CVE-2026-26209

• 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 decode_array.
  3. decode_array loops and calls self.decode() for each item, leading to deep recursion when parsing a payload like [...[...[1]...]...].

PoC

import cbor2

DEPTH = 1000

payload = b'\x81' * DEPTH + b'\x01'
print(f"[*] Payload size: {len(payload) / 1024:.2f} KB")
print("[*] Triggering decoder...")

try:
    cbor2.loads(payload)
    print("[+] Parsed successfully (Not Vulnerable)")
except RecursionError:
    print("\n[!] VULNERABLE: RecursionError triggered!")
except Exception as e:
    print(f"\n[-] Unexpected Error: {type(e).__name__}: {e}")

Credit

This issue was discovered by Kevin Tu of TMIR at ByteDance. The patch was developed by @agronholm.

• Scope: This vulnerability affects any application using cbor2 to parse untrusted data. Common use cases include IoT data processing, WebAuthn (FIDO2) authentication flows, and inter-service communication over COSE (CBOR Object Signing and Encryption).
• Attack Vector: A remote, unauthenticated attacker can achieve a full Denial of Service with a highly efficient, low-bandwidth attack. A payload under 100KB is sufficient to reliably terminate a Python worker process.

CVE-2026-26209 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (5.9.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-26209? CVE-2026-26209 is a high-severity security vulnerability in cbor2 (pip), affecting versions <= 5.8.0. It is fixed in 5.9.0.
2. How severe is CVE-2026-26209? CVE-2026-26209 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of cbor2 are affected by CVE-2026-26209? cbor2 (pip) versions <= 5.8.0 is affected.
4. Is there a fix for CVE-2026-26209? Yes. CVE-2026-26209 is fixed in 5.9.0. Upgrade to this version or later.
5. Is CVE-2026-26209 exploitable, and should I be worried? Whether CVE-2026-26209 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-26209 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-26209? Upgrade cbor2 to 5.9.0 or later.