CVE-2019-14859

Python-ecdsa < 0.13.3 accepted malformed signatures, enabling false transactions

• CVSS 9.1
• CWE-347 Improper Verification of Cryptographic Signature
• Input Validation and Sanitization
• Remote

A flaw was found in all python-ecdsa versions before 0.13.3, where it did not correctly verify whether signatures used DER encoding. Without this verification, a malformed signature could be accepted, making the signature malleable. Without proper verification, an attacker could use a malleable signature to create false transactions.

CVE

CVE-2019-14859

CWE

CWE-347 Improper Verification of Cryptographic Signature

CVSS base score

9.1

Published

2020-01-02

OWASP

A08 Software and Data Integrity Failures

Orthogonal defect classification

Checking

Code defect classification

Incorrect Check

Category

Input Validation and Sanitization

Subcategory

Insecure Parsing or Deserialization

Accessibility scope

Remote

Impact

Unauthorized Access

Fixed by upgrading

Yes

Solution

Update python-ecdsa to version 0.13.3 or higher.

from ecdsa import SigningKey, VerifyingKey, BadSignatureError

def verify_signature(signature, message, public_key):
    try:
        if public_key.verify(signature, message):
            print("Signature is valid.")
        else:
            raise BadSignatureError("Signature verification failed.")
    except BadSignatureError:
        print("Signature verification failed.")

private_key = SigningKey.generate()
public_key = private_key.get_verifying_key()
message = b"Sample message"
signature = private_key.sign(message)

verify_signature(signature, message, public_key)

from ecdsa import SigningKey, VerifyingKey, BadSignatureError
from ecdsa.util import string_to_number, number_to_string
from ecdsa.der import der_decode

def verify_signature(der_signature, message, public_key):
    try:
        r, s = der_decode(der_signature)
        
        if not public_key.verify((r, s), message):
            raise BadSignatureError("Signature verification failed.")
    except Exception as e:
        raise BadSignatureError("Invalid signature format or verification failed.") from e

private_key = SigningKey.generate()
public_key = private_key.get_verifying_key()
message = b"Sample message"
signature = private_key.sign(message)

try:
    verify_signature(signature, message, public_key)
    print("Signature is valid.")
except BadSignatureError as e:
    print(f"Signature verification failed: {e}")

@misc{vaitp:cve201914859,
  title        = {{Python-ecdsa < 0.13.3 accepted malformed signatures, enabling false transactions}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2020},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2019-14859},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2019-14859/}}
}