CVE-2020-9840Network attack vector

Remote denial of service via decompression size mischeck

Published May 11, 2020 · Updated Aug 4, 2024

Denial of service in SwiftNIO Extras 1.3.0 through 1.4.0 allows remote attackers to interrupt clients or servers via HTTP bodies. The NIOHTTPRequestDecompressor and NIOHTTPResponseDecompressor .size limit compares the compressed byte count instead of the decompressed output size. Exposure requires an application to use either decompressor with the .size limit on attacker-controlled HTTP traffic; the reported effect is availability loss only.

CVSS severity7.5
High
EPSS probability1.02%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Denial of service in SwiftNIO Extras 1.3.0 through 1.4.0 allows remote attackers to interrupt clients or servers via HTTP bodies. The NIOHTTPRequestDecompressor and NIOHTTPResponseDecompressor .size limit compares the compressed byte count instead of the decompressed output size. Exposure requires an application to use either decompressor with the .size limit on attacker-controlled HTTP traffic; the reported effect is availability loss only.

The record

CVE
CVE-2020-9840
Published
May 11, 2020
Updated
Aug 4, 2024
Vendor
Swift Project
Product
SwiftNIO Extras
Classifications
T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. May 11, 2020CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

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

  1. Affected versionversion=next of SwiftNIO Extras 1.2.0 <unspecified
  2. Affected versionversion=unspecified <SwiftNIO Extras 1.4.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Swift Project · SwiftNIO Extrasversion=next of SwiftNIO Extras 1.2.0 <unspecified; version=unspecified <SwiftNIO Extras 1.4.1

Published CVSS scores

7.5NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
5.0NIST NVDAV:N/AC:L/Au:N/C:N/I:N/A:P

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.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

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

T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo