External risk intelligence

Apache Tomcat Always-Incorrect Control Flow Implementation Vulnerability

CVE advisorySeverity: CRITICAL (CVSS 9.1)

CVE-2026-55276

Apache Tomcat is a widely deployed web server and servlet container that is commonly positioned as an internet-facing service or an edge-facing application gateway to handle web traffic, making its configuration and management components frequently reachable from the public internet.

Apache Tomcat

before 9.0.11910.1.0 to before 10.1.5611.0.0 to before 11.0.23

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A vulnerability in Apache Tomcat could allow unauthorized access to system information due to an incorrect implementation of access controls. This issue affects various versions of Tomcat, and while the direct business impact requires confirmation of relevance, it highlights the importance of verifying system configurations.

Access control logic flaw in Tomcat.
Crucial for web infrastructure security.
Confirm relevance and assess exposure.

Attack Path

How an attacker could exploit the issue

An attacker could exploit this vulnerability by accessing specific logging features within Apache Tomcat. If these features are improperly configured, the attacker might be able to manipulate the logged information, potentially leading to a compromise of sensitive data or unauthorized access.

Requires access to logging features.
Triggered by incorrect role and authorization handling.
Leads to information disclosure and access risks.

Live Threat

Current exploitation, exposure, and threat context

This vulnerability in Apache Tomcat could impact the logging of effective web.xml files, potentially leading to unauthorized access or disclosure of sensitive configuration details when special roles or empty authorization constraints are present. The issue arises from an incorrect implementation of control flow during the logging process.

Configuration details may be exposed.
Improper logging can reveal settings.
Unauthorized access could occur.

Operational Fix

Recommended remediation, mitigation, and detection steps

Application owners and infrastructure teams are likely responsible for addressing this vulnerability in Apache Tomcat, given its role as a web server and servlet container. The immediate priority is to identify all instances of affected Tomcat deployments, confirm their accessibility and business criticality, and locate the accountable owner to plan remediation.

Identify affected Tomcat instances.
Verify exposure and business impact.
Plan upgrade or mitigation.

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 an open-source web server and servlet container that executes Java-based web applications. It serves as the foundation for many enterprise environments, acting as a gateway that processes incoming web traffic and handles communication between users and backend services.

How does CVE-2026-55276 represent a control flow weakness?

This vulnerability, classified as CWE-670: Always-Incorrect Control Flow Implementation, occurs when the software fails to follow its intended logic path. In this instance, Tomcat incorrectly skips specific authorization constraints and roles when logging the effective web.xml file, leading the system to behave in a way that does not match its actual security configuration.

When does this vulnerability trigger?

The flaw is triggered during the process where Tomcat logs the effective web.xml configuration. It is specifically linked to how the server handles special roles and empty authorization constraints. Simply accessing the server does not trigger the bug; it manifests when the logging mechanism incorrectly processes these specific authorization settings, potentially revealing sensitive configuration details that should remain hidden.

Why should I care about this if my server is internet-facing?

According to Halo Surface Signal, Apache Tomcat is frequently deployed as an internet-facing service or edge-facing gateway. Because it is designed to handle external web traffic, any flaw involving unauthorized access or configuration disclosure is more likely to be reached by unauthorized parties if your specific instance is exposed to the public network.

Do I need to patch my Tomcat installation?

Yes, upgrading is the recommended response. You should identify all running instances of the affected versions and coordinate an upgrade to the fixed releases—specifically 11.0.23, 10.1.56, or 9.0.119. Before patching, work with your infrastructure team to inventory your deployments and prioritize those that are business-critical or directly accessible from the network.

References

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.