CVE-2026-55601 in MultiTail
Summary
by MITRE • 10/13/2026
MultiTail is a utility for monitoring multiple log files in one terminal. In version 7.1.5 and earlier, the -ex and -eX execute-on-match feature appended attacker-controlled matched input to a configured command inside double quotes, then check_filter_exec() in mt.c passed the constructed command through execute_program() in exec.c to sh -c. Partial escaping left shell metacharacters such as command substitution active, allowing an attacker who could influence monitored input to execute commands as the user running MultiTail. No fixed release is available as of this review.
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Analysis
by VulDB Data Team • 10/13/2026
MultiTail serves as a widely utilized utility for monitoring multiple log files simultaneously within a single terminal interface, offering system administrators and developers a convenient way to track real-time events across various data streams. The vulnerability identified in versions 7.1.5 and earlier centers on the execution-on-match feature, specifically triggered by the -ex and -eX command-line flags. This functionality is designed to execute a user-defined shell command whenever specific patterns are detected within the monitored log files. While this feature provides significant operational utility for automated alerting and response workflows, its implementation contains a critical flaw in how it constructs and executes these commands, leading to severe security implications.
The core technical flaw lies in the insufficient sanitization of matched input before passing it to the shell interpreter. When MultiTail detects a pattern match configured via -ex or -eX, it appends the attacker-controlled matched text into a command string enclosed within double quotes. This constructed string is then passed through the execute_program function in exec.c to sh -c for execution. Although the input is wrapped in quotes, which typically prevents word splitting and globbing, this measure fails to neutralize shell metacharacters that remain active inside double-quoted strings in POSIX-compliant shells such as bash or dash. Specifically, characters like backticks and $() are not escaped, allowing them to be interpreted by the shell during command execution rather than being treated as literal data.
This partial escaping mechanism creates a classic Command Injection vulnerability, formally categorized under CWE-78: Improper Neutralization of Special Elements used in an OS Command. An attacker who can influence or inject content into the monitored log files can exploit this flaw to achieve arbitrary code execution with the privileges of the user running MultiTail. For instance, by injecting text containing backtick command substitution syntax such as `id` or $(whoami), the injected payload is executed directly by the shell spawned via sh -c. This bypasses the intended isolation between log data and system commands, effectively turning a passive monitoring tool into an active execution vector for remote code execution if the logs are sourced from untrusted inputs.
The operational impact of this vulnerability is significant, particularly in environments where MultiTail is used to monitor logs generated by external services or user-facing applications that may allow controlled input injection. If an attacker can manipulate log entries through a web application, API endpoint, or other service monitored by MultiTail, they can escalate their access from mere data observation to full command execution on the host system. This could lead to complete compromise of the underlying operating system, depending on the permissions assigned to the user account running the MultiTail process. The lack of a fixed release as of the review date exacerbates this risk, leaving organizations reliant on workarounds or alternative monitoring tools until an official patch is provided by the vendor.
Mitigation strategies for this vulnerability primarily involve avoiding the use of the -ex and -eX flags with untrusted log sources until a patched version becomes available. Administrators should ensure that MultiTail only monitors logs from trusted, internal services where input cannot be manipulated by external actors. Additionally, implementing strict access controls to limit who can write to monitored log files reduces the attack surface significantly. Organizations may also consider using more robust monitoring solutions that employ sandboxed execution environments or explicit parameterization of command arguments rather than shell string concatenation. Until a fixed release is published, continuous vigilance and restriction of MultiTail usage to secure contexts are essential to prevent exploitation through this command injection flaw.