# Multiple Fluentd Vulnerabilities: RCE via Tag Placeholder (CVE-2026-44024), Info Disclosure (CVE-2026-44025), Decompression Bomb DoS (CVE-2026-44160), and SSRF (CVE-2026-44161)

> Fluentd 1.19.3 patches four newly disclosed flaws: a critical (CVSS 9.8) path-traversal-to-RCE bug in the ${tag} placeholder used by output plugins such as out_file, a high-severity (CVSS 7.5) Monitor Agent API information disclosure exposing plugin instance variables (credentials, API keys), a high-severity (CVSS 7.5) gzip decompression-bomb DoS in in_http/in_forward, and a high-severity (CVSS 7.2) SSRF in out_http via unsafe placeholder expansion in the endpoint parameter that can reach cloud metadata services. Two older unpatched issues, CVE-2022-39379 (insecure Oj deserialization RCE in non-default config) and CVE-2021-41186 (ReDoS in parser_apache2), remain relevant to unpatched fleets.

- **Published:** 2026-07-01T00:00:00Z
- **Last reviewed:** 2026-07-01T00:00:00Z
- **Canonical:** https://intel.threadlinqs.com/threat/TL-2026-1041
- **ID:** TL-2026-1041
- **Severity:** HIGH (CVSS 9.8)
- **Category:** VULNERABILITY
- **Status:** ACTIVE
- **Detections:** 9 · **IOCs:** 18 (full data via the Threadlinqs MCP server — Purple tier)
- **CVEs:** CVE-2026-44024, CVE-2026-44025, CVE-2026-44160, CVE-2026-44161, CVE-2022-39379, CVE-2021-41186

## Description

Fluentd, the CNCF-graduated log collector widely deployed as the logging backbone of Kubernetes clusters and cloud observability pipelines, shipped version 1.19.3 to remediate four newly assigned CVEs and to re-surface risk from two long-standing unpatched issues.

CVE-2026-44024 (CVSS 9.8, CWE-22/CWE-94) is the most severe: Fluentd's output plugins (e.g. out_file) let administrators construct file paths dynamically using the ${tag} placeholder, but validation of the resulting tag value was insufficient. An untrusted log source (any client able to reach in_http, in_forward, or another accepting input plugin) can send a record whose tag contains path-traversal sequences such as ../../. When that tag is substituted into a path template like /var/log/${tag}.log, Fluentd will write to attacker-chosen paths outside the intended log directory. Because Fluentd can be configured to write files with attacker-controlled content and, depending on formatting options, attacker-controlled extensions, this primitive escalates from arbitrary file write to full remote code execution — e.g. by dropping a cron file, an SSH authorized_keys entry, a web shell into a served directory, or a malicious Fluentd plugin/config file that is subsequently loaded. No authentication is required if the vulnerable input plugin is reachable from an untrusted network.

CVE-2026-44025 (CVSS 7.5, CWE-200/CWE-306) affects the in_monitor_agent plugin, which exposes a REST API (default TCP/24220) for internal metrics such as /api/plugins.json. The API response unintentionally serializes and returns internal Ruby instance variables of every loaded plugin. If any plugin stores secrets in instance variables — database passwords, API keys, cloud credentials used for output destinations — those values are returned in plaintext to any unauthenticated client that can reach the Monitor Agent port, which is commonly bound to 0.0.0.0 in container/Kubernetes deployments.

CVE-2026-44160 (CVSS 7.5, CWE-409) is a denial-of-service in in_http and in_forward. Both plugins correctly cap the size of the compressed (gzip) payload accepted over the wire, but neither enforces a limit on the size of the data after decompression. A small, highly compressed payload (a classic "zip bomb" / decompression-bomb pattern) can expand to gigabytes in memory during processing, exhausting available RAM and triggering an OOM kill of the Fluentd process — halting all log collection on the affected node/pod.

CVE-2026-44161 (CVSS 7.2, CWE-918) is an SSRF in the out_http output plugin. The endpoint configuration parameter supports placeholder expansion (${tag} and record fields) so the destination URL can be built dynamically from log content. When the placeholder value is derived from untrusted, attacker-supplied log data, the attacker can redirect Fluentd's outbound HTTP requests to arbitrary internal hosts — including cloud instance metadata services (e.g. AWS IMDS at 169.254.169.254), internal admin panels, or other services not intended to be internet-reachable — potentially harvesting IAM/cloud credentials or pivoting further into the network.

Two older, still-relevant CVEs compound the risk picture for unpatched fleets. CVE-2022-39379 (CWE-502) is a remote code execution via insecure deserialization: when the non-default environment variable FLUENT_OJ_OPTION_MODE=object is set, Fluentd's Oj-based JSON parsing deserializes arbitrary Ruby objects from attacker-supplied JSON, enabling RCE without authentication (default configuration is not affected). CVE-2021-41186 (CWE-1333) is a ReDoS in the parser_apache2 plugin: crafted Apache-format log lines exploiting catastrophic backtracking in the path/referer/agent capture groups can consume excessive CPU, causing a denial of service for any deployment parsing untrusted or attacker-influenced Apache logs.

Collectively these flaws threaten confidentiality (credential exposure via Monitor Agent, SSRF to metadata endpoints), integrity (arbitrary file write/RCE), and availability (decompression bomb, ReDoS) of any Fluentd-based logging pipeline — a category of infrastructure that is frequently exposed to semi-trusted or multi-tenant log producers (Kubernetes DaemonSets ingesting pod logs, shared SaaS logging endpoints, CI/CD log forwarders) making several of these bugs practically reachable without insider access.

## MITRE ATT&CK

- T1595 Active Scanning
- T1588 Obtain Capabilities
- T1190 Exploit Public-Facing Application
- T1059 Command and Scripting Interpreter
- T1505 Server Software Component
- T1053 Scheduled Task/Job
- T1098 Account Manipulation
- T1068 Exploitation for Privilege Escalation
- T1211 Exploitation for Stealth
- T1036 Masquerading
- T1552 Unsecured Credentials
- T1046 Network Service Discovery
- T1083 File and Directory Discovery
- T1210 Exploitation of Remote Services
- T1005 Data from Local System
- T1071 Application Layer Protocol
- T1567 Exfiltration Over Web Service
- T1499 Endpoint Denial of Service
- T1489 Service Stop

## Sources

- [Fluentd Vulnerabilities](https://cybersecuritynews.com/fluentd-vulnerabilities/)
- [GHSA-44hj-4m45-frj3: Fluentd RCE via Arbitrary File Write in ${tag} Placeholder](https://github.com/advisories/GHSA-44hj-4m45-frj3)
- [GHSA-pr7j-96cj-549h: Fluentd Exposure of Sensitive Information via Monitor Agent API](https://github.com/advisories/GHSA-pr7j-96cj-549h)
- [GHSA-j9cw-hwqf-85w7: Fluentd DoS via Gzip Decompression Bomb in in_http and in_forward](https://github.com/advisories/GHSA-j9cw-hwqf-85w7)
- [GHSA-72f5-rr8c-r6gr: Fluentd SSRF via Placeholder Expansion in out_http](https://github.com/advisories/GHSA-72f5-rr8c-r6gr)
- [CVE-2026-44024: Fluentd is Vulnerable to RCE via Arbitrary File Write in ${tag} Placeholder](https://advisories.gitlab.com/gem/fluentd/CVE-2026-44024/)
- [CVE-2026-44025: Fluentd is Vulnerable to Exposure of Sensitive Information via Monitor Agent API](https://advisories.gitlab.com/gem/fluentd/CVE-2026-44025/)
- [CVE-2026-44160: Fluentd is Vulnerable to DoS via Gzip Decompression Bomb](https://advisories.gitlab.com/gem/fluentd/CVE-2026-44160/)
- [CVE-2026-44161: Fluentd is Vulnerable to SSRF via Placeholder Expansion in out_http](https://advisories.gitlab.com/gem/fluentd/CVE-2026-44161/)
- [Fluentd Patches Critical RCE, SSRF, DoS, and Sensitive Data Exposure Flaws](https://cyberpress.org/critical-fluentd-flaws-patches/)
- [GHSA-fppq-mj76-fpj2: fluentd vulnerable to RCE due to insecure deserialization (non-default config)](https://github.com/advisories/GHSA-fppq-mj76-fpj2)
- [GHSL-2022-067: Remote Code Execution (RCE) in Fluentd - CVE-2022-39379](https://securitylab.github.com/advisories/GHSL-2022-067_Fluentd/)
- [CVE-2022-39379 Detail](https://nvd.nist.gov/vuln/detail/CVE-2022-39379)
- [GHSA-hwhf-64mh-r662: ReDoS vulnerability in parser_apache2](https://github.com/advisories/GHSA-hwhf-64mh-r662)
- [GHSL-2021-102: ReDoS (Regular Expression Denial of Service) in Fluentd - CVE-2021-41186](https://securitylab.github.com/advisories/GHSL-2021-102-fluent-fluentd/)

## Full data

Detection queries (Splunk SPL / Microsoft KQL / Sigma) and IOC values require the Threadlinqs MCP server (Purple tier): https://intel.threadlinqs.com/mcp

Canonical: https://intel.threadlinqs.com/threat/TL-2026-1041