PatchTheCVECVE-2020-12403: A flaw was found in the way CHACHA20-POLY1305 was implemented in NSS in versions before 3.55
CVE-2020-12403 is a critical-severity flaw (CVSS 9.1) affecting nss. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.
A flaw was found in the way CHACHA20-POLY1305 was implemented in NSS in versions before 3.55. When using multi-part Chacha20, it could cause out-of-bounds reads. This issue was fixed by explicitly disabling multi-part ChaCha20 (which was not functioning correctly) and strictly enforcing tag length. The highest threat from this vulnerability is to confidentiality and system availability.
Why this priority
- Critical severity
- Remotely exploitable, no login, no user action
A fixed release or distro patch exists.
Risk facts
| CVSS score | 9.1 (Critical) |
| Attack vector | Network (remote) |
| Attack complexity | Low |
| Privileges needed | None |
| User interaction | None |
| EPSS exploit chance | 1.5% (top 25%) |
| CISA exploited list | No |
| Weakness | CWE-125 |
| Published | 2021-05-27 |
| Last modified | 2026-06-17 |
Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score
Am I affected?
Timeline
- Published CVE record published
- Updated Record last modified
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.
Quick answers
How do I fix CVE-2020-12403?
nss: upgrade to 3.55. Exact commands for each OS are on this page.
Is CVE-2020-12403 being exploited?
CVE-2020-12403 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.
What does CVE-2020-12403 affect?
CVE-2020-12403 affects nss. Severity is critical (CVSS 9.1). A flaw was found in the way CHACHA20-POLY1305 was implemented in NSS in versions before 3.55. When using multi-part Chacha20, it could cause out-of-bounds reads. This issue was fixed by explicitly disabling multi-part ChaCha20 (which was no
How urgent is CVE-2020-12403?
Priority 56/100. Critical severity; Remotely exploitable, no login, no user action
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