Impact Analysis

This vulnerability can cause denial of service due to disk exhaustion on the victim's server.

When the decompression bomb expands to a very large file, it fills up the disk, potentially causing cascading failures in co-located services.

Recovery can be difficult because partial files remain if the extraction process is killed mid-way, and repeated exploitation is possible on restarts or signals if auto-download is enabled.

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Executive Summary

CVE-2026-39396 is a Denial of Service (DoS) vulnerability in OpenBao's OCI plugin downloader, specifically in the ExtractPluginFromImage() function.

This function extracts a plugin binary from a container image by streaming decompressed tar data using io.Copy without any upper bound on the number of bytes written.

An attacker who controls or compromises the OCI registry referenced by the victim can serve a crafted image containing a decompression bomb that expands to an arbitrarily large file, causing disk exhaustion on the victim's server.

The vulnerability exists because the tar header's Size field is never validated before copying, and all gzip layers are decompressed in memory or streaming without limits, resulting in unbounded decompression to disk.

The SHA256 integrity check is performed only after the full file is written, so the hash mismatch is detected too late to prevent disk exhaustion.

This allows attackers to replace legitimate plugin images without needing to alter their signatures.

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Detection Guidance

This vulnerability can be detected by monitoring for unusually large disk usage or disk exhaustion events during the extraction of OCI plugin images in OpenBao. Since the vulnerability involves unbounded decompression of tar data, signs include rapid disk space consumption and presence of very large extracted files from plugin images.

There is no direct command provided to detect the vulnerability itself, but you can check for large files created during plugin extraction or monitor disk usage with standard system tools.

• Use disk usage commands such as `df -h` to monitor available disk space.
• Use `du -sh /path/to/plugin/extraction` to check the size of extracted plugin directories.
• Monitor running OpenBao processes and check logs for errors related to disk exhaustion or failed plugin extraction.
• If possible, inspect the OCI registry configuration to verify the source of plugin images and ensure they are from trusted registries.

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Mitigation Strategies

The immediate mitigation step is to upgrade OpenBao to version 2.5.3 or later, where the vulnerability has been patched.

The patch includes validating the tar header Size field against a configurable maximum before extraction and enforcing size limits on decompression to prevent disk exhaustion.

Additionally, operators should configure the max_size field (default 1 GiB) in PluginConfig to limit the maximum allowed plugin size.

Until the upgrade is applied, avoid using OCI registries that are not fully trusted or that could be compromised, as attackers can serve crafted images containing decompression bombs.

Monitor disk usage closely and consider implementing disk quotas or alerts to detect abnormal disk consumption during plugin extraction.

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Compliance Impact

The provided information does not specify any direct impact of this vulnerability on compliance with common standards and regulations such as GDPR or HIPAA.

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