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Fluentd’s `out_http` output plugin, introduced in v1.7.0， allows logs to be forwarded to an HTTP/HTTPS endpoint. To enable dynamic routing, the `endpoint` configuration parameter supports placeholders such as `${tag}` and record fields, allowing the destination URL to be constructed from log data.
A vulnerability was discovered in how Fluentd handles these placeholders. If an attacker can control the value substituted into a placeholder (e.g., by injecting a malicious tag into the log stream), they can manipulate the hostname of the outbound HTTP request. This occurs because the plugin does not sufficiently validate the final, interpolated URL before making the connection.
Impact
This flaw enables a Server-Side Request Forgery (SSRF) attack. An unauthenticated attacker can force the Fluentd node to send crafted HTTP requests to arbitrary internal or external services. This can lead to:
– Unauthorized access to internal APIs and services.
– Exfiltration of sensitive data from internal networks.
– Compromise of cloud metadata endpoints (e.g., AWS IMDS at 169.254.169.254).
Patches
The vulnerability is fixed in Fluentd v1.19.3. The fix is also included in Fluent Package v6.0.4.
Workarounds
If an immediate upgrade is not possible, apply these mitigations:
1. Avoid Dynamic Hostnames: Do not use placeholders in the `endpoint` parameter to define the hostname.
2. Restrict Network Access: Use firewall rules (e.g., iptables, AWS Security Groups) to block the Fluentd node from accessing sensitive internal IP addresses.
3. Restrict Allowed Hosts: Implement a filter or proxy to validate and restrict the resolved hostname to a predefined allowlist.
DailyCVE Form:
Platform: ……. Fluentd
Version: …….. < v1.19.3
Vulnerability :…… Server-Side Request Forgery (SSRF)
Severity: ……. 7.5 (High)
date: ………. 2026-06-25
Prediction: …… 2026-06-26
What Undercode Say: Analytics
The core of the vulnerability lies in the interpolation of user-controlled data into the `endpoint` URL. To understand the risk, system administrators can audit their configurations.
– Check for Vulnerable Configuration:
grep -r "endpoint" /etc/fluentd/ | grep -E '\${.}'
This command searches for `endpoint` directives that use placeholders. If the placeholder sources (like ${tag}) are not strictly controlled, the configuration is vulnerable.
– Audit Allowed Hosts (Workaround):
While not a direct code fix, restricting outbound traffic is a critical mitigation. The following `iptables` rules can be used as an example to block access to the AWS metadata service:
iptables -A OUTPUT -d 169.254.169.254 -j DROP iptables -A OUTPUT -d 192.168.0.0/16 -j DROP Example for internal networks
Exploit
An attacker can exploit this vulnerability by injecting a malicious value into a log field that is used as a placeholder in the `endpoint` configuration.
1. Identify a Target: The attacker first identifies a Fluentd instance with a vulnerable configuration, for example:
<match pattern>
@type http
endpoint http://${tag}.internal.api/v1/log
...
</match>
2. Craft Malicious Payload: The attacker sends a log record with a specially crafted `tag` that points to an internal service. For example, a tag of `169.254.169.254` would cause the Fluentd node to make a request to `http://169.254.169.254.internal.api/v1/log`.
3. Execute SSRF: The Fluentd node resolves the hostname and makes an outbound HTTP request to the attacker-controlled or internal target, potentially exfiltrating sensitive data like cloud metadata.
Protection
Protection against this vulnerability involves a combination of upgrading and implementing network controls.
– Upgrade: The primary and most effective protection is to upgrade to Fluentd v1.19.3 or later.
– Sanitize Input: If placeholders must be used, ensure that the data populating them (e.g., tags, record fields) is strictly validated against an allowlist of expected values.
– Network Segmentation: Isolate Fluentd nodes and restrict their outbound network access using firewalls. Only allow connections to known, trusted log aggregation endpoints.
– Use a Proxy: Route all outbound HTTP requests from Fluentd through a forward or reverse proxy that can enforce allowlists and block malicious destinations.
Impact
- Confidentiality: High. An attacker can read sensitive data from internal services or cloud metadata.
- Integrity: Medium. An attacker could potentially modify data by sending requests to internal APIs.
- Availability: Low. The primary impact is data leakage and unauthorized access, not service disruption.
- Scope: This vulnerability affects all Fluentd deployments that use the `out_http` plugin with placeholders in the `endpoint` parameter and where the placeholder data is not strictly controlled. This is a common pattern in dynamic logging environments, making the potential impact widespread.
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Sources:
Reported By: github.com
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