CVE-2021-43572

Python Charmers Future 0.18.2 and earlier: Remote attacker causes denial of service via malicious Set-Cookie header

• CVSS 9.8
• CWE-347 Improper Verification of Cryptographic Signature
• Cryptographic
• Remote

The verify function in the Stark Bank Python ECDSA library (aka starkbank-escada or ecdsa-python) before 2.0.1 fails to check that the signature is non-zero, which allows attackers to forge signatures on arbitrary messages.

CVE

CVE-2021-43572

CWE

CWE-347 Improper Verification of Cryptographic Signature

CVSS base score

9.8

Published

2021-11-09

OWASP

A08 Software and Data Integrity Failures

Orthogonal defect classification

Checking

Code defect classification

Incorrect Check

Category

Cryptographic

Subcategory

Cryptographic Implementation Error

Accessibility scope

Remote

Impact

Arbitrary Code Execution

Fixed by upgrading

Yes

Solution

Update starkbank-ecdsa to version 2.0.1 or higher.

from ecdsa import VerifyingKey, BadSignatureError

def verify_signature(public_key: bytes, signature: bytes, message: bytes) -> bool:
    vk = VerifyingKey.from_string(public_key)
    try:
        return vk.verify(signature, message)
    except BadSignatureError:
        return False

from ecdsa import VerifyingKey, BadSignatureError

def verify_signature(public_key: bytes, signature: bytes, message: bytes) -> bool:
    vk = VerifyingKey.from_string(public_key)
    if all(x == 0 for x in signature):
        raise ValueError("Signature cannot be zero")
    try:
        return vk.verify(signature, message)
    except BadSignatureError:
        return False

@misc{vaitp:cve202143572,
  title        = {{Python Charmers Future 0.18.2 and earlier: Remote attacker causes denial of service via malicious Set-Cookie header}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2021},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2021-43572},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2021-43572/}}
}