CVE-2021-4437

Service interruption via unbounded JSON Content-Type matching

Published Feb 12, 2024 · Updated May 6, 2025

Inefficient regular expression handling in lambda-middleware frameguard 1.0.4 and earlier allows attackers to interrupt service. The JSON MIME-type handler in JsonDeserializer.ts applies an unbounded regular expression to the Content-Type header, allowing crafted nonmatching values to trigger excessive backtracking. Reachability requires an application to pass an attacker-controlled HTTP event through this middleware; repeated expensive matches consume CPU and delay or block Lambda request handling.

CVSS severity6.5
Medium
EPSS probability0.45%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Inefficient regular expression handling in lambda-middleware frameguard 1.0.4 and earlier allows attackers to interrupt service. The JSON MIME-type handler in JsonDeserializer.ts applies an unbounded regular expression to the Content-Type header, allowing crafted nonmatching values to trigger excessive backtracking. Reachability requires an application to pass an attacker-controlled HTTP event through this middleware; repeated expensive matches consume CPU and delay or block Lambda request handling.

The record

CVE
CVE-2021-4437
Published
Feb 12, 2024
Updated
May 6, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-1333, T1499.002
Attack vector
adjacent
Privileges
authenticated

Timeline

How it unfolded

  1. Feb 12, 2024CVE publishedPublication date reported by the CVE source.
  2. May 6, 2025Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectoradjacent
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

3.5VulDBCVSS:3.1/AV:A/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L
2.7VulDBAV:A/AC:L/Au:S/C:N/I:N/A:P
6.5NIST NVDCVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

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No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectoradjacent
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-1333Inefficient Regular Expression Complexity
T1499.002Service Exhaustion

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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