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CVE-2020-5249· NVD / CVE Program· CNA GitHub_M

HTTP Response Splitting (Early Hints) in Puma

Noroxi assessment

Priority: Monitor (action score 26/100)

  • The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
  • Probability of exploitation in the next 30 days: 1.5% (FIRST EPSS); higher than 74.9% of all records.
  • Anyone who can reach it over the internet can trigger it. A low-privileged account is enough. No user action is required. No special conditions are required; it is repeatable.
  • Fixed version: puma Puma 4.3.3.

Low priority for now. Upgrade to puma Puma 4.3.3 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 6.5 · v3.1—No exploit Fix available
Published
Mar 2, 2020
Updated
Jun 16, 2026
EPSS
1.5% · 74th percentile

Report tools

JSON

Action score

26

Monitor

Low priority for now.

CVSS
26 / 40 · 6.5 / 10
CISA KEV
0 / 30 · Not listed
EPSS
0 / 30 · 1.5%

CNA vs NVD score

NVD
6.5
CNA · GitHub_M
6.5
agree

CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:L

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

  • Not in KEV: no record of exploitation in the wild
  • No public exploit code on record
  • A fix has been released

If any of these changes, we'll tell you in the daily email digest.

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When you'll hear from us

  • when it enters CISA KEV
  • when exploit code appears or is weaponized
  • when the record is updated (including a fixed version)
  • when EPSS rises 20 points in a week

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Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

VendorProduct
pumapuma

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

  • puma pumaup to 3.12.3 inclusive
  • puma puma4.0.0 and later · up to 4.3.2 inclusive

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

  • puma Puma

    • < 3.12.4, >= 4.0.0, < 4.3.3affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

EcosystemPackageAffected rangeFix
Debian:12pumabefore 3.12.4-13.12.4-1
openSUSE:Leap 15.1rmt-serverbefore 2.6.5-lp151.2.18.22.6.5-lp151.2.18.2
openSUSE:Leap 15.2rmt-serverbefore 2.6.5-lp152.2.3.12.6.5-lp152.2.3.1
RubyGemspumafrom 4.0.0 · before 4.3.34.3.3
RubyGemspumabefore 3.12.43.12.4
SUSE:Linux Enterprise High Performance Computing 15-ESPOSrmt-serverbefore 2.6.5-3.34.12.6.5-3.34.1
SUSE:Linux Enterprise Module for Public Cloud 15 SP1rmt-serverbefore 2.6.5-3.18.12.6.5-3.18.1
SUSE:Linux Enterprise Module for Public Cloud 15 SP2rmt-serverbefore 2.6.5-3.3.12.6.5-3.3.1

Same product

puma: all records

Other highest-scoring records for the same primary product.

Published before and after this one in the same product

  • ← EarlierCVE-2019-16770Dec 5, 2019 · Potential DOS attack in Puma
    31Monitor
  • ← EarlierCVE-2020-5247Feb 28, 2020 · HTTP Response Splitting in Puma
    31Monitor
  • Later →CVE-2020-11076May 22, 2020 · HTTP Smuggling via Transfer-Encoding Header in Puma
    31Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / packageFixed versionSource
puma Puma4.3.3 · descriptionVendor (CNA)
debian:puma3.12.4-1 · Debian:12Package registry (OSV)
opensuse:rmt-server2.6.5-lp152.2.3.1 · openSUSE:Leap 15.2Package registry (OSV)
RubyGems:puma3.12.4Package registry (OSV)
RubyGems:puma4.3.3Package registry (OSV)
suse:rmt-server2.6.5-3.3.1 · SUSE:Linux Enterprise Module for Public Cloud 15 SP2Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

EPSS, last 120 days

FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

  • Anyone who can reach it over the internet can trigger it.
  • A low-privileged account is enough.
  • No user action is required.
  • No special conditions are required; it is repeatable.

If successful

Confidentiality
none
Integrity
high · data or configuration can be modified
Availability
none
Attack vector
Network
Attack complexity
Low
Privileges required
Low
User interaction
None
Scope
Unchanged
Confidentiality impact
None
Integrity impact
High
Availability impact
None

Weakness class (CWE)

CWE-113 · Improper Neutralization of CRLF Sequences in HTTP Headers ('HTTP Request/Response Splitting')

CVSS vector

CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:H/A:N

nvd-primary

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Change log

  1. Fix✗ → ✓

For records you follow, these changes also arrive as notifications. →

References

Show all references (6 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records