected | See vendor advisory |
| undertow-servletRed Hat | Default status: unaffected | See vendor advisory |
| undertow-servletRed Hat | Default status: affected | See vendor advisory |
Version information comes from the source record and can include unaffected products and exceptions. Distribution packages may contain backported fixes: an older upstream version alone does not prove a server is affected.
Is it being exploited?
This CVE was not listed in the CISA catalogue at our last retrieval. This does not establish that exploitation is absent.
How to respond
- Confirm the product, installed package version, and any configuration requirements in the vendor advisory.
- Follow the vendor’s remediation or workaround guidance in the references below. Structured fixed-version data is unavailable here; the publisher’s description may include a fix or workaround.
- Restart affected services if required, then verify the running version. Known exploitation may warrant investigation as well as patching.