CVE-2020-5247 | HTTP Response Splitting in Puma

In Puma (RubyGem) before 4.3.2 and before 3.12.3, if an application using Puma allows untrusted input in a response header, an attacker can use newline characters (i.e. `CR`, `LF` or`/r`, `/n`) to end the header and inject malicious content, such as additional headers or an entirely new response body. This vulnerability is known as HTTP Response Splitting. While not an attack in itself, response splitting is a vector for several other attacks, such as cross-site scripting (XSS). This is related to CVE-2019-16254, which fixed this vulnerability for the WEBrick Ruby web server. This has been fixed in versions 4.3.2 and 3.12.3 by checking all headers for line endings and rejecting headers with those characters.

Published: 2020-02-28 Last update: 2024-11-21 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2020-5247 is rated Moderate Risk (54.3/100): CVSS Medium severity, with medium exploitation likelihood (EPSS 2.09%). Review affected assets and schedule remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2020-5247

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-03-04 3.07% 2.09% -0.97%
2 2026-03-01 2.09% 3.07% +0.97%
3 2026-02-04 — 2.09% —

Full EPSS history (49 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2020-5247

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
6.5 3.1 MEDIUM
CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:L Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:R)
A real person has to do something—click, install, enable—otherwise it doesn’t land.
Scope (S:C)
Breaking this can reach past the original component and bite other resources—bigger blast radius.
Confidentiality (C:L)
Some sensitive info could get out, but not a total data dump.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:L)
Might cause slowdowns, glitches, or partial disruption—not a full brick.
 2.3 3.7 [email protected]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:N)
Service keeps running; no real outage angle.
 3.9 3.6 [email protected]
5.0 2.0 MEDIUM
AV:N/AC:L/Au:N/C:N/I:P/A:N Click to expand
Access vector (AV:N)
Can be exploited remotely over network reachability.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:N)
No confidentiality impact.
Integrity impact (I:P)
Partial integrity impact.
Availability impact (A:N)
No availability impact.
 10.0 2.9 [email protected]

Weakness enumeration for CVE-2020-5247

GitHub Security Advisory for CVE-2020-5247

GHSA-84j7-475p-hp8v · Severity: medium · Ecosystem: rubygems — HTTP Response Splitting in Puma

OS Trackers for CVE-2020-5247

vendor priority summary link
alpine high CVE-2020-5247: 1 source package rows (ruby); 20 state rows across 5 repos (3.19-main, 3.20-main, 3.21-main, 3.22-main, edge-main); fixed 0, open 20. https://security.alpinelinux.org/vuln/CVE-2020-5247
debian not yet assigned CVE-2020-5247 not yet assigned priority: Debian including 1 source packages (puma), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2020-5247
redhat medium — https://access.redhat.com/security/cve/CVE-2020-5247
suse medium CVE-2020-5247 severity moderate: SUSE including 294 source package names (ansible-2.2.3.0-12.2, ardana-ansible-8.0+git.1583432621.24fa60e-3.70.1, …), 500 product×package rows across 24 product lines (HPE Helion OpenStack 8, SUSE Linux Enterprise High Performance Computing 15-ESPOS, … (24 product lines)): Fixed 500. https://www.suse.com/security/cve/CVE-2020-5247/
ubuntu medium CVE-2020-5247 medium priority: Ubuntu including 1 source packages (puma), 6 status rows across 6 suites (bionic, eoan, focal, trusty, upstream, xenial): DNE 3, ignored 1, needs-triage 1, not-affected 1. https://ubuntu.com/security/CVE-2020-5247

Affected software / configurations for CVE-2020-5247

Vendor Product Version Raw CPE
puma puma <= 3.12.3 cpe:2.3:a:puma:puma:*:*:*:*:*:ruby:*:*
puma puma >= 4.0.0, <= 4.3.2 cpe:2.3:a:puma:puma:*:*:*:*:*:ruby:*:*
ruby-lang ruby <= 2.3.0 cpe:2.3:a:ruby-lang:ruby:*:*:*:*:*:*:*:*
ruby-lang ruby >= 2.4.0, <= 2.4.7 cpe:2.3:a:ruby-lang:ruby:*:*:*:*:*:*:*:*
ruby-lang ruby >= 2.5.0, <= 2.5.6 cpe:2.3:a:ruby-lang:ruby:*:*:*:*:*:*:*:*
ruby-lang ruby >= 2.6.0, <= 2.6.4 cpe:2.3:a:ruby-lang:ruby:*:*:*:*:*:*:*:*
ruby-lang ruby 2.7.0 cpe:2.3:a:ruby-lang:ruby:2.7.0:preview1:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
fedoraproject fedora 30 cpe:2.3:o:fedoraproject:fedora:30:*:*:*:*:*:*:*
fedoraproject fedora 31 cpe:2.3:o:fedoraproject:fedora:31:*:*:*:*:*:*:*
fedoraproject fedora 32 cpe:2.3:o:fedoraproject:fedora:32:*:*:*:*:*:*:*

References for CVE-2020-5247

URL Tags
https://github.com/puma/puma/security/advisories/GHSA-84j7-475p-hp8v Mitigation Third Party Advisory
https://lists.debian.org/debian-lts-announce/2022/05/msg00034.html Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/BMJ3CGZ3DLBJ5WUUKMI5ZFXFJQMXJZIK/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/DIHVO3CQMU7BZC7FCTSRJ33YDNS3GFPK/
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/NJ3LL5F5QADB6LM46GXZETREAKZMQNRD/
https://owasp.org/www-community/attacks/HTTP_Response_Splitting Third Party Advisory
https://www.ruby-lang.org/en/news/2019/10/01/http-response-splitting-in-webrick-cve-2019-16254 Vendor Advisory
cvelogic Threat Intelligence