In ONNX before 1.21.0, the 'save_external_data' function builds the external-data file path from the model's external_data location field and opens it for writing without 'O_NOFOLLOW/O_EXCL', after a non-atomic 'os.path.isfile()' check. A local attacker with write access to the directory where a victim serializes external data can deterministically pre-plant a symlink that is being followed, causing the victim's write to append to any file the victim can write, e.g. ~/.ssh/authorized_keys, cron files, or application configs. Fixed in 1.21.0.

CVE-2026-49114 affects ONNX versions before 1.21.0, where the 'save_external_data' function constructs external-data file paths without safe file-opening flags (O_NOFOLLOW/O_EXCL). A non-atomic os.path.isfile() check creates a TOCTOU race condition. A local attacker with write access to the serialization directory can pre-plant a symlink before the victim writes data. This allows the attacker to redirect writes to arbitrary files the victim can write, such as ~/.ssh/authorized_keys, cron files, or application configs. The vulnerability enables privilege escalation or persistent access through file manipulation. It is classified as a local symlink-follow attack. The issue has been fixed in ONNX version 1.21.0.