External risk intelligence

Apache Tomcat Improper Input Validation Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-65637

Apache Tomcat is a widely deployed web server and servlet container designed to serve web applications and APIs directly to network users. As a foundational piece of infrastructure for web hosting, it is commonly deployed in internet-facing configurations, making its components inherently exposed to public network traffic by design.

Apache Tomcat

9.0.115 to before 9.0.12110.1.53 to before 10.1.5811.0.20 to before 11.0.25

Halo Surface Signal: 5 out of 5 — more likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in Apache Tomcat, a widely used web server technology. This issue could allow unauthorized access and manipulation of systems, potentially impacting web applications and APIs. The main concern at this time is to confirm if our environment utilizes the affected versions and assess any potential exposure.

Flaw in Tomcat could enable unauthorized system access.
Widely used server technology; widely deployed and internet-facing.
Confirm relevance and assess exposure to this critical flaw.

Attack Path

How an attacker could exploit the issue

An attacker could reach this vulnerability through the network without any special access or user interaction. The issue lies within Apache Tomcat, specifically in how it handles certain inputs. If exploited, this vulnerability could allow an attacker to gain significant control, potentially leading to data compromise, system modification, or service disruption.

Entry condition: Network access.
Trigger point: Improper input validation.
Resulting risk: Data compromise, modification, or disruption.

Live Threat

Current exploitation, exposure, and threat context

This Improper Input Validation vulnerability in Apache Tomcat, when exploited, could allow an unauthenticated attacker to affect the service's behavior. The impact on system or user data is not specified in the provided context.

Affects Tomcat service behavior.
Exposure through improper input validation.
Unspecified impact on data.

Operational Fix

Recommended remediation, mitigation, and detection steps

This vulnerability impacts Apache Tomcat, a widely used web server and servlet container. Ownership likely falls to the infrastructure or platform teams managing Tomcat deployments, with coordination from network/security teams for exposure assessment and vendor-management if using a managed service. The first practical step is to identify all Tomcat instances, assess their network reachability and business criticality, and confirm the accountable owner for each.

Infrastructure/platform teams own the issue.
Verify exposure and business criticality first.
Plan remediation based on identified risk.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

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Frequently asked questions

What is Apache Tomcat?

Apache Tomcat is a widely deployed open-source web server and servlet container. It serves as the foundational infrastructure for hosting Java-based web applications and APIs, allowing them to process requests and communicate with network users.

What does Improper Input Validation mean for CVE-2026-65637?

This refers to a CWE-20 weakness where the software fails to properly check or sanitize incoming data. In the context of this CVE, it means the system does not adequately verify inputs, which could allow an attacker to trigger unintended behaviors or gain unauthorized control over the server.

How is this CVE-2026-65637 vulnerability triggered?

The vulnerability is triggered when an attacker sends specifically crafted network traffic to an affected Apache Tomcat instance. It does not require the attacker to have prior authentication or special user interaction to initiate the exploit.

Why should I care about this if my server is internal?

Halo Surface Signal indicates that while Apache Tomcat is commonly internet-facing, any deployment reachable over a network is a potential target. Even internal instances should be evaluated to understand if the server can be reached by unauthorized actors or compromised systems within your network.

Do I need to update my Apache Tomcat installation?

Yes, if you are running versions 11.0.20 through 11.0.24, 10.1.53 through 10.1.57, or 9.0.115 through 9.0.120, you should upgrade to version 11.0.25, 10.1.58, or 9.0.121 respectively. Begin by identifying all instances in your environment and confirming which are affected.

References

lists.apache.org/thread/djog953z1ohsyt25bdvhfzbmsy22vgcj

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.