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MEDIUM severity

CVE-2021-23339 — akka-http-core

MEDIUMFix: akka/akka-http#3754

CVE-2021-23339 is a medium-severity (CVSS 6.5) CWE-444 vulnerability in com.typesafe.akka:akka-http-core. A fix is available for com.typesafe.akka:akka-http-core — see the affected versions and patch details below.

HTTP Request Smuggling in akka-http-core

Also known asGHSA-2w7w-2j92-44hxSNYK-JAVA-COMTYPESAFEAKKA-1075043
Published
Updated
Affected
2 pkgs
Patched
2 / 2
Exploits
None indexed
Exploitation data as of Oct 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
0.7%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs52th percentile — riskier than 52% of all scored CVEsHighest risk
0.00%0.40%0.81%1.21%0.2%0.7%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-23339 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

2 pkgs affected
☕com.typesafe.akka:akka-http-core☕com.typesafe.akka:akka-http-core

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

A vulnerable Akka HTTP server will accept a malformed message and hand it over to the user. If the user application proxies this message to another server unchanged and that server also accepts that message but interprets it as two HTTP messages, the second message has reached the second server without having been inspected by the proxy.

Affected Packages

2 total 2 fixed
EcosystemPackageVulnerable rangeFix
☕Mavencom.typesafe.akka:akka-http-core≥ 10.2.0&&< 10.2.410.2.4com.typesafe.akka:akka-http-core:10.2.4
☕Mavencom.typesafe.akka:akka-http-coreall versions10.1.14com.typesafe.akka:akka-http-core:10.1.14

Affected Products

1 product · 2 configurations
Application
akka-httplightbend
≥ 10.2.0 && < 10.2.4
range

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for com.typesafe.akka:akka-http-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update com.typesafe.akka:akka-http-core to 10.2.4 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-23339 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

A vulnerable Akka HTTP server will accept a malformed message and hand it over to the user. If the user application proxies this message to another server unchanged and that server also accepts that message but interprets it as two HTTP messages, the second message has reached the second server without having been inspected by the proxy.
O3 Security · Impact-Aware SCA

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CVE-2021-23339: akka-http-core — Fixed in 10.2.4