CVE-2026-44161: Fluentd: Server-Side Request Forgery (SSRF) via Placeholder Expansion in `out_http`
Fluentd collects events from various data sources and writes them to files, RDBMS, NoSQL, IaaS, SaaS, Hadoop and so on. Prior to 1.19.3, the Fluentd outhttp output plugin allows placeholders such as ${tag} in the endpoint configuration parameter, and if a placeholder value is derived from untrusted input an attacker can control the destination hostname of outbound HTTP requests and force requests to arbitrary internal services. This issue is fixed in version 1.19.3.
Other sources
The outhttp output plugin allows the use of placeholders (such as ${tag}) in the endpoint configuration parameter. It was discovered that if the placeholder value is derived from untrusted user input, an attacker can maliciously control the destination hostname of the outbound HTTP requests made by Fluentd.
Impact This vulnerability allows for a Server-Side Request Forgery (SSRF) attack. An unauthenticated attacker can force the Fluentd node to send HTTP requests to arbitrary internal services. This can lead to unauthorized access to internal APIs, data exfiltration, or the compromise of cloud metadata endpoints (e.g., AWS IMDS 169.254.169.254).
Patches v1.19.3
Workarounds If an immediate upgrade is not possible, users are strongly advised to apply the following mitigations:
1. Avoid Dynamic Hostnames Do not use the placeholder in the endpoint parameter as hostname.
2. Restrict Network Access Use firewall rules (e.g., iptables, AWS Security Groups) to block the Fluentd node from accessing sensitive internal IP addresses, specifically the cloud provider's metadata service and other internal microservices that Fluentd does not explicitly need to access.
3. Restrict allowed hosts Inject filter to accept allowed hosts in placeholders explicitly if possible.
— GitHub
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
rubygems/fluentdto a version that resolves this vulnerability.Fixed in 1.19.3 - Upgrade
Upgrade
Fluentd out_http output pluginto a version that resolves this vulnerability.Fixed in 1.19.3 - Configuration
Ensure the `endpoint` parameter in `out_http` does not use placeholder values (such as `${tag}`) when the placeholder content is derived from untrusted user input.
Fluentd out_http output plugin endpoint = Avoid using placeholders derived from untrusted user input (e.g., do not use placeholders like ${tag} in endpoint when their values come from untrusted input) - Configuration
If possible, configure an explicit allow-list filter for hosts used in placeholder-expanded destinations (so only intended hosts are accepted when expanding placeholders in `endpoint`).
Fluentd out_http output plugin endpoint placeholders = If placeholders must be used, inject an allow-list filter for explicitly allowed hosts - Compensating control
Use firewall rules (e.g., iptables or AWS Security Groups) to block the Fluentd node from accessing sensitive internal IP addresses that Fluentd does not need, specifically including the cloud provider metadata service (e.g., AWS IMDS at `169.254.169.254`) and other internal microservices.
Event History
Frequently Asked Questions
What is the severity of CVE-2026-44161?
CVE-2026-44161 has a high severity rating of 7.2.
How do I fix CVE-2026-44161?
To fix CVE-2026-44161, ensure that the endpoint configuration parameter does not accept untrusted user input.
What type of attack is associated with CVE-2026-44161?
CVE-2026-44161 is associated with Server-Side Request Forgery (SSRF) attacks.
Which software is affected by CVE-2026-44161?
CVE-2026-44161 affects the rubygems/fluentd software.
What is the main issue in CVE-2026-44161?
The main issue in CVE-2026-44161 is that untrusted user input can manipulate the destination hostname in HTTP requests.