CVE-2021-33197Public exploitNetwork attack vector

Arbitrary header dropping via duplicate Connection fields

Published Aug 2, 2021 · Updated Aug 3, 2024

Header manipulation in Go before 1.15.13 and 1.16.x before 1.16.5 allows remote attackers to drop arbitrary headers. ReverseProxy.ServeHTTP fails to remove hop-by-hop headers when an empty Connection field precedes another Connection field naming attacker-chosen headers. Exploitation requires the ReverseProxy target to be another reverse proxy, which can then remove headers added by ReverseProxy.Director.

CVSS severity5.3
Medium
EPSS probability2.28%
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

Header manipulation in Go before 1.15.13 and 1.16.x before 1.16.5 allows remote attackers to drop arbitrary headers. ReverseProxy.ServeHTTP fails to remove hop-by-hop headers when an empty Connection field precedes another Connection field naming attacker-chosen headers. Exploitation requires the ReverseProxy target to be another reverse proxy, which can then remove headers added by ReverseProxy.Director.

The record

CVE
CVE-2021-33197
Published
Aug 2, 2021
Updated
Aug 3, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-862
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 2, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record 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

4.3NIST NVDAV:N/AC:M/Au:N/C:N/I:P/A:N
5.3NIST NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/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 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-862Missing Authorization

Public exploit references

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