CVE-2020-26521 is a high-severity (CVSS 7.5) NULL Pointer Dereference vulnerability in github.com/nats-io/jwt. A fix is available for github.com/nats-io/jwt — see the affected versions and patch details below.
Nil dereference in NATS JWT, DoS of nats-server
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
CVE-2020-26521 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.
Where this sits among everything scored
Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.
Real-World Exposure
github.com/nats-io/jwtReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.
Description
Problem Description
The NATS account system has an Operator trusted by the servers, which signs Accounts, and each Account can then create and sign Users within their account. The Operator should be able to safely issue Accounts to other entities which it does not fully trust.
A malicious Account could create and sign a User JWT with a state not created by the normal tooling, such that decoding by the NATS JWT library (written in Go) would attempt a nil dereference, aborting execution.
The NATS Server is known to be impacted by this.
Affected versions
JWT library
- all versions prior to 1.1.0
NATS Server
- Version 2 prior to 2.1.9
Impact
JWT library
- Programs would nil dereference and panic, aborting execution by default.
NATS server
- Denial of Service caused by process termination
Workaround
If your NATS servers do not trust any accounts which are managed by untrusted entities, then malformed User credentials are unlikely to be encountered.
Solution
Upgrade the JWT dependency in any application using it.
Upgrade the NATS server if using NATS Accounts.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/nats-io/jwt | all versions | 1.1.0go get github.com/nats-io/jwt@v1.1.0 |
Affected Products
fedorafedoraprojectnats-serverlinuxfoundationDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/nats-io/jwt, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/nats-io/jwt to 1.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-26521 is resolved across your whole dependency graph.
Workarounds
Cap what an attacker can consume: apply request size, rate and timeout limits in front of the affected component, and run it with memory and CPU limits so exhaustion degrades one worker rather than the whole service.
Frequently Asked Questions
Is CVE-2020-26521 in your dependencies?
Find it across Go, including transitive dependencies.