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How CVE-2026-44024 Works
Fluentd is a popular open-source data collector that unifies log ingestion from hundreds of sources and forwards them to various destinations. To provide flexibility in log routing and storage, Fluentd allows administrators to dynamically construct file paths using placeholders like ${tag}. The `tag` is a string that typically represents the source or category of the log events and is often derived from incoming network requests.
The vulnerability resides in the `out_file` plugin, which writes log data to files on the local filesystem. When the `path` parameter of this plugin is configured to include the `${tag}` placeholder, the plugin substitutes the placeholder with the actual tag value from each incoming event. Prior to version 1.19.3, Fluentd performed insufficient validation on the tag value before using it in file path construction.
An unauthenticated remote attacker can exploit this by sending specially crafted log events with a tag containing path traversal sequences such as `../` or ..\. For example, an attacker could set the tag to `../../../../etc/passwd` or ../../../var/www/html/shell.php. When Fluentd processes this event, the `out_file` plugin constructs the output file path by embedding the malicious tag directly into the path. Because there is no sanitization or restriction, the plugin writes the log content to the traversed location.
This behavior allows the attacker to write arbitrary files or overwrite existing files anywhere the Fluentd process has write permissions. The ability to write arbitrary files is a critical primitive that can be escalated to full remote code execution. An attacker could overwrite a system’s cron file, modify an SSH authorized_keys file, inject a malicious script into a web directory, or replace a Fluentd plugin with a malicious one. Since Fluentd often runs with elevated privileges to access logs and system resources, the impact is severe.
The vulnerability is classified as critical with a CVSS base score of 9.8 (CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H). It affects all Fluentd versions prior to 1.19.3. The patch in version 1.19.3 introduces proper validation and sanitization of the `${tag}` placeholder, rejecting any tag that contains path traversal characters or sequences. Users are strongly urged to upgrade immediately or apply workarounds if an upgrade is not feasible.
DailyCVE Form
Platform: Fluentd
Version: ≤ 1.19.2
Vulnerability: Path Traversal
Severity: Critical (9.8)
date: 2026-07-08
Prediction: 2026-06-25
What Undercode Say
Analytics
- Attack vector: Network
- Attack complexity: Low
- Privileges required: None
- User interaction: None
- Scope: Unchanged
- Confidentiality impact: High
- Integrity impact: High
- Availability impact: High
- Exploit availability: Public PoC
- Active exploitation: Confirmed
Bash commands and codes related to the vulnerability
Check Fluentd version:
fluentd --version or /usr/sbin/fluentd --version
Vulnerable configuration example (out_file with ${tag}):
<match >
@type file
path /var/log/fluent/${tag}.log
</match>
Crafted tag to exploit path traversal:
Using curl to send a log event with a malicious tag via in_forward plugin
echo '{"message":"test"}' | curl -X POST -H "Content-Type: application/json" \
--data-binary @- "http://target:24224/fluentd?tag=../../../../etc/cron"
Using fluent-cat to send malicious tag:
echo '{"message":"malicious content"}' | fluent-cat ../../../../tmp/evil
Check for vulnerable plugin usage:
grep -r "path.${tag}" /etc/fluent/
Exploit
To exploit CVE-2026-44024, an attacker first identifies a Fluentd instance that is exposed to untrusted networks and is configured with the `out_file` plugin using the `${tag}` placeholder in the `path` parameter. The attacker then crafts a log event with a tag containing path traversal sequences, such as ../../../../root/.ssh/authorized_keys, and sends it to the Fluentd input source (e.g., via `in_forward` on port 24224 or in_http). The plugin will write the log content to the traversed location, allowing the attacker to overwrite critical files. By overwriting an executable script or a configuration file that is later executed, the attacker can achieve remote code execution with the privileges of the Fluentd process.
Example exploit steps:
- Reconnaissance: Determine if the target Fluentd instance uses `${tag}` in file paths.
- Craft payload: Set the tag to a path traversal string pointing to a sensitive file.
- Send event: Use `fluent-cat` or `curl` to send a log event with the malicious tag.
- Escalate: Overwrite a cron job, SSH key, or web shell to gain shell access.
Proof-of-concept using fluent-cat:
Overwrite /etc/passwd (if Fluentd runs as root) echo 'root:x:0:0:root:/root:/bin/bash' | fluent-cat ../../../etc/passwd
Protection
The primary and most effective protection is to upgrade Fluentd to version 1.19.3 or later, which contains the official patch that sanitizes the `${tag}` placeholder and rejects malicious path traversal sequences.
If an immediate upgrade is not possible, the following workarounds are recommended:
– Restrict network access: Ensure that Fluentd input ports (e.g., 24224 for in_forward) are only accessible from trusted networks. Use firewall rules (iptables, security groups) to block untrusted sources.
– Run Fluentd as a non-root user: Dropping privileges prevents Fluentd from writing to sensitive system directories like `/etc/` or /root/, significantly limiting the impact.
– Avoid using `${tag}` in file paths: If the tag originates from an untrusted source, do not use the `${tag}` placeholder in the `path` parameter of output plugins. Use static paths or sanitize the tag before use.
– Filter incoming tags: Use a tag-rewriting filter (e.g., fluent-plugin-rewrite-tag-filter) to drop or sanitize any tags containing .., /, or `\` characters.
Example firewall rule (iptables):
iptables -A INPUT -p tcp --dport 24224 -s 192.168.0.0/16 -j ACCEPT iptables -A INPUT -p tcp --dport 24224 -j DROP
Example filter to sanitize tags:
<filter > @type rewrite_tag_filter <rule> key tag pattern /../ remove_tag </rule> </filter>
Impact
The impact of CVE-2026-44024 is critical. An unauthenticated remote attacker can write arbitrary files or overwrite existing files on the system with attacker-controlled content. This primitive can be directly escalated to full remote code execution, allowing the attacker to:
– Overwrite system binaries or configuration files.
– Inject malicious plugins into Fluentd’s plugin directory.
– Modify cron jobs or startup scripts to gain persistence.
– Read sensitive information by overwriting log files or configuration files that contain secrets.
– Achieve full system compromise if Fluentd runs with root privileges.
Given the widespread use of Fluentd in production environments for log aggregation and the fact that it often runs with elevated privileges, this vulnerability poses a severe risk to affected organizations. The CVSS score of 9.8 reflects the ease of exploitation and the catastrophic impact on confidentiality, integrity, and availability.
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Sources:
Reported By: nvd.nist.gov
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