CVE-2021-4437 is a medium-severity (CVSS 6.5) CWE-1333 vulnerability in @lambda-middleware/json-deserializer. A fix is available for @lambda-middleware/json-deserializer — see the affected versions and patch details below.
lambda-middleware Inefficient Regular Expression Complexity vulnerability
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
Exploitation and automatability from CISA’s SSVC triage for CVE-2021-4437.
EPSS Exploitation Probability
Probability of exploitation in the next 30 days, from FIRST.org EPSS.
How urgent is this, really
Where this sits among everything scored
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
@lambda-middleware/json-deserializernpmDescription
A vulnerability, which was classified as problematic, has been found in dbartholomae lambda-middleware frameguard up to 1.0.4. Affected by this issue is some unknown functionality of the file packages/json-deserializer/src/JsonDeserializer.ts of the component JSON Mime-Type Handler. The manipulation leads to inefficient regular expression complexity. Upgrading to version 1.1.0 is able to address this issue. The patch is identified as f689404d830cbc1edd6a1018d3334ff5f44dc6a6. It is recommended to upgrade the affected component. VDB-253406 is the identifier assigned to this vulnerability.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | @lambda-middleware/json-deserializer | all versions | 1.1.0npm install @lambda-middleware/json-deserializer@1.1.0 |
Affected Products
lambda-middlewaredbartholomaeDetection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @lambda-middleware/json-deserializer, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @lambda-middleware/json-deserializer to 1.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-4437 is resolved across your whole dependency graph.
Workarounds
Do not deserialise data from untrusted sources: where the format allows it, restrict deserialisation to an explicit allowlist of expected types, and prefer a data-only format (JSON, Protobuf) over one that can reconstruct arbitrary objects until you can upgrade.
Frequently Asked Questions
Is CVE-2021-4437 in your dependencies?
Find it across npm, including transitive dependencies.