CVE-2020-26263: RSA vulnerability in tslite-ng

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 1.5% probability of exploitation in the next 30 days
Public exploit or detection template: yes
Published: 2020-12-21 · updated 2026-06-17
Weakness: CWE-326

Description

tlslite-ng is an open source python library that implements SSL and TLS cryptographic protocols. In tlslite-ng before versions 0.7.6 and 0.8.0-alpha39, the code that performs decryption and padding check in RSA PKCS#1 v1.5 decryption is data dependant. In particular, the code has multiple ways in which it leaks information about the decrypted ciphertext. It aborts as soon as the plaintext doesn't start with 0x00, 0x02. All TLS servers that enable RSA key exchange as well as applications that use the RSA decryption API directly are vulnerable. This is patched in versions 0.7.6 and 0.8.0-alpha39. Note: the patches depend on Python processing the individual bytes in side-channel free manner, this is known to not the case (see reference). As such, users that require side-channel resistance are recommended to use different TLS implementations, as stated in the security policy of tlslite-ng.

Affected products

tlsfuzzer tlslite-ng: before 0.7.6; 0.8.0-alpha1, 0.8.0-alpha10, 0.8.0-alpha11, 0.8.0-alpha12, 0.8.0-alpha13, 0.8.0-alpha14 (+32) — fixed in 0.7.6

How to fix

Upgrade to a fixed version: tlsfuzzer tlslite-ng 0.7.6. Check the vendor advisory for mitigations.

References

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