Legacy protocols continue to create modern security gaps. In February 2026, the Apache Software Foundation disclosed CVE-2026-24733, a vulnerability in Apache Tomcat that allows attackers to bypass security constraints using crafted HTTP/0.9 requests.
While rated Low severity, this issue highlights a recurring problem in enterprise environments: outdated protocol handling interacting unpredictably with access-control rules.
For CISOs, security engineers, and DevOps teams running Java-based web infrastructure, understanding this flaw is essential. In this article, we break down:
- What CVE-2026-24733 is
- How HTTP/0.9 parsing enables the bypass
- Which versions of Tomcat are affected
- Real-world risk implications
- Mitigation and hardening best practices
What Is CVE-2026-24733?
CVE-2026-24733 is a security constraint bypass vulnerability in Apache Tomcat.
The issue stems from Tomcat’s handling of HTTP/0.9 requests, an obsolete version of HTTP that predates modern method enforcement and header validation.
Under specific configurations, an attacker can:
- Send a crafted HTTP/0.9-style request
- Exploit improper method restriction handling
- Bypass configured access controls
Although the vulnerability is situational, it exposes weaknesses in legacy protocol compatibility and access rule assumptions.
Understanding HTTP/0.9 and Why It Matters
What Is HTTP/0.9?
HTTP/0.9 is the earliest version of HTTP, originally supporting only:
- The
GETmethod - No headers
- No status codes
- A minimal request-response format
Modern HTTP versions (1.1, 2, 3) enforce strict method and header structures. However, backward compatibility sometimes allows older protocol parsing — creating edge cases.
Why This Becomes a Security Risk
Tomcat did not restrict HTTP/0.9 requests exclusively to GET. As a result:
- When security constraints allowed
HEADbut deniedGET - An attacker could craft a specification-invalid
HEADrequest using HTTP/0.9 - The configured
GETdenial could be bypassed
This creates an unexpected enforcement gap in method handling.
How the Security Constraint Bypass Works
The vulnerability manifests when both conditions are true:
- A security constraint is configured to:
- Allow
HEAD - Deny
GET
- Allow
- HTTP/0.9 parsing is accepted along the request path
Attack Flow
- Attacker identifies a protected endpoint.
- They craft an HTTP/0.9-style
HEADrequest. - Tomcat processes the request in a way that bypasses the
GETrestriction. - Resource access controls are effectively undermined.
Under normal HTTP versions, this bypass would not occur.
Why This Matters for Enterprise Environments
Although CVE-2026-24733 is rated Low severity, the impact depends on:
- Application architecture
- Reverse proxy behavior
- Legacy client support
- Security rule granularity
High-Risk Scenarios
- Legacy integrations that still permit HTTP/0.9
- Misconfigured reverse proxies that allow protocol downgrades
- Applications relying on method-based authorization
- Environments without strict request normalization
Security teams must treat this as a configuration-dependent risk amplifier rather than a theoretical issue.
Affected Versions of Apache Tomcat
| Tomcat Branch | Versions Affected | Fixed Version |
|---|---|---|
| 11 | 11.0.0-M1 to 11.0.14 | 11.0.15+ |
| 10.1 | 10.1.0-M1 to 10.1.49 | 10.1.50+ |
| 9.0 | 9.0.0.M1 to 9.0.112 | 9.0.113+ |
| Older (EOL) | Also affected | Upgrade to supported branch |
Organizations running End-of-Life (EOL) versions should prioritize migration. Backporting security patches to unsupported branches is risky and often impractical.
Common Misconceptions
“Low Severity Means Low Risk”
Severity ratings assume typical deployments. Custom access-control configurations may elevate practical impact.
“HTTP/0.9 Isn’t Used Anymore”
While rarely used intentionally, unexpected protocol handling can occur due to:
- Misconfigured load balancers
- Incomplete proxy normalization
- Internal service mesh anomalies
“Reverse Proxies Always Normalize Traffic”
Not all reverse proxies enforce strict HTTP version validation. In some architectures, malformed requests may pass through unchanged.
Mitigation and Hardening Recommendations
1. Upgrade Immediately
Upgrade to patched releases:
- 11.0.15+
- 10.1.50+
- 9.0.113+
Patch management remains the most effective defense.
2. Disable or Reject HTTP/0.9 Requests
Where possible:
- Configure reverse proxies to reject HTTP/0.9 traffic
- Enforce strict protocol validation
- Log malformed or downgrade attempts
3. Review Method-Based Access Controls
Audit security constraints that:
- Allow
HEAD - Deny
GET
Ensure access control logic aligns with business intent.
4. Harden Reverse Proxy and Load Balancer Configurations
Implement:
- Protocol normalization
- Strict HTTP parsing
- WAF rules to detect invalid method structures
- Zero-trust segmentation for internal services
5. Align With Security Frameworks
Map this risk into governance and compliance models:
- NIST SP 800-53 – Access Control (AC) controls
- ISO 27001 Annex A – Secure system configuration
- OWASP ASVS – HTTP method validation
Risk Impact Analysis
| Factor | Impact Level |
|---|---|
| Public-facing Tomcat | Moderate |
| Internal-only deployment | Low |
| Legacy proxy chain | Elevated |
| EOL Tomcat version | High |
This vulnerability demonstrates how protocol edge cases can undermine carefully designed access-control rules.
Detection and Monitoring Guidance
Security teams should monitor for:
- HTTP/0.9-style request signatures
- Unusual method handling in logs
- Access attempts on endpoints with HEAD/GET discrepancies
- Reverse proxy downgrade behavior
SIEM correlation rules should include malformed HTTP request patterns.
Frequently Asked Questions (FAQs)
What is CVE-2026-24733?
A security constraint bypass vulnerability in Apache Tomcat caused by improper handling of HTTP/0.9 requests.
Does this vulnerability allow remote code execution?
No. It enables access-control bypass under specific configurations, not arbitrary code execution.
Who is most at risk?
Organizations using method-based access restrictions and allowing HTTP/0.9 parsing across their request path.
How can I test if my system is vulnerable?
Review:
- Tomcat version
- Security constraint rules
- Reverse proxy protocol handling
- Application logs for malformed HTTP traffic
Is upgrading enough?
Yes, upgrading to patched versions resolves the parsing issue. However, additional hardening is recommended for defense-in-depth.
Conclusion
CVE-2026-24733 is a reminder that legacy protocol compatibility can introduce modern security risks. While rated Low severity, the vulnerability can create enforcement gaps in specific access-control configurations.
Organizations should:
- Upgrade immediately
- Audit HEAD vs. GET permissions
- Harden reverse proxy configurations
- Enforce strict protocol validation
In enterprise environments, even subtle parsing inconsistencies can undermine access controls. Proactive patching and architectural hygiene remain essential.
Next Step: Conduct a Tomcat configuration audit today and validate that legacy protocol handling does not weaken your security posture.
