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How CVE-2026-44161 Works
Fluentd is a popular open-source data collector that ingests events from diverse sources—logs, metrics, databases, cloud services—and forwards them to various destinations via its rich plugin ecosystem. One of its core output plugins, out_http, is designed to send events to HTTP endpoints. To offer flexibility, this plugin supports placeholders (e.g., ${tag}, ${time}, ${hostname}) within the `endpoint` configuration parameter, allowing dynamic URL construction based on event metadata.
The vulnerability arises because the plugin evaluates these placeholders without sufficient validation or sanitization of their source. If an attacker can inject arbitrary data into a placeholder—for instance, by crafting a log message with a malicious `tag` value—that value is directly interpolated into the HTTP destination URL. This means the attacker can control the hostname (and potentially the path) of the outbound request.
Because Fluentd often runs inside trusted network perimeters with access to internal services, this flaw transforms into a classic Server-Side Request Forgery (SSRF) . An unauthenticated remote attacker can force the Fluentd node to send HTTP requests to any internal IP address or hostname reachable from that node. Common targets include:
– Cloud metadata endpoints (e.g., AWS IMDS at 169.254.169.254), which can expose instance credentials and sensitive configuration.
– Internal databases, REST APIs, monitoring dashboards, or Kubernetes API servers.
– Any internal service that relies on network segmentation for protection.
The vulnerability is tracked as CWE-918 (Server-Side Request Forgery) and aligns with MITRE ATT&CK technique T1071.1004 (Application Layer Protocol Manipulation) . It affects all Fluentd versions prior to 1.19.3. The fix in v1.19.3 introduces strict validation of placeholder-derived hostnames, ensuring they conform to expected patterns and rejecting malicious inputs.
While the CVSS v3.1 base score is 7.2 (High) , the real-world impact depends on the internal services exposed. In environments where Fluentd has broad network access, this vulnerability can lead to data exfiltration, privilege escalation, or denial of service by overwhelming internal endpoints.
DailyCVE Form
| Field | Value |
|–|–|
| Platform | Fluentd |
| Version | <= 1.19.2 |
| Vulnerability | SSRF |
| Severity | High (7.2) |
| Date | 2026-07-08 |
| Prediction| Patch 2026-06-25 |
What Undercode Say
Analytics & Detection Commands
To determine if your Fluentd instance is vulnerable, check the installed version:
Check Fluentd version fluentd --version or for Ruby gems gem list fluentd
If the version is 1.19.2 or lower, the system is at risk.
Identifying Vulnerable Configurations
Search your Fluentd configuration files for `out_http` plugins that use placeholders in the `endpoint` parameter:
grep -r "out_http" /etc/fluentd/
grep -r "endpoint" /etc/fluentd/ | grep "\${"
A typical vulnerable configuration might look like:
<match pattern>
@type http
endpoint http://${tag}.internal.service.local/api
... other parameters
</match>
Monitoring for Exploitation Attempts
Monitor outbound HTTP requests from the Fluentd host for unexpected destinations. Use `tcpdump` or audit firewall logs:
tcpdump -i any -n "host 169.254.169.254" AWS metadata probe tcpdump -i any -n "dst net 10.0.0.0/8" internal subnet traffic
Exploit
An attacker can exploit this vulnerability by injecting a malicious payload into a placeholder that is used as the hostname. For example, if Fluentd receives a log event with a `tag` field controlled by the attacker, and the configuration uses `${tag}` in the endpoint, the attacker can set `tag` to:
attacker.com
or
169.254.169.254/latest/meta-data/
The resulting HTTP request would be sent to:
http://attacker.com/...
or
http://169.254.169.254/latest/meta-data/...
This allows the attacker to:
- Probe internal networks – scan for open ports and services.
- Read cloud metadata – retrieve AWS IAM credentials, user-data, etc.
- Access internal APIs – interact with internal dashboards, databases, or orchestration tools.
- Cause denial of service – flood internal endpoints with repeated requests.
No authentication is required, and the attack can be performed remotely if Fluentd accepts untrusted input (e.g., via `in_forward` or `in_http` plugins).
Protection
Immediate Mitigation (Upgrade)
The only complete fix is to upgrade to Fluentd v1.19.3 or later. This version includes proper validation of placeholder values in the `out_http` endpoint.
Using Ruby gems gem update fluentd Using package manager (e.g., for fluent-package) apt-get update && apt-get install fluentd=1.19.3-
Workarounds (if upgrade is not possible)
- Avoid dynamic hostnames – Do not use placeholders in the `endpoint` parameter. Hardcode the destination hostname.
- Restrict network access – Use firewall rules (iptables, security groups) to block the Fluentd node from reaching sensitive internal IP ranges (e.g.,
169.254.169.254,10.0.0.0/8,172.16.0.0/12,192.168.0.0/16).iptables -A OUTPUT -d 169.254.169.254 -j DROP iptables -A OUTPUT -d 10.0.0.0/8 -j DROP
- Allow-list valid hosts – If placeholders are necessary, implement a filter that validates the resolved hostname against a predefined list of allowed domains.
- Isolate Fluentd – Run Fluentd in a network namespace or container with restricted egress policies.
Impact
- Confidentiality – An attacker can exfiltrate sensitive data from internal services, including cloud credentials, configuration secrets, and application data.
- Integrity – While the CVSS vector lists no integrity impact, an attacker could potentially modify data via internal APIs (e.g., altering configurations or deleting resources).
- Availability – The attacker can overwhelm internal services with a flood of requests, leading to denial of service (DoS).
- Lateral Movement – SSRF can serve as a stepping stone for further attacks within the internal network, bypassing perimeter defenses.
- Compliance – Organizations subject to regulations (GDPR, HIPAA, PCI-DSS) may face violations if internal data is exposed through this flaw.
In cloud environments, the most critical risk is access to the instance metadata service, which can grant the attacker full control over the compromised instance and, in some cases, the entire cloud account.
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Sources:
Reported By: nvd.nist.gov
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