What is CVE-2021-23339?

CVE-2021-23339 is a medium-severity vulnerability in Lightbend Akka-Http, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 5.0/10. Published 2021-02-17.

How severe is CVE-2021-23339?

Medium severity. CVSS v3 base score is 5.0 out of 10.

Vulnerability in Lightbend Akka-Http

CVE-2021-23339

This affects all versions before 10.1.14 and from 10.2.0 to 10.2.4 of package com.typesafe.akka:akka-http-core. It allows multiple Transfer-Encoding headers.

Vulnerability class: HTTP Request Smuggling

Published 2021-02-17 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 5.0 (Medium). Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:L.

EPSS exploit prediction

EPSS: 0.007 (51.8th percentile), scored 2026-10-01.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 52nd percentile — 51.8% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.007 · newest: 0.007 · change: 0.000

Affected products

Lightbend Akka-Http
Com.typesafe.akka:akka-Http-Core — versions all versions before 10.1.14 and from 10.2.0 to 10.2.4

Weakness classification (CWE)

CWE-444 — Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling)

References

snyk.io/vuln/SNYK-JAVA-COMTYPESAFEAKKA-1075043 (Broken Link)
github.com/akka/akka-http/pull/3754#issuecomment-779265201 (Broken Link)

Frequently asked questions

What is CVE-2021-23339?: CVE-2021-23339 is a medium-severity vulnerability in Lightbend Akka-Http, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 5.0/10. Published 2021-02-17.
How severe is CVE-2021-23339?: Medium severity. CVSS v3 base score is 5.0 out of 10.