Summary
ONNX: Null Pointer Dereference in ScanInferenceFunction Due to Missing Required Attribute
ONNX shape inference did not safely validate the required Scan num_scan_inputs attribute. Two malformed values could terminate or disrupt the process:
- If the attribute was missing,
ctx.getAttribute("num_scan_inputs")returned null and was dereferenced. - If the attribute was negative, conversion to
size_tproduced a very large value that reached a vector allocation and raisedstd::length_error.
The negative-value variant passed onnx.checker.check_model() before reaching shape inference.
Technical details
The vulnerable Scan inference implementations used the following pattern:
auto num_scan_inputs =
narrow_cast<size_t>(ctx.getAttribute("num_scan_inputs")->i());
This code neither checked the returned pointer nor rejected a negative signed value before converting it to an unsigned size. The pattern was present in the current Scan inference function and the older opset 8 and 9 paths.
The fix uses the required-attribute helper and checked narrowing:
auto num_scan_inputs =
narrow<size_t>(getRequiredAttributeInt(ctx, "num_scan_inputs"));
Affected versions
- Affected: ONNX 1.4.0 through 1.21.0
- Patched: ONNX 1.22.0
ONNX 1.4.0 is the first release containing the vulnerable Scan shape-inference implementation.
Severity
The vulnerable ONNX component processes the malicious model through a local library API or file boundary and is not itself bound to a network stack. Applications that expose ONNX model processing over a network should calculate an Environmental score with Modified Attack Vector set to Network.
The two triggers remain in one advisory because they affect the same required attribute, share the same unsafe conversion path and remediation, and were fixed in the same release.
Impact
An application that performs shape inference on an untrusted model can be terminated or forced into an exceptional allocation path, causing denial of service. No confidentiality or integrity impact was demonstrated.
The application dereferences a null pointer, causing a crash. Typical impact: denial of service via crash.
CVE-2026-115921 has a CVSS score of 6.2 (Medium). The vector is requires local access, no privileges required, and no user interaction. 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 (1.22.0); 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.
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Upgrade to ONNX 1.22.0 or later.
- Fix: onnx/onnx#7916
- Commit:
dd6bc92ea
Both reported models were re-tested against the published ONNX 1.23.2 wheel and correctly raise InferenceError.
Frequently Asked Questions
- What is CVE-2026-115921? CVE-2026-115921 is a medium-severity null pointer dereference vulnerability in onnx (pip), affecting versions >= 1.4.0, <= 1.21.0. It is fixed in 1.22.0. The application dereferences a null pointer, causing a crash.
- How severe is CVE-2026-115921? CVE-2026-115921 has a CVSS score of 6.2 (Medium). 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 onnx are affected by CVE-2026-115921? onnx (pip) versions >= 1.4.0, <= 1.21.0 is affected.
- Is there a fix for CVE-2026-115921? Yes. CVE-2026-115921 is fixed in 1.22.0. Upgrade to this version or later.
- Is CVE-2026-115921 exploitable, and should I be worried? Whether CVE-2026-115921 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-2026-115921 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-2026-115921? Upgrade
onnxto 1.22.0 or later.