CVE-2026-49114 ONNX CVE debrief

Who should care

Defenders responsible for securing systems that use ONNX, particularly those with write access to directories where external data is serialized, should assess exposure and prioritize updating to version 1.21.0 or later.

Why it matters

CVE-2026-49114 is a medium-severity vulnerability in ONNX that allows a local attacker to append to any file a victim can write, potentially leading to privilege escalation. Defenders responsible for securing systems that use ONNX should assess exposure and prioritize updating to version 1.21.0 or later.

• A local attacker can append to any file a victim can write, potentially leading to privilege escalation.
• The vulnerability allows for unauthorized modification of sensitive files.
• Defenders need to verify if their systems are using affected versions of ONNX and prioritize updates.

Technical summary

The 'save_external_data' function in ONNX before 1.21.0 builds external-data file paths without using 'O_NOFOLLOW/O_EXCL', allowing a local attacker to pre-plant a symlink and append to any file a victim can write. This can lead to unauthorized modification of sensitive files and potentially privilege escalation. The vulnerability is fixed in version 1.21.0. Defenders should verify if their systems are using affected versions of ONNX and prioritize updates. The vulnerability allows for unauthorized modification of sensitive files, and defenders need to assess exposure and prioritize updating to version 1.21.0 or later.

Defensive priority

High

Recommended defensive actions

• Update ONNX to version 1.21.0 or later
• Restrict write access to directories where external data is serialized
• Monitor for suspicious file modifications
• Review compensating controls for exposed systems while remediation is scheduled and verified
• Check relevant monitoring, detection, and logs for exposed assets that need extra review
• Track exceptions, retest remediated assets, and close the item only after evidence is documented
• Confirm whether affected product deployments exist in managed environments and assign an owner for follow-up

Evidence notes

The vulnerability exists in the 'save_external_data' function of ONNX, which does not use 'O_NOFOLLOW/O_EXCL' when opening external-data files for writing. A local attacker with write access to the directory can pre-plant a symlink, causing the victim's write to append to any file the victim can write.

Sources and references

Methodology and review provenance

AI-assisted synthesis based on stored public vulnerability evidence. System validation, approval state, and publication status do not by themselves establish human review of this revision. PatchSiren helps prioritize defensive review and does not prove exposure or remediation on any system.