CVE-2020-11077 [Medium] | Security Vulnerability in Puma

In Puma (RubyGem) before 4.3.5 and 3.12.6, a client could smuggle a request through a proxy, causing the proxy to send a response back to another unknown client. If the proxy uses persistent connections and the client adds another request in via HTTP pipelining, the proxy may mistake it as the first request's body. Puma, however, would see it as two requests, and when processing the second request, send back a response that the proxy does not expect. If the proxy has reused the persistent connection to Puma to send another request for a different client, the second response from the first client will be sent to the second client. This is a similar but different vulnerability from CVE-2020-11076. The problem has been fixed in Puma 3.12.6 and Puma 4.3.5.

#	Date	Old EPSS score	New EPSS score	Delta (New - Old)
1	2026-03-04	0.49%	0.82%	+0.33%
2	2026-03-01	0.82%	0.49%	-0.33%
3	2026-02-04	—	0.82%	—

Date

Old EPSS score

New EPSS score

Delta (New - Old)

2026-03-04

0.49%

0.82%

+0.33%

2026-03-01

0.82%

0.49%

-0.33%

2026-02-04

—

0.82%

—

Base score	Version	Severity	Vector	Exploitability	Impact	Score source
6.8	3.1	MEDIUM	`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/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:H) Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work. 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:C) Breaking this can reach past the original component and bite other resources—bigger blast radius. 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.	2.2	4.0	[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]

Base score

Version

Severity

Vector

Exploitability

Impact

Score source

6.8

3.1

MEDIUM

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/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:H): Even with access, the exploit needs extra luck, timing, or a fussy environment to actually work.
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:C): Breaking this can reach past the original component and bite other resources—bigger blast radius.
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.

2.2

4.0

[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]

OS Trackers for CVE-2020-11077

vendor	priority	summary	link
`debian`	not yet assigned	CVE-2020-11077 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-2020-11077
`redhat`	medium	—	https://access.redhat.com/security/cve/CVE-2020-11077
`suse`	medium	—	https://www.suse.com/security/cve/CVE-2020-11077/
`ubuntu`	medium	CVE-2020-11077 medium priority: Ubuntu including 1 source packages (puma), 16 status rows across 16 suites (bionic, eoan, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, trusty, upstream, xenial): not-affected 7, ignored 4, DNE 3, needs-triage 1, released 1.	https://ubuntu.com/security/CVE-2020-11077

Affected software / configurations for CVE-2020-11077

Vendor	Product	Version	Raw CPE
puma	puma	>= 3.0.0, < 3.12.6	cpe:2.3:a:puma:puma::::::ruby::*
puma	puma	>= 4.0.0, < 4.3.5	cpe:2.3:a:puma:puma::::::ruby::*
fedoraproject	fedora	33	cpe:2.3:o:fedoraproject:fedora:33:::::::*
debian	debian_linux	9.0	cpe:2.3:o:debian:debian_linux:9.0:::::::*
opensuse	leap	15.1	cpe:2.3:o:opensuse:leap:15.1:::::::*
opensuse	leap	15.2	cpe:2.3:o:opensuse:leap:15.2:::::::*

References for CVE-2020-11077

URL	Tags
http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00034.html	Mailing List Third Party Advisory
http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00038.html	Mailing List Third Party Advisory
https://github.com/puma/puma/blob/master/History.md#434435-and-31253126--2020-05-22	Release Notes
https://github.com/puma/puma/security/advisories/GHSA-w64w-qqph-5gxm	Vendor Advisory
https://lists.debian.org/debian-lts-announce/2020/10/msg00009.html	Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/SKIY5H67GJIGJL6SMFWFLUQQQR3EMVPR/

URL

CVE-2020-11077 | HTTP Smuggling via Transfer-Encoding Header in Puma

Exploit prediction scoring system (EPSS) score for CVE-2020-11077

Common vulnerability scoring system (CVSS) metrics for CVE-2020-11077

Weakness enumeration for CVE-2020-11077

GitHub Security Advisory for CVE-2020-11077

OS Trackers for CVE-2020-11077

Affected software / configurations for CVE-2020-11077

References for CVE-2020-11077