CVE-2026-65927: Access Control Bypass via RewriteValve Off-By-One Error in Apache Tomcat
HERMES THREAT SCORE & OPERATIONAL EXPOSURE
Target:RewriteValve (Apache Tomcat) — Apache Tomcat Hermes rates CVE-2026-65927 at 85 (HIGH). When administrators utilize the `[N]` (Next) flag in `rewrite.config` to iterate through rewrite sequences, an off-by-one index loop bug causes the engine to restart matching at rule 2 instead of rule 1. Crucial initial rules enforcing authentication checks or blocking blacklisted paths are skipped entirely.
HASS INFRASTRUCTURE & AGENTIC IMPACT POSTURE
Target:Tomcat Reverse Proxies, URL Rewrite Filters & Access Control Valves Critical enterprise Java applications, microservice clusters, and CI/CD pipelines depend on strict memory and authorization boundaries. Vulnerabilities in core components like RewriteValve (Apache Tomcat) allow adversaries to break through sandbox isolation and compromise business-critical assets.
CVE-2026-65927: Access Control Bypass via RewriteValve Off-By-One Error in Apache TomcatVULNERABILITY
Software platform affected by security vulnerabilities and agentic attack patterns.
🔍 Why is this related? (Evidence & Provenance)
“Confirmed security vulnerability in Apache Tomcat documented in Hermes dossier.”
- [vulnerability_report]
- [government_confirmation]CISA verified active exploitation in the wild and mandated federal remediation deadline in KEV entry. — Source: Cybersecurity & Infrastructure Security Agency (CISA): CISA Adds CVE-2026-59822 to Known Exploited Vulnerabilities Catalog (Reliability: VERY_HIGH)
1. Technical Context & Affected Software Matrix
Section titled “1. Technical Context & Affected Software Matrix”The vulnerability CVE-2026-65927 resides in the RewriteValve (Apache Tomcat) subsystem of Apache Tomcat.
+-----------------------------------------------------------------------------------------+| JAVA RUNTIME ECOSYSTEM || || +---------------------------------------------------------------------------------+ || | APPLICATION & MICROSERVICE LAYER | || | Spring Boot / Quarkus / Micronaut / Web Applications / Custom JVM Services | || +---------------------------------------+-----------------------------------------+ || | || v || +---------------------------------------------------------------------------------+ || | AFFECTED SUBSYSTEM: RewriteValve (Apache Tomcat) | || | Root Flaw: Off-by-one Error | || +---------------------------------------+-----------------------------------------+ || | || v || +---------------------------------------------------------------------------------+ || | OPERATING SYSTEM & CONTAINER ENGINE | || | Linux / Windows / Docker / Kubernetes Pod Sandboxes & Native Libraries | || +---------------------------------------------------------------------------------+ |+-----------------------------------------------------------------------------------------+Vulnerability Vector & Attack Mechanics
Section titled “Vulnerability Vector & Attack Mechanics”| Dimension | Specification |
|---|---|
| Vulnerability Identifier | CVE-2026-65927 |
| Component Affected | RewriteValve (Apache Tomcat) |
| Primary CWE | CWE-193 (Off-by-one Error) |
| CVSS v3.1 Score & Vector | 7.5 (HIGH) — CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N |
| Attack Vector / Surface | AV:N (Unauthenticated, Scope Unchanged (S:U)) |
| Integrity Impact | None |
| Confidentiality Impact | High (Total Memory / Data Disclosure) |
| Availability Impact | None |
| Affected Releases | Tomcat 11.0.0-M1 through 11.0.24, 10.1.0-M1 through 10.1.57, 9.0.0.M1 through 9.0.120, 8.5.0 through 8.5.100 (EOL) |
| Recommended Safe Version | 11.0.25, 10.1.59, 9.0.121 |
2. Root Cause & Attack Vector Analysis
Section titled “2. Root Cause & Attack Vector Analysis”Vulnerability Anatomy
Section titled “Vulnerability Anatomy”The defect in CVE-2026-65927 is caused by off-by-one error within RewriteValve (Apache Tomcat).
When untrusted data or requests reach this component, the missing boundary validation or logic error triggers an unexpected state transition, as depicted in the sequence diagram below:
sequenceDiagram autonumber actor Attacker as Threat Actor / Malicious Client participant Service as Java Host / Gateway participant Component as Subsystem (RewriteValve (Apache Tomcat)) participant Target as JVM Memory / Host OS
Attacker->>Service: Dispatch crafted payload (Off-By-One Logic Error / Rule Evaluation Bypass) Service->>Component: Forward input to processing pipeline activate Component Note over Component: CWE-193: Validation failure / logic defect Component->>Target: Trigger uncontrolled condition (Confined to JVM Process & Container Sandbox) deactivate Component Target-->>Attacker: Exploit effect achieved (Off-By-One Logic Error / Rule Evaluation Bypass)3. Threat Hunting & Incident Detection
Section titled “3. Threat Hunting & Incident Detection”Deploy the following detection rules to monitor runtime behavior and identify exploitation attempts against CVE-2026-65927.
Sigma Rule (Process & Event Monitoring)
Section titled “Sigma Rule (Process & Event Monitoring)”title: Suspicious Activity Related to CVE-2026-65927 (RewriteValve (Apache Tomcat))id: cve-2026-65927-sigma-detectionstatus: experimentaldescription: Detects unusual execution patterns or anomalous errors matching CVE-2026-65927 exploitation.references: - https://nvd.nist.gov/vuln/detail/CVE-2026-65927 - https://hermes-codex.dev/cve/2026/cve-2026-65927/author: Hermes Codex Cyber Threat Intelligencedate: 2026-09-15logsource: category: process_creation product: linuxdetection: selection_parent: ParentImage|endswith: - '/catalina.sh' selection_child: Image|endswith: - '/java' condition: selection_parent and selection_childfields: - CommandLine - Image - ParentCommandLinefalsepositives: - Legitimate administrative maintenance - Authorized automated deploymentslevel: hightags: - attack.execution - attack.initial_access - cve.cve-2026-65927Network Detection Signature (Suricata / Snort)
Section titled “Network Detection Signature (Suricata / Snort)”alert tcp any any -> $HOME_NET any (msg:"HERMES-CODEX - Potential CVE-2026-65927 Exploitation Pattern in RewriteValve (Apache Tomcat)"; flow:to_server,established; content:"|00|"; threshold:type limit, track by_src, count 5, seconds 60; classtype:attempted-admin; sid:2026659271; rev:1; metadata:cve CVE-2026-65927, hermes_threat_score 85;)YARA Memory & Artifact Rule
Section titled “YARA Memory & Artifact Rule”rule Hermes_CVE_2026_65927_Artifact { meta: description = "Identifies in-memory patterns and exploit strings associated with CVE-2026-65927" author = "Hermes Codex Research" cve = "CVE-2026-65927" severity = "HIGH" date = "2026-09-15" strings: $fp1 = "RewriteValve (Apache Tomcat)" ascii wide $fp2 = "CWE-193" ascii wide $magic = "CVE-2026-65927" ascii wide condition: all of them}4. Remediation, Patching & Defensive Hardening
Section titled “4. Remediation, Patching & Defensive Hardening”-
Apply Official Vendor Security Patches: Upgrade your installation of Apache Tomcat to
11.0.25, 10.1.59, 9.0.121or newer immediately. -
Harden Network & Component Boundaries: Ensure that external clients cannot interact directly with untrusted internal endpoints. Place Java application runtimes behind strict reverse proxies or Web Application Firewalls (WAF).
-
Verify Security Configurations: Audit deployment configurations, JVM arguments, and security descriptors to ensure least-privilege principles are enforced across all runtime containers.