RSA weakness in tslite-ng
Impact
The code that performs decryption and padding check in RSA PKCS#1 v1.5 decryption is data dependant. In particular, code in current (as of 0.8.0-alpha38) master https://github.com/tlsfuzzer/tlslite-ng/blob/0812ed60860fa61a6573b2c0e18771414958f46d/tlslite/utils/rsakey.py#L407-L441 and code in 0.7.5 branch https://github.com/tlsfuzzer/tlslite-ng/blob/acdde3161124d6ae37c506b3476aea9996d12e97/tlslite/utils/rsakey.py#L394-L425 has multiple ways in which it leaks information (for one, it aborts as soon as the plaintext doesn't start with 0x00, 0x02) about the decrypted ciphertext (both the bit length of the decrypted message as well as where the first unexpected byte lays).
All TLS servers that enable RSA key exchange as well as applications that use the RSA decryption API directly are vulnerable.
All previous versions of tlslite-ng are vulnerable.
Patches
The patches to fix it are proposed in https://github.com/tlsfuzzer/tlslite-ng/pull/438 https://github.com/tlsfuzzer/tlslite-ng/pull/439
Note: the patches depend on Python processing the individual bytes in side-channel free manner, this is known to not be the case: https://securitypitfalls.wordpress.com/2018/08/03/constant-time-compare-in-python/ As such, users that require side-channel resistance are recommended to use different TLS implementations, as stated in the security policy of tlslite-ng.
Workarounds
There is no way to workaround this issue.
References
https://securitypitfalls.wordpress.com/2018/08/03/constant-time-compare-in-python/
For more information
If you have any questions or comments about this advisory please open an issue in tlslite-ng.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Python (PyPI) | tlslite-ng | < 0.7.6 | 0.7.6 |
References
- https://pypi.org/project/tlslite-ng/
- https://github.com/tlsfuzzer/tlslite-ng/pull/439
- https://github.com/tlsfuzzer/tlslite-ng/pull/438
- https://github.com/tlsfuzzer/tlslite-ng/commit/c28d6d387bba59d8bd5cb3ba15edc42edf54b368
- https://github.com/tlsfuzzer/tlslite-ng/security/advisories/GHSA-wvcv-832q-fjg7
- https://securitypitfalls.wordpress.com/2018/08/03/constant-time-compare-in-python/
- https://nvd.nist.gov/vuln/detail/CVE-2020-26263
- https://github.com/pypa/advisory-database/tree/main/vulns/tlslite-ng/PYSEC-2020-143.yaml
- https://github.com/tlsfuzzer/tlslite-ng
- https://pypi.org/project/tlslite-ng
- https://securitypitfalls.wordpress.com/2018/08/03/constant-time-compare-in-python
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N- Weakness (CWE)
CWE-326- Platform
- Python (PyPI)
- Package
tlslite-ng- Disclosed
- 21.12.2020
- Updated
- 11.10.2026 13:19
Sources and licenses
OSV / GitHub Advisory Database - PYSEC-2020-143 - PyPA Advisory Database; CC BY 4.0
OSV / GitHub Advisory Database - GHSA-wvcv-832q-fjg7 - CC BY 4.0
Vulnerability history: tlslite-ng
All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 2
- First → latest
- 04.2018 12.2020
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- High 2
Disclosures per year
- 2018: 1
- 2019: 0
- 2020: 1
- 2021: 0
- 2022: 0
- 2023: 0
- 2024: 0
- 2025: 0
- 2026: 0
-
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