CyberSecBulletin
CVE-2026-39821NVD status: Modified

net/http: Improper Validation of Unsafe Equivalence in Input

Published Updated By the CyberSecBulletin desk

CVE-2026-39821 is a high-severity improper validation of unsafe equivalence in input vulnerability in net/http from Go standard library, with a CVSS 3.1 base score of 8.2 out of 10. It can be exploited remotely over the network by an attacker with a low-privileged account, without any action from a user. The record lists versions 1.25.13, 1.26.6, 1.27.0-rc.3 as the first without the flaw, so updating net/http to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 1.83% chance of exploitation within 30 days, higher than 79% of scored CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update net/http to 1.25.13 or 1.26.6 or 1.27.0-rc.3, whichever matches your release line, or a later version.
  2. Update net/http/internal/http2 to 1.25.13 or 1.26.6 or 1.27.0-rc.3, whichever matches your release line, or a later version.
  3. Update golang.org/x/net/idna to 0.55.0 or a later version.
  4. Read the advisory on pkg.go.dev for workarounds and exact fixed builds.

Affected products and versions

VendorProductAffected versionsFixed in
Go standard librarynet/httpbefore 1.25.13; 1.26.0-0 up to, but not including, 1.26.6; 1.27.0-0 up to, but not including, 1.27.0-rc.31.25.13, 1.26.6, 1.27.0-rc.3
Go standard librarynet/http/internal/http2before 1.25.13; 1.26.0-0 up to, but not including, 1.26.6; 1.27.0-0 up to, but not including, 1.27.0-rc.31.25.13, 1.26.6, 1.27.0-rc.3
Golang.org/x/netgolang.org/x/net/idnabefore 0.55.00.55.0
Red HatRed Hat Ansible Automation Platform 2.5 for RHEL 8Not specified in the recordNot stated
Red HatRed Hat Ansible Automation Platform 2.5 for RHEL 9Not specified in the recordNot stated
Red HatRed Hat Ansible Automation Platform 2.6 for RHEL 10Not specified in the recordNot stated
Red HatRed Hat Ansible Automation Platform 2.6 for RHEL 9Not specified in the recordNot stated
Red HatRed Hat Ansible Automation Platform 2.7 for RHEL 10Not specified in the recordNot stated
Red HatRed Hat Ansible Automation Platform 2.7 for RHEL 9Not specified in the recordNot stated
Red HatRed Hat Enterprise Linux 10Not specified in the recordNot stated
Red HatRed Hat Enterprise Linux 10.0 Extended Update SupportNot specified in the recordNot stated
Red HatRed Hat Enterprise Linux 7 Extended Lifecycle SupportNot specified in the recordNot stated
Red HatRed Hat Enterprise Linux 8Not specified in the recordNot stated
Red HatRed Hat Enterprise Linux 9Not specified in the recordNot stated
Red HatRed Hat Enterprise Linux 9.2 Update Services for SAP SolutionsNot specified in the recordNot stated
Red HatRed Hat Enterprise Linux 9.4 Update Services for SAP SolutionsNot specified in the recordNot stated
Red HatRed Hat Enterprise Linux 9.6 Extended Update SupportNot specified in the recordNot stated
Red HatRed Hat OpenShift Container Platform 4.16Not specified in the recordNot stated
Red HatRed Hat OpenShift Container Platform 4.20Not specified in the recordNot stated
Red HatRed Hat OpenShift Container Platform 4.21Not specified in the recordNot stated
Red HatRed Hat OpenShift Container Platform 4.22Not specified in the recordNot stated
Red HatRed Hat OpenStack Platform 16.2Not specified in the recordNot stated
Red HatRed Hat Satellite 6.16 for RHEL 8Not specified in the recordNot stated
Red HatRed Hat Satellite 6.16 for RHEL 9Not specified in the recordNot stated
Red HatRed Hat Satellite 6.17 for RHEL 9Not specified in the recordNot stated

Version data comes from the organisation that assigned the CVE; 35 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
Attack complexity
HighSuccess depends on conditions outside the attacker's control.
Privileges required
LowA basic user account is enough.
User interaction
NoneNo action by a victim is required.
Scope
ChangedThe attack can reach beyond the vulnerable component into other systems.
Confidentiality
HighAll data handled by the component can be read.
Integrity
HighData or code can be modified at will.
Availability
NoneAvailability is not affected.

Vector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:N. Scored by CNA.

Scoring systemScoreSeverityScored by
CVSS 3.19.6CriticalCISA (ADP)
CVSS 3.18.2HighCNA

Official description

The ToASCII and ToUnicode functions incorrectly accept Punycode-encoded labels that decode to an ASCII-only label. For example, ToUnicode("xn--example-.com") incorrectly returns the name "example.com" rather than an error. This behavior can lead to privilege escalation in programs using the idna package. For example, a program which performs privilege checks on the ASCII hostname may reject "example.com" but permit "xn--example-.com". If that program subsequently converts the ASCII hostname to Unicode, it will inadvertently permits access to the Unicode name "example.com".

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Improper Validation of Unsafe Equivalence in Input

The product receives an input value that is used as a resource identifier or other type of reference, but it does not validate or incorrectly validates that the input is equivalent to a potentially-unsafe value.

More Improper Validation of Unsafe Equivalence in Input vulnerabilities (CWE-1289)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
  3. Record last changed materially (score, affected products, status or references).

References

Other vulnerabilities in net/http

All net/http vulnerabilities

Recent Improper Validation of Unsafe Equivalence in Input vulnerabilities

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