CVE-2020-26263

Name: CVE-2020-26263
Creator: NVD / NIST
Published: 2020-12-21T17:15:12.637+00:00
License: https://creativecommons.org/publicdomain/zero/1.0/

HIGHCVSS 7.5/10EPSS 1.28%

Last modified Jun 17, 2026

CVE-2020-26263 is a high-severity vulnerability rated 7.5/10 on the CVSS scale. 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. EPSS estimates a 1.28% chance of exploitation in the next 30 days.

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.

Metrics

CVSS 3.1

7.5/10

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:N

EPSS Probability

1.28%

66.3th percentile

Probability of exploitation in the next 30 days. Learn more

Weakness Enumeration

CWE-326

Affected Software

Vendor	Product	Versions	Update
Tlslite-Ng Project	Tlslite-Ng	< 0.7.6	—
Tlslite-Ng Project	Tlslite-Ng	0.8.0	Alpha1

References

https://github.com/tlsfuzzer/tlslite-ng/commit/c28d6d387bba59d8bd5cb3ba15edc42edf54b368Patch, Third Party Advisory
https://github.com/tlsfuzzer/tlslite-ng/pull/438Patch, Third Party Advisory
https://github.com/tlsfuzzer/tlslite-ng/pull/439Patch, Third Party Advisory
https://github.com/tlsfuzzer/tlslite-ng/security/advisories/GHSA-wvcv-832q-fjg7Exploit, Patch, Third Party Advisory
https://pypi.org/project/tlslite-ng/Product, Third Party Advisory
https://securitypitfalls.wordpress.com/2018/08/03/constant-time-compare-in-python/Exploit, Third Party Advisory
https://github.com/tlsfuzzer/tlslite-ng/commit/c28d6d387bba59d8bd5cb3ba15edc42edf54b368Patch, Third Party Advisory
https://github.com/tlsfuzzer/tlslite-ng/pull/438Patch, Third Party Advisory
https://github.com/tlsfuzzer/tlslite-ng/pull/439Patch, Third Party Advisory
https://github.com/tlsfuzzer/tlslite-ng/security/advisories/GHSA-wvcv-832q-fjg7Exploit, Patch, Third Party Advisory
https://pypi.org/project/tlslite-ng/Product, Third Party Advisory
https://securitypitfalls.wordpress.com/2018/08/03/constant-time-compare-in-python/Exploit, Third Party Advisory

Timeline

Published: Dec 21, 2020
Last Modified: Jun 17, 2026
Status: Modified

Frequently Asked Questions

What is CVE-2020-26263?

How severe is CVE-2020-26263?

CVE-2020-26263 has a CVSS score of 7.5/10 (HIGH severity). The EPSS model estimates a 1.28% probability of exploitation in the next 30 days.

How do I fix CVE-2020-26263?

Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.

Are you affected by CVE-2020-26263?

Run a free Strix scan to check your systems for this vulnerability.

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Source: NVD / NIST