CVE-2021-41819Network attack vector

Security-prefix spoofing via decoded cookie names

Published Jan 1, 2022 · Updated May 22, 2025

Cookie prefix spoofing in Ruby 2.6.8 and earlier and CGI gem 0.3.0 and earlier allows remote attackers to alter application trust decisions. CGI::Cookie.parse URL-decodes attacker-controlled cookie names, allowing encoded names to acquire the reserved __Host- or __Secure- security prefixes after parsing. Exploitation requires an application to parse a crafted Cookie header and treat one of those prefixes as proof of browser-enforced security constraints.

CVSS severity7.5
High
EPSS probability2.93%
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

Cookie prefix spoofing in Ruby 2.6.8 and earlier and CGI gem 0.3.0 and earlier allows remote attackers to alter application trust decisions. CGI::Cookie.parse URL-decodes attacker-controlled cookie names, allowing encoded names to acquire the reserved __Host- or __Secure- security prefixes after parsing. Exploitation requires an application to parse a crafted Cookie header and treat one of those prefixes as proof of browser-enforced security constraints.

The record

CVE
CVE-2021-41819
Published
Jan 1, 2022
Updated
May 22, 2025
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-565, T1606.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jan 1, 2022CVE publishedPublication date reported by the CVE source.
  2. May 22, 2025Record 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?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

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

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

CWE-565Reliance on Cookies without Validation and Integrity Checking
T1606.004Forge Web Credentials: Web Cookies

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