CVE-2021-20153 in AC2600 TEW-827DRU
Summary
by MITRE • 12/31/2021
Trendnet AC2600 TEW-827DRU version 2.08B01 contains a symlink vulnerability in the bittorrent functionality. If enabled, the bittorrent functionality is vulnerable to a symlink attack that could lead to remote code execution on the device. If an end user inserts a flash drive with a malicious symlink on it that the bittorrent client can write downloads to, then a user is able to download arbitrary files to any desired location on the devices filesystem, which could lead to remote code execution. Example directories vulnerable to this include "config", "downloads", and "torrents", though it should be noted that "downloads" is the only vector that allows for arbitrary files to be downloaded to arbitrary locations.
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Analysis
by VulDB Data Team • 01/05/2022
The CVE-2021-20153 vulnerability affects Trendnet AC2600 TEW-827DRU routers running firmware version 2.08B01 and represents a critical symlink attack vector within the bittorrent client functionality. This vulnerability stems from improper handling of symbolic links during file operations, creating a path traversal condition that allows attackers to manipulate the filesystem through maliciously crafted symlink structures. The flaw specifically manifests when the bittorrent client is enabled and configured to write downloads to user-accessible directories on the device's storage, making it exploitable through physical media insertion attacks. The vulnerability is categorized under CWE-367 which identifies Time-of-Check to Time-of-Use (TOCTOU) flaws and symlink attacks as significant security weaknesses that can lead to privilege escalation and arbitrary code execution.
The technical implementation of this vulnerability exploits the bittorrent client's failure to properly validate or sanitize symbolic links during file download operations. When a user inserts a flash drive containing malicious symlinks into the router, the bittorrent client processes these links without adequate verification, allowing attackers to create symbolic references that point to critical system directories. The attack vector specifically targets the downloads directory as the only location where arbitrary file placement is possible, enabling attackers to overwrite system configuration files, inject malicious binaries, or modify critical components of the router's operating system. This represents a sophisticated attack pattern that combines physical access vectors with network-based exploitation, leveraging the trust relationship between the router's bittorrent client and user-inserted storage media.
The operational impact of CVE-2021-20153 extends beyond simple unauthorized file access to encompass complete system compromise and persistent backdoor capabilities. Successful exploitation allows attackers to achieve remote code execution with the privileges of the bittorrent client process, which typically runs with elevated permissions on the device. This vulnerability can be leveraged to install persistent malware, modify network configurations, redirect traffic through malicious proxies, or establish covert communication channels. The attack requires minimal user interaction beyond inserting the malicious storage device, making it particularly dangerous in environments where physical security controls are inadequate. The vulnerability's impact aligns with ATT&CK technique T1059.007 for command and scripting interpreter and T1543.003 for create or modify system process, as it enables attackers to establish persistent execution and modify system behavior through compromised bittorrent functionality.
Mitigation strategies for CVE-2021-20153 should focus on both immediate defensive measures and long-term architectural improvements. The most effective immediate solution involves disabling the bittorrent client functionality entirely when not required, as this eliminates the attack surface entirely. Network administrators should implement strict access controls to prevent unauthorized physical access to network devices and establish policies prohibiting the insertion of unknown storage media into network equipment. Additionally, firmware updates from Trendnet addressing this specific vulnerability should be deployed immediately, as the company has acknowledged the issue and provided patched versions. Security monitoring should include detection of unusual file creation patterns, particularly in system directories, and implementation of file integrity monitoring to detect unauthorized modifications to critical configuration files. The vulnerability demonstrates the importance of proper input validation and secure file handling practices, aligning with industry standards that emphasize the need for robust sanitization of all user-supplied data and the principle of least privilege in system design.