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How CVE-2026-44162 Works
The `fluent-plugin-s3` gem provides both input (in_s3) and output (out_s3) plugins for Fluentd, enabling integration with Amazon S3. The `in_s3` input plugin is designed to read objects from a configured S3 bucket and supports transparent decompression of several compressed formats, including gzip, lzma2, and lzop.
When `in_s3` processes a new object, it streams the file from S3, detects the compression format, and decompresses the payload entirely into memory before any further processing or size validation takes place. The plugin does not enforce a maximum decompressed size limit, nor does it stream the decompressed data incrementally – it reads the full expanded content into a single in‑memory buffer.
An attacker who has write (PUT) permissions to the monitored S3 bucket can upload a maliciously crafted file that is extremely small when compressed but expands to a massive size (a classic “decompression bomb” or “zip bomb”) upon decompression. For example, a tiny gzip file can contain repeated patterns that expand to gigabytes or even terabytes of data.
When Fluentd’s `in_s3` plugin polls the bucket and encounters this malicious object, it initiates the decompression routine. The plugin reads the entire compressed file, decompresses it in one shot, and attempts to allocate a contiguous memory block large enough to hold the fully expanded data. As the decompression proceeds, the plugin’s memory footprint grows rapidly, often exceeding the available system RAM or the process’s configured memory limit.
The operating system’s Out‑of‑Memory (OOM) killer eventually terminates the Fluentd process to prevent system instability. This results in a complete crash of the Fluentd daemon. All log collection, buffering, and forwarding on that node cease immediately, leading to a Denial of Service (DoS) condition for the entire logging pipeline.
Because the attack does not require authentication to Fluentd itself – only write access to the target S3 bucket – it can be exploited remotely by any user or service that has been granted PUT permissions on that bucket. The vulnerability is particularly dangerous in environments where bucket policies are overly permissive or where public write access is enabled.
The flaw was assigned CVE-2026-44162 with a CVSS v3 base score of 2.7 (Low). Although the severity is rated Low, the practical impact can be severe in production logging systems, as it can completely halt observability and monitoring until the Fluentd service is manually restarted and the malicious object is removed from the bucket.
DailyCVE Form
| Field | Value |
|-|-|
| Platform | Fluentd / fluent-plugin-s3 |
| Version | prior to 1.8.5 (≤ 1.8.4) |
| Vulnerability | Decompression Bomb (Memory Exhaustion) |
| Severity | Low (CVSS 2.7) |
| Date | 2026‑06‑26 |
| Prediction | Already patched in v1.8.5 |
What Undercode Say
Analytics & Detection
To determine whether your Fluentd instance is vulnerable, check the installed version of fluent-plugin-s3:
Check the installed gem version gem list fluent-plugin-s3 Or, if using Bundler bundle show fluent-plugin-s3 For td-agent / Fluent Package users td-agent-gem list fluent-plugin-s3
If the version is older than 1.8.5, the system is vulnerable.
To monitor for potential exploitation attempts, enable debug logging on the `in_s3` plugin and watch for unusually large memory allocations or sudden OOM kills:
Enable debug logging for in_s3 in Fluentd config <match pattern> @type s3 ... @log_level debug </match> Monitor Fluentd memory usage in real time watch -n 2 'ps aux | grep fluentd | grep -v grep' Check system logs for OOM events dmesg | grep -i "out of memory" journalctl -k | grep -i "oom"
You can also audit your S3 bucket permissions to identify any overly broad write policies:
List bucket policies (using AWS CLI) aws s3api get-bucket-policy --bucket YOUR_BUCKET_NAME Check bucket ACLs aws s3api get-bucket-acl --bucket YOUR_BUCKET_NAME List all IAM roles/users with write access aws iam list-attached-role-policies --role-name YOUR_ROLE
Code Snippet – Vulnerable Decompression Flow (Simplified)
The vulnerable code in `in_s3.rb` essentially performs:
Vulnerable pseudocode – reads entire decompressed payload into memory def process_object(s3_object) compressed_data = s3_object.get.body.read decompressed = Zlib::GzipReader.new(StringIO.new(compressed_data)).read No size limit! process_logs(decompressed) end
The fix in v1.8.5 introduces a configurable size limit and streams decompression to avoid loading everything at once.
Exploit
Prerequisites:
- Write (PUT) access to the S3 bucket monitored by
in_s3. - Ability to upload a file with a compression format supported by the plugin (gzip, lzma2, or lzop).
Attack Vector:
- Create a highly compressed file that expands to an enormous size (e.g., a 1 MB gzip that decompresses to 10 GB).
- Upload this file to the target S3 bucket using any S3 client:
aws s3 cp malicious.gz s3://YOUR_BUCKET/path/
- Wait for the next `in_s3` polling cycle (or trigger an S3 event notification if configured).
- When Fluentd processes the object, the decompression routine consumes all available memory, triggering an OOM kill and crashing the Fluentd process.
- The logging pipeline is disrupted until the process is restarted and the malicious object is removed.
Note: The attack does not require any direct network access to the Fluentd host – it only needs write privileges to the S3 bucket.
Protection
1. Upgrade to the patched version (Recommended)
- Update `fluent-plugin-s3` to v1.8.5 or later:
gem update fluent-plugin-s3
- For td-agent / Fluent Package users, upgrade to Fluent Package v6.0.4 (which bundles the fixed plugin).
2. Restrict S3 Bucket Write Permissions (Workaround)
- Apply strict IAM policies to ensure that only trusted services and administrators have PUT access to the bucket monitored by
in_s3. - Remove any public write policies or overly permissive bucket ACLs.
- Use S3 Object Lock or versioning to retain a history of uploaded objects for forensic analysis.
- Implement Decompression Size Limits (If Upgrade Is Not Possible)
– While the plugin does not natively support a size limit in older versions, you can implement a custom wrapper script that validates file sizes before they are placed in the bucket, or use S3 Event Notifications to trigger a Lambda function that scans and rejects suspicious objects.
4. Monitor and Alert
- Set up memory usage alerts for the Fluentd process.
- Configure your orchestration system (e.g., Kubernetes, systemd) to automatically restart Fluentd if it crashes, and notify the operations team.
Impact
Immediate Effects:
- Denial of Service (DoS) – The Fluentd process is terminated by the OOM killer, halting all log collection, buffering, and forwarding on the affected node.
- Loss of Observability – Monitoring, alerting, and troubleshooting capabilities are lost until the service is restored.
- Data Loss – Any logs that were in transit or buffered at the time of the crash may be lost, depending on the buffer configuration.
Long‑Term Consequences:
- Repeated exploitation can lead to persistent service disruption, requiring manual intervention to remove malicious objects and restart the daemon.
- In containerized or auto‑scaling environments, the attack may cause frequent pod restarts, leading to resource thrashing and increased cloud costs.
- If the bucket contains sensitive logs, the attacker’s ability to upload arbitrary files could also be leveraged for other attacks (e.g., storing malware or exfiltrating data via misconfigured bucket policies).
CVSS Score:
- Base Score: 2.7 (Low) – due to the requirement of write access to the S3 bucket and the fact that the attack does not compromise data confidentiality or integrity.
- However, the practical severity in production logging environments is often higher, as it can completely cripple observability with a relatively simple attack.
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Sources:
Reported By: github.com
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