CVE-2026-26158
Executive Summary
CVE-2026-26158 is a high-severity vulnerability in BusyBox, a widely used multi-call binary in embedded Linux systems and containers. The flaw resides in the tar extraction functionality, where the application fails to validate hardlink and symlink entries within a crafted tar archive. An attacker with the ability to supply a malicious tar file can cause files to be written outside the intended extraction directory. When extraction is performed with elevated privileges (e.g., root), this can lead to privilege escalation, allowing overwriting of critical system files such as /etc/passwd, /etc/shadow, or binaries. The vulnerability is currently unpatched, and no public exploit code exists, but the attack vector is well-understood and poses a significant risk to systems using BusyBox tar for automated extraction tasks.
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CVSS Breakdown & Threat Level
- CVSS Score: 7.0 (High)
- Attack Vector: Local (AV:L) – requires local access or ability to deliver a tar file.
- Attack Complexity: Low (AC:L) – no special conditions needed.
- Privileges Required: Low (PR:L) – attacker needs a low-privileged account or ability to upload a tar file.
- User Interaction: Required (UI:R) – a user or process must extract the malicious archive.
- Scope: Unchanged (S:U) – the vulnerable component and the impacted component are the same.
- Confidentiality Impact: High (C:H) – sensitive files can be read via symlinks.
- Integrity Impact: High (I:H) – critical system files can be overwritten.
- Availability Impact: High (A:H) – overwriting binaries or configuration can cause denial of service.
Threat Level: High. The vulnerability is easy to exploit once a malicious tar file is delivered, and the impact includes full system compromise.
Deep Dive & Mechanics
BusyBox’s tar implementation, like many tar utilities, supports hardlinks and symlinks to preserve file system structure. When extracting an archive, the code typically resolves the target path by concatenating the extraction directory with the entry name. However, if a tar entry contains a symlink or hardlink pointing to an absolute path (e.g., /etc/passwd) or a relative path with ../ sequences (e.g., ../../etc/passwd), the extraction routine may follow the link and write data outside the intended directory.
The vulnerability arises because BusyBox does not validate that the resolved path of a symlink or hardlink target stays within the extraction root. An attacker can craft a tar archive with:
- A symlink entry named
link_to_etcthat points to/etc. - A regular file entry named
link_to_etc/passwdthat contains malicious content.
When extracted, the symlink is created, and then the file is written through the symlink, effectively overwriting /etc/passwd. If extraction runs as root, the attacker gains root privileges.
The core issue is missing path sanitization in the tar applet, specifically in the data_extract_all or similar function that handles link resolution.
Vulnerable vs. Patched Code
Since no official patch is available, the vulnerable code pattern is described conceptually.
Vulnerable Code (Conceptual):
// In tar.c, during extraction of a symlink entry:
if (S_ISLNK(tar_header.typeflag)) {
// No check that target_path is within extraction directory
symlink(tar_header.linkname, full_path);
}
// For regular files, the path is constructed but not validated against symlinks
Patched Code (Conceptual):
// After resolving the full path, check it is under the extraction root
char *resolved = realpath(full_path, NULL);
if (resolved == NULL || strncmp(resolved, extraction_root, strlen(extraction_root)) != 0) {
// Reject the entry
free(resolved);
return -1;
}
free(resolved);
The patch would involve using realpath() to canonicalize the target path and verifying it starts with the extraction directory. Additionally, symlink creation should be restricted or validated to prevent pointing outside the extraction root.
Exploitation & Lab Setup
Red Team
Objective: Escalate privileges from a low-privileged user to root by crafting a malicious tar archive.
Lab Setup:
- Target: A Linux system with BusyBox (version vulnerable to CVE-2026-26158) and a low-privileged user account.
- Attacker machine: Any system with tar and netcat.
Steps:
- Create a malicious tar archive that overwrites
/etc/passwdto add a new root user. - Create a symlink entry pointing to
/etc. - Create a file entry named
passwd(which will be written through the symlink) containing a new root user line. - Transfer the archive to the target (e.g., via SCP, web download, or email).
- On the target, extract the archive with BusyBox tar:
busybox tar -xf malicious.tar - If extraction runs as root (e.g., via cron job or systemd timer), the
/etc/passwdfile is overwritten. - Log in as the new root user.
Example Malicious Tar Creation (Conceptual):
# Create a symlink entry pointing to /etc
echo -n 'link_to_etc' | tar -cf malicious.tar --transform 's/.*/link_to_etc/' --owner=0 --group=0 --mode=777 --no-recursion --files-from -
# Add a file that will be written through the symlink
echo 'newroot:x:0:0:root:/root:/bin/sh' > passwd
tar -rf malicious.tar --transform 's/.*/link_to_etc\/passwd/' --owner=0 --group=0 --mode=644 passwd
Blue Team
Detection:
- Monitor for tar extraction processes running as root or with elevated privileges.
- Audit file system changes in sensitive directories like
/etc,/usr/bin,/bin. - Use file integrity monitoring (e.g., AIDE, Tripwire) to detect unauthorized changes to critical files.
Prevention:
- Avoid extracting untrusted tar archives with BusyBox tar. Use GNU tar with
--no-overwrite-dirand--no-same-permissionsas a safer alternative. - Run extraction in a sandboxed environment (e.g., Docker container with read-only root filesystem).
- Apply the principle of least privilege: do not run tar extraction as root unless absolutely necessary.
- Use AppArmor or SELinux to confine BusyBox and restrict its write access.
Hunting:
- Search for tar archives containing symlinks or hardlinks with absolute paths or
../sequences. - Analyze system logs for unusual file writes to
/etcor/usrduring tar extraction.
Indicators of Compromise (IOCs)
- File System Changes: Unexpected modifications to
/etc/passwd,/etc/shadow,/etc/sudoers, or system binaries. - Process Anomalies: BusyBox tar process running as root with arguments containing suspicious archive names.
- Network Artifacts: Inbound transfer of tar archives from untrusted sources.
- Log Entries: Syslog or auditd entries showing tar extraction events with high privileges.
- YARA Rule Matches: Detection of tar archives with embedded symlinks pointing to absolute paths.
Detection Engineering
YARA Rule
rule CVE_2026_26158_BusyBox_Tar_Symlink_Escape {
meta:
description = "Detects malicious tar archives exploiting CVE-2026-26158 via symlink escape"
author = "Valters IT"
date = "2025-04-10"
hash = "unknown"
strings:
$symlink_magic = { 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 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