CVE-2026-44162: 
Ruby vulnerability analysis and mitigation

CVE-2026-44162 is a Denial of Service (DoS) vulnerability via decompression bomb in the in_s3 input plugin of the fluent-plugin-s3 RubyGem. The plugin reads and decompresses files (gzip, lzma2, lzop) from Amazon S3 without enforcing a size limit on the decompressed payload, allowing an attacker with S3 write permissions to trigger memory exhaustion on the Fluentd host. Affected versions are >= 0.7.0 and <= 1.8.4; version 1.8.5 contains the fix. It carries a CVSS v3.1 base score of 2.7 (Low) (GitHub Advisory). The advisory was published on June 26, 2026 (GitHub Security Advisory).

The root cause is CWE-770 (Allocation of Resources Without Limits or Throttling): the in_s3 plugin's extractor classes (GzipExtractor, TextExtractor, s3_extractor_lzma2, s3_extractor_lzo) previously called io.read or accumulated decompressed chunks into memory without checking the total size against any threshold. The fix in commit e085aee introduces a configurable decompression_size_limit parameter (defaulting to 256 MiB) and adds chunked reading with a SizeLimitError exception that terminates decompression if the output exceeds the limit (GitHub Commit). Exploitation requires the attacker to have AWS IAM permissions to upload (PUT) objects to the S3 bucket monitored by the in_s3 plugin — a high-privilege precondition that limits the attack surface (GitHub Advisory).

Successful exploitation causes memory exhaustion on the node running Fluentd, which can trigger an Out-of-Memory (OOM) kill of the Fluentd process by the operating system. This results in complete disruption of log collection on the affected node, with no impact on confidentiality or data integrity. There is no known path to lateral movement or data exfiltration from this vulnerability alone (GitHub Advisory).

No public exploit code, exploit kits, or in-the-wild exploitation have been reported for CVE-2026-44162. The CVE status is listed as "Reserved" and no EPSS score or CISA KEV catalog entry is currently available. Exploitation is constrained by the requirement for high privileges (S3 bucket write access), significantly reducing the practical risk (GitHub Advisory, Feedly).

1. Obtain S3 Write Access: Acquire AWS IAM credentials or a role with s3:PutObject permission on the S3 bucket monitored by the target's in_s3 Fluentd plugin.
2. Craft a Decompression Bomb: Create a highly compressed file (e.g., gzip) that expands to a very large size upon decompression — for example, a file containing gigabytes of repeated null bytes compressed to a small archive.
3. Upload the Malicious File: Use the AWS CLI or SDK to upload the crafted compressed file to the monitored S3 bucket: aws s3 cp bomb.gz s3://<target-bucket>/.
4. Trigger Decompression: Wait for or trigger the Fluentd in_s3 plugin to poll and process the uploaded object. The plugin will attempt to decompress the file and load the entire payload into memory.
5. Achieve DoS: The Fluentd process exhausts available system memory, causing the OS to OOM-kill the process and halting all log collection on the affected node (GitHub Advisory, GitHub Commit).

• Logs: Fluentd logs containing "Extracted data exceeds limit of N bytes" (post-patch) or sudden absence of Fluentd log output indicating process crash; OS-level OOM killer messages in /var/log/syslog or dmesg referencing the Fluentd/Ruby process (e.g., Out of memory: Kill process <pid> (ruby)).
• Process: Unexpected termination of the fluentd or ruby process; rapid spike in memory consumption by the Fluentd process visible in top, htop, or system monitoring tools immediately after an S3 object is processed.
• File System: Presence of unexpected or anomalously large compressed files in the monitored S3 bucket, particularly with unusual upload timestamps or IAM principals.
• Network: AWS CloudTrail logs showing PutObject API calls to the monitored S3 bucket from unexpected IAM principals, IP addresses, or at unusual times.

Upgrade fluent-plugin-s3 to version 1.8.5, which introduces a decompression_size_limit parameter (default: 256 MiB) that enforces chunked reading and raises a SizeLimitError if the decompressed output exceeds the configured threshold (GitHub Commit). If an immediate upgrade is not possible, restrict AWS IAM s3:PutObject permissions on the monitored S3 bucket to only trusted services and administrators, and block any public or untrusted uploads (GitHub Advisory). After upgrading, the decompression_size_limit configuration parameter can be tuned in the Fluentd configuration to match operational requirements.