CVE-2021-23339Network attack vector

Request smuggling via multiple Transfer-Encoding acceptance

Published Feb 17, 2021 · Updated Sep 16, 2024

HTTP request smuggling in Lightbend Akka HTTP before 10.1.14 and 10.2.0 through 10.2.3 allows remote attackers to bypass proxy inspection. The HTTP message parser accepts multiple Transfer-Encoding values, ignores that header, and uses Content-Length to delimit and forward the request unchanged. Exploitation requires an application acting as a reverse proxy and a backend that parses the malformed request as two messages, allowing the second to reach the backend without proxy inspection.

CVSS severity6.5
Medium
EPSS probability0.71%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

HTTP request smuggling in Lightbend Akka HTTP before 10.1.14 and 10.2.0 through 10.2.3 allows remote attackers to bypass proxy inspection. The HTTP message parser accepts multiple Transfer-Encoding values, ignores that header, and uses Content-Length to delimit and forward the request unchanged. Exploitation requires an application acting as a reverse proxy and a backend that parses the malformed request as two messages, allowing the second to reach the backend without proxy inspection.

The record

CVE
CVE-2021-23339
Published
Feb 17, 2021
Updated
Sep 16, 2024
Vendor
Lightbend
Product
Akka HTTP
Classifications
CWE-444, T1190
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 17, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 16, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=all versions before 10.1.14 and from 10.2.0 to 10.2.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Lightbend · Akka HTTPversion=all versions before 10.1.14 and from 10.2.0 to 10.2.4

Published CVSS scores

6.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:L/I:L/A:N
5.0SnykCVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:L

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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-444Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')
T1190Exploit Public-Facing Application

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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