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Alta

Denial of Service in pyasn1 via Unbounded Recursion

  • Publicada: 17.03.2026
  • Actualizado: 11.10.2026 15:17

Summary

The pyasn1 library is vulnerable to a Denial of Service (DoS) attack caused by uncontrolled recursion when decoding ASN.1 data with deeply nested structures. An attacker can supply a crafted payload containing nested SEQUENCE (0x30) or SET (0x31) tags with Indefinite Length (0x80) markers. This forces the decoder to recursively call itself until the Python interpreter crashes with a RecursionError or consumes all available memory (OOM), crashing the host application.

Details

The vulnerability exists because the decoder iterates through the input stream and recursively calls decodeFun (the decoding callback) for every nested component found, without tracking or limiting the recursion depth. Vulnerable Code Locations:

  1. indefLenValueDecoder (Line 998): for component in decodeFun(substrate, asn1Spec, allowEoo=True, **options): This method handles indefinite-length constructed types. It sits inside a while True loop and recursively calls the decoder for every nested tag.

  2. valueDecoder (Lines 786 and 907): for component in decodeFun(substrate, componentType, **options): This method handles standard decoding when a schema is present. It contains two distinct recursive calls that lack depth checks: Line 786: Recursively decodes components of SEQUENCE or SET types. Line 907: Recursively decodes elements of SEQUENCE OF or SET OF types.

  3. _decodeComponentsSchemaless (Line 661): for component in decodeFun(substrate, **options): This method handles decoding when no schema is provided.

In all three cases, decodeFun is invoked without passing a depth parameter or checking against a global MAX_ASN1_NESTING limit.

PoC

python
import sys
from pyasn1.codec.ber import decoder

sys.setrecursionlimit(100000)

print("[*] Generating Recursion Bomb Payload...")
depth = 50_000
chunk = b'\x30\x80' 
payload = chunk * depth

print(f"[*] Payload size: {len(payload) / 1024:.2f} KB")
print("[*] Triggering Decoder...")

try:
    decoder.decode(payload)
except RecursionError:
    print("[!] Crashed: Recursion Limit Hit")
except MemoryError:
    print("[!] Crashed: Out of Memory")
except Exception as e:
    print(f"[!] Crashed: {e}")
[*] Payload size: 9.77 KB
[*] Triggering Decoder...
[!] Crashed: Recursion Limit Hit

Impact

  • This is an unhandled runtime exception that typically terminates the worker process or thread handling the request. This allows a remote attacker to trivially kill service workers with a small payload (<100KB), resulting in a Denial of Service. Furthermore, in environments where recursion limits are increased, this leads to server-wide memory exhaustion.
  • Service Crash: Any service using pyasn1 to parse untrusted ASN.1 data (e.g., LDAP, SNMP, Kerberos, X.509 parsers) can be crashed remotely.
  • Resource Exhaustion: The attack consumes RAM linearly with the nesting depth. A small payload (<200KB) can consume hundreds of megabytes of RAM or exhaust the stack.

Credits

Vulnerability discovered by Kevin Tu of TMIR at ByteDance.

Software afectado

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PlataformaPaqueteVersiones vulnerablesCorregida en
Python (PyPI) pyasn1 < 0.6.3 0.6.3

Referencias

Puntuación CVSS 7.5/10
Vector CVSS
CVSS:3.0/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Debilidad (CWE)
CWE-674, CWE-835
Plataforma
Python (PyPI)
Paquete
pyasn1
Publicada
17.03.2026
Actualizado
11.10.2026 15:17

Fuentes y licencias

OSV / GitHub Advisory Database - PYSEC-2026-2263 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-jr27-m4p2-rc6r - CC BY 4.0

Historial de vulnerabilidades: pyasn1

Todas las vulnerabilidades publicadas de este paquete (Python (PyPI)), de la más reciente a la más antigua. Muestra si es un fallo aislado o un paquete que hay que vigilar.

Entradas en total
5
Primera → última
01.2026 07.2026
Sin corrección
0
Explotada en ataques
0

Distribución por gravedad

  • Alta 5

Divulgaciones por año

  1. pyasn1: Uncontrolled resource consumption when converting decoded REAL values
    Alta · 7.5 CVE-2026-59886 < 0.6.4 0.6.4
  2. pyasn1 BER/CER/DER decoder denial of service via unbounded long-form tag IDs
    Alta · 7.5 CVE-2026-59884 < 0.6.4 0.6.4
  3. pyasn1: Quadratic complexity in OBJECT IDENTIFIER and RELATIVE-OID processing allows denial of service
    Alta · 7.5 CVE-2026-59885 < 0.6.4 0.6.4
  4. Denial of Service in pyasn1 via Unbounded Recursion esta entrada
    Alta · 7.5 CVE-2026-30922 < 0.6.3 0.6.3
  5. pyasn1 has a DoS vulnerability in decoder
    Alta · 7.5 CVE-2026-23490 >= 0.6.1, < 0.6.2 0.6.2

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