CVE-2026-105840 in lrzsz
Summary
by MITRE • 10/06/2026
lrzsz before 0.13.0 contains a path traversal vulnerability in the lrz receive utility's restricted mode that allows malicious ZMODEM senders to write files outside the current directory using absolute pathnames. Because checkpath() in src/lrz.c only rejects '../' sequences unless built with --enable-pubdir, attackers can send files named with absolute paths to overwrite any file writable by the receiving user.
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Analysis
by VulDB Data Team • 10/06/2026
The lrzsz software suite, specifically prior to version 0.13.0, contains a critical path traversal vulnerability within its receive utility when operating in restricted mode. This flaw affects the handling of ZMODEM protocol transfers, allowing an attacker with the ability to initiate or control a file transfer from a remote system to write files outside the intended target directory. The core technical issue lies in the implementation of the checkpath function located in src/lrz.c. While this function is designed to prevent path traversal by rejecting sequences such as ../, it fails to adequately sanitize absolute paths provided during the ZMODEM transmission process. Consequently, if a malicious sender specifies an absolute pathname for the file being transmitted, the receiving lrz utility will accept and write that file directly to the specified location on the filesystem rather than within the current working directory or its designated subdirectories.
This vulnerability is particularly severe because it bypasses the intended security boundary of restricted mode, which is supposed to confine file operations to a specific sandboxed area. The logic flaw indicates that the validation mechanism primarily targets relative path traversal attempts but neglects absolute path injection scenarios unless the software was explicitly compiled with the --enable-pubdir configuration flag. In standard installations where this flag is not enabled, an attacker can exploit this gap by sending files named with absolute paths such as /etc/passwd or other sensitive system files that are writable by the user executing lrzsz. This capability effectively transforms a simple file transfer utility into a tool for arbitrary file write attacks, which can lead to significant security compromises including privilege escalation if the receiving process runs with elevated privileges, or data corruption and defacement in standard user contexts.
From an operational impact perspective, this vulnerability undermines the integrity of systems relying on lrzsz for secure file transfers over serial lines or emulated terminals. An attacker could overwrite configuration files, inject malicious scripts into executable directories, or replace critical system binaries depending on the permissions of the target process. The exploitability is relatively high given that ZMODEM is often used in trusted environments such as console access via modems or virtual consoles, where users might not anticipate file transfer attacks from potentially compromised remote hosts. Furthermore, because the vulnerability exists in the default build configuration for many distributions, a large number of systems remain exposed without requiring any special compilation flags to be exploited.
Mitigation strategies primarily involve upgrading lrzsz to version 0.13.0 or later where this path traversal flaw has been addressed through improved input validation and stricter enforcement of directory confinement rules. For environments that cannot immediately upgrade, administrators should consider compiling the software with the --enable-pubdir option if supported by their build system, although relying on compile-time flags is less robust than a patched binary. Additionally, restricting file transfer capabilities to only necessary users and ensuring that processes running lrzsz operate under minimal privilege principles can reduce the blast radius of any successful exploitation. Monitoring for unusual ZMODEM traffic patterns or unexpected file creations in sensitive directories may also aid in early detection of attempted exploits.
This vulnerability aligns with CWE-22, which describes Improper Limitation of a Pathname to a Restricted Directory, commonly known as path traversal. It is further categorized under the MITRE ATT&CK framework technique T1048, specifically Exfiltration Over Alternative Protocol or potentially T1565 Data Manipulation if used for overwriting files to disrupt operations. The failure to properly validate absolute paths against a restricted root directory represents a classic implementation error in input sanitization logic that has been historically prevalent in network service utilities handling user-supplied filenames.