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CVE-2026-26209 — cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads

Affected: agronholm cbor2

cbor2 provides encoding and decoding for the Concise Binary Object Representation (CBOR) serialization format. Versions prior to 5.9.0 are vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. This vulnerability affects both the pure Python implementation and the C extension `_cbor2`. The C extension relies on Python's internal recursion limits `Py_EnterRecursiveCall` rather than a data-driven depth limit, meaning it still raises `RecursionError` and crashes the worker process when the limit is hit. While the library handles moderate nesting levels, it lacks a hard depth limit. An attacker can supply a crafted CBOR payload containing approximately 100,000 nested arrays `0x81`. When `cbor2.loads()` attempts to parse this, it hits the Python interpreter's maximum recursion depth or exhausts the stack, causing the process to crash with a `RecursionError`. Because the library does not enforce its own limits, it allows an external attacker to exhaust the host application's stack resource. In many web application servers (e.g., Gunicorn, Uvicorn) or task queues (Celery), an unhandled `RecursionError` terminates the worker process immediately. By sending a stream of these small (<100KB) malicious packets, an attacker can repeatedly crash worker processes, resulting in a complete Denial of Service for the application. Version 5.9.0 patches the issue.

GHSA-3c37-wwvx-h642 — cbor2 has a Denial of Service via Uncontrolled Recursion in cbor2.loads

Affected: PyPI: cbor2

### Summary

- The `cbor2` library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding deeply nested CBOR structures. - This vulnerability affects both the pure Python implementation and the C extension (`_cbor2`). The C extension correctly uses Python's C-API for recursion protection (`Py_EnterRecursiveCall`), but this mechanism is designed to prevent a stack overflow by raising a `RecursionError`. In some environments, this exception is not caught, thus causing the service process to terminate. - While the library handles moderate nesting, it lacks a configurable, data-driven depth limit independent of Python's global recursion setting. An attacker can supply a crafted CBOR payload containing thousands of nested arrays (e.g., `0x81`). When `cbor2.loads()` attempts to parse this, it hits the interpreter's recursion limit, causing the call to raise a `RecursionError`. - By sending a stream of small (<100KB) malicious packets, an attacker can repeatedly crash worker processes faster than they can be restarted, resulting in a complete and sustained Denial of Service.

### Details

- The vulnerability stems from the recursive design of the `CBORDecoder` class, specifically how it decodes nested container types like Arrays and Maps. - Inside `decode_array` (and similarly `decode_map`), the decoder iterates through the number of elements specified in the CBOR header. For each element, it calls `self.decode()` again to parse the nested…

PYSEC-2026-2123

Affected: PyPI: cbor2