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CVE-2020-25658

Python-rsa vulnerable to Bleichenbacher timing attack: RSA decryption can be partially exploited by attackers

  • CVSS 5.9
  • CWE-385
  • Cryptographic
  • Remote

It was found that python-rsa is vulnerable to Bleichenbacher timing attacks. An attacker can use this flaw via the RSA decryption API to decrypt parts of the cipher text encrypted with RSA.

CVE
CVE-2020-25658
CWE
CWE-385
CVSS base score
5.9
Published
2020-11-12
OWASP
A02 Cryptographic Failures
Orthogonal defect classification
Timing/Serialization
Code defect classification
Timing Issues
Category
Cryptographic
Subcategory
Cryptographic Implementation Error
Accessibility scope
Remote
Impact
Information Disclosure
Fixed by upgrading
Yes

Solution

Update python-rsa to a version that mitigates Bleichenbacher timing attacks.

Vulnerable code sample

import rsa

def decrypt(private_key, ciphertext):
    decrypted_message = rsa.decrypt(ciphertext, private_key)
    return decrypted_message

if __name__ == "__main__":
    (public_key, private_key) = rsa.newkeys(512)
    message = b'Hello, RSA!'
    ciphertext = rsa.encrypt(message, public_key)

    decrypted_message = decrypt(private_key, ciphertext)
    print(decrypted_message)

Patched code sample

from cryptography.hazmat.primitives.asymmetric import rsa, padding
from cryptography.hazmat.primitives import serialization, hashes

def decrypt(private_key, ciphertext):
    try:
        return private_key.decrypt(
            ciphertext,
            padding.OAEP(
                mgf=padding.MGF1(algorithm=hashes.SHA256()),
                algorithm=hashes.SHA256(),
                label=None
            )
        )
    except Exception:
        return b"Decryption failed"

if __name__ == "__main__":
    private_key = rsa.generate_private_key(public_exponent=65537, key_size=2048)
    public_key = private_key.public_key()

    message = b'Hello, RSA!'
    ciphertext = public_key.encrypt(
        message,
        padding.OAEP(
            mgf=padding.MGF1(algorithm=hashes.SHA256()),
            algorithm=hashes.SHA256(),
            label=None
        )
    )

    decrypted_message = decrypt(private_key, ciphertext)
    print(decrypted_message)

Cite this entry

@misc{vaitp:cve202025658,
  title        = {{Python-rsa vulnerable to Bleichenbacher timing attack: RSA decryption can be partially exploited by attackers}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2020},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2020-25658},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2020-25658/}}
}
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