CVE-2020-5249 [Medium]

In Puma (RubyGem) before 4.3.3 and 3.12.4, if an application using Puma allows untrusted input in an early-hints header, an attacker can use a carriage return character 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-2020-5247, which fixed this vulnerability but only for regular responses. This has been fixed in 4.3.3 and 3.12.4.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2025-05-04	0.40%	0.51%	+0.12%
2	2025-05-01	0.51%	0.40%	-0.12%
3	2025-03-30	—	0.51%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2025-05-04

0.40%

0.51%

+0.12%

2025-05-01

0.51%

0.40%

-0.12%

2025-03-30

—

0.51%

—

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]
6.5	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:L/PR:L/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:L) A normal user session is enough; they don’t have to be admin. 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.	2.8	3.6	[email protected]
4.0	2.0	MEDIUM	`AV:N/AC:L/Au:S/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:S) A single authentication is required. Confidentiality impact (C:N) No confidentiality impact. Integrity impact (I:P) Partial integrity impact. Availability impact (A:N) No availability impact.	8.0	2.9	[email protected]

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]

6.5

3.1

MEDIUM

CVSS:3.1/AV:N/AC:L/PR:L/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:L): A normal user session is enough; they don’t have to be admin.
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.

2.8

3.6

[email protected]

4.0

2.0

MEDIUM

AV:N/AC:L/Au:S/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:S): A single authentication is required.
Confidentiality impact (C:N): No confidentiality impact.
Integrity impact (I:P): Partial integrity impact.
Availability impact (A:N): No availability impact.

8.0

2.9

[email protected]

OS Trackers for CVE-2020-5249

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2020-5249 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-2020-5249
`redhat`	medium	—	https://access.redhat.com/security/cve/CVE-2020-5249
`ubuntu`	medium	CVE-2020-5249 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-2020-5249

Affected software / configurations for CVE-2020-5249

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::*

References for CVE-2020-5249

URL	Tags
https://github.com/puma/puma/commit/c22712fc93284a45a93f9ad7023888f3a65524f3	Patch Third Party Advisory
https://github.com/puma/puma/security/advisories/GHSA-33vf-4xgg-9r58	Third Party Advisory
https://github.com/puma/puma/security/advisories/GHSA-84j7-475p-hp8v	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

URL

CVE-2020-5249 | HTTP Response Splitting (Early Hints) in Puma

Exploit prediction scoring system (EPSS) score for CVE-2020-5249

Common vulnerability scoring system (CVSS) metrics for CVE-2020-5249

Weakness enumeration for CVE-2020-5249

GitHub Security Advisory for CVE-2020-5249

OS Trackers for CVE-2020-5249

Affected software / configurations for CVE-2020-5249

References for CVE-2020-5249