CVE-2026-104721 in Logback-classic
Summary
by MITRE • 10/02/2026
Path-traversal vulnerability in QOS.CH Sarl Logback-classic on Java (logback-classic module) allows path-traversal vulnerability. More specifically, an MDC-based discriminator value flows unsanitized into a nested FileAppender path, letting an attacker who influences that MDC value (e.g. via an HTTP header) create and append log files outside the intended directory.
This issue affects Logback-classic: from 0.9.14 through 1.6.4. This vulnerability is similar to CVE-2026-19880 but involves other attack techniques.
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Analysis
by VulDB Data Team • 10/02/2026
The identified security flaw resides within the logback-classic module, a widely utilized logging framework for Java applications developed by QOS.CH Sarl. The vulnerability specifically impacts versions ranging from 0.9.14 through 1.6.4 and represents a critical path traversal issue rooted in improper input validation of dynamic configuration parameters. In typical Java enterprise architectures, the Logging Context (MDC) is frequently populated with contextual data such as user identifiers, session tokens, or request metadata to facilitate structured logging. When this MDC content is subsequently utilized by logback-classic components without adequate sanitization, it creates a direct vector for file system manipulation attacks. The core technical deficiency lies in the handling of discriminator values within appenders that write output to local files. Specifically, when an MDC-based discriminator value flows unsanitized into the path configuration of a nested FileAppender, the framework fails to validate or escape special characters and directory traversal sequences embedded within that input.
This architectural oversight allows an attacker who can influence the content of the MDC context to dictate the destination file paths for log entries. By injecting standard Unix-style path traversal sequences such as dot-dot-slash into HTTP headers or other request parameters that are mapped to the MDC, a malicious actor can break out of the intended logging directory structure. The application then proceeds to create and append log files at arbitrary locations on the underlying operating system file hierarchy. This capability effectively bypasses the security boundary established by the application's configuration, granting the attacker write access to sensitive areas of the server filesystem where they may not otherwise have permissions or visibility.
The operational impact of this vulnerability is severe due to its potential for remote code execution and data exfiltration. An attacker can leverage this path traversal capability to overwrite critical system files, inject malicious scripts into executable directories, or plant backdoors within configuration files that are read by the application upon restart. Furthermore, because log files often contain sensitive information such as session tokens, passwords, or personal identifiable information, an attacker could also manipulate existing logs to cover their tracks or create false evidence for forensic analysis. The ability to write arbitrary files significantly expands the attack surface beyond simple data leakage, enabling persistent access and deeper compromise of the host environment. This issue is distinct yet related in nature to CVE-2026-19880, sharing similar underlying mechanisms but exploiting different specific techniques within the logging pipeline.
From a classification perspective, this vulnerability aligns with CWE-22: Improper Limitation of a Pathname to a Restricted Directory, which describes flaws where software does not properly neutralize special elements that can cause paths to traverse outside the intended directory structure. Additionally, it maps to MITRE ATT&CK technique T1048: Exfiltration Over Alternative Protocol if used for data theft, or more critically T1562: Impair Defenses such as Disable or Modify Tools when used to overwrite log files to hide activity. The exploitation vector is typically classified under CWE-79: Improper Neutralization of Input During Web Page Generation if the input originates from web requests, though in this specific context, it manifests through server-side processing logic rather than direct client rendering.
Mitigation strategies must focus on both immediate patching and long-term architectural improvements. The primary remediation is to upgrade the logback-classic library to a version that has addressed this path traversal flaw, ensuring that all instances of the vulnerable software stack are updated across the environment. For applications where upgrading is not immediately feasible, developers should implement strict input validation on any data entering the MDC context. This includes sanitizing HTTP headers and other external inputs by stripping or encoding characters such as dots, slashes, and backslashes before they are used in log file path configurations. Additionally, configuring FileAppender instances to use absolute paths that do not rely on dynamic user-supplied input for directory resolution can prevent traversal attempts. Implementing a whitelist approach for allowed MDC values further reduces the risk by ensuring only predefined, safe strings are processed as part of logging operations.