CVE-2026-26209

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 item. This recursive call lacks a depth-tracking mechanism.
• Vulnerable Code Locations:
  • cbor2/decoder.py (Pure Python implementation)
  • source/decoder.c (C extension implementation)
• Execution Flow:
  1. The cbor2.loads() function initializes a CBORDecoder and calls its decode() method.
  2. The decode() method reads the initial byte and dispatches control to a specific handler based on the major type. For an Array (Major Type 4), it calls .