CVE-2020-26263 : Security Advisory and Response

tlslite-ng is an open-source Python library that implements SSL and TLS cryptographic protocols. Versions before 0.7.6 and 0.8.0-alpha39 are affected by an RSA vulnerability due to data-dependent decryption and padding checks in RSA PKCS#1 v1.5 decryption.

Understanding CVE-2020-26263

What is CVE-2020-26263?

In tlslite-ng versions prior to 0.7.6 and 0.8.0-alpha39, the RSA decryption code leaks information about decrypted ciphertext, making TLS servers with RSA key exchange and applications using RSA decryption API vulnerable.

The Impact of CVE-2020-26263

The vulnerability has a CVSS base score of 7.5 (High severity) with low attack complexity and network attack vector. It affects integrity but not confidentiality or availability.

Technical Details of CVE-2020-26263

Vulnerability Description

Data-dependent decryption and padding checks in RSA PKCS#1 v1.5 decryption
Leaks information about decrypted ciphertext
Vulnerable TLS servers and applications using RSA decryption API

Affected Systems and Versions

Product: tlslite-ng
Vendor: tlsfuzzer
Versions: < 0.7.6, >= 0.8.0-alpha1, < 0.8.0-alpha39

Exploitation Mechanism

Code aborts if plaintext doesn't start with 0x00, 0x02
Vulnerable to side-channel attacks

Mitigation and Prevention

Immediate Steps to Take

Update tlslite-ng to versions 0.7.6 or 0.8.0-alpha39
Consider using alternative TLS implementations for side-channel resistance

Long-Term Security Practices

Regularly monitor for security advisories and updates
Implement secure coding practices to prevent similar vulnerabilities

Patching and Updates

Apply patches provided in versions 0.7.6 and 0.8.0-alpha39