VulnerabilityPublished 22 May 2026Updated 17 Sep 2026

CVE-2026-39821: Go standard library net/http · Improper Validation of Unsafe Equivalence in Input

Affects Go standard library net/httpSource CVE Database V5

Not known to be exploited · CVSS 9.6 Critical · fixed in 1.27.0-rc.3 : patch Go standard library net/http as a priority.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 3.1 base score

9.6Critical

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 1.8% chance of exploitation in 30 days – higher than 78% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.27.0-rc.3, 1.26.6, 1.25.13 +1

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Changed
Confidentiality: High
Integrity: High
Availability: None

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: High: There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact.
Integrity: High: There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
Availability: None: There is no impact to availability within the impacted component.

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 22 May 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade net/http and net/http/internal/http2 to 1.27.0-rc.3, 1.26.6 or 1.25.13; golang.org/x/net/idna to 0.55.0.

Affected software

Product	Affected	Fixed in
Go standard librarynet/http	Affected >=1.27.0-0 <1.27.0-rc.3	Fixed in 1.27.0-rc.3
Go standard librarynet/http	Affected >=1.26.0-0 <1.26.6	Fixed in 1.26.6
Go standard librarynet/http	Affected <1.25.13	Fixed in 1.25.13
Go standard librarynet/http/internal/http2	Affected >=1.27.0-0 <1.27.0-rc.3	Fixed in 1.27.0-rc.3
Go standard librarynet/http/internal/http2	Affected >=1.26.0-0 <1.26.6	Fixed in 1.26.6
Go standard librarynet/http/internal/http2	Affected <1.25.13	Fixed in 1.25.13

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A fix is available. Users and administrators should upgrade to a fixed release of golang.org/x/net that includes the corrected idna package. This can be achieved by updating the Go toolchain or rebuilding dependent packages with the patched version. Red Hat customers should apply the relevant security updates provided in their product advisories. No alternative mitigations are specified, so patching is the recommended remediation.

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".

Impact

This vulnerability can lead to privilege escalation by allowing attackers to bypass hostname-based access controls. Applications that rely on ASCII hostname validation may incorrectly permit access to restricted hostnames after Unicode normalization, potentially exposing sensitive resources. The impact includes high confidentiality and integrity risks but no availability impact. The vulnerability affects a broad range of products that include the affected Go packages, increasing the potential attack surface.

Summary

The vulnerability arises from improper validation of Punycode-encoded labels in the ToASCII and ToUnicode functions of golang.org/x/net/idna. These functions incorrectly accept Punycode labels that decode to ASCII-only hostnames, returning the ASCII name instead of an error. This behavior can cause privilege escalation in applications that first validate ASCII hostnames and then convert them to Unicode, potentially granting access to hostnames that should have been blocked. The issue is tracked as CWE-1289 (Improper Validation of Unsafe Equivalence in Input). Red Hat's advisory confirms broad exposure across products shipping the Go toolchain or bundling golang.org/x/net, including RHEL, OpenShift, and Ceph components. The vulnerability has a CVSS v3.1 score of 9.6 (critical) with network attack vector, low privileges required, no user interaction, and high confidentiality and integrity impact. Red Hat recommends upgrading to fixed versions of golang.org/x/net that include the idna correction.

CWE-1289Improper Validation of Unsafe Equivalence in Input (opens in a new tab)
Base weakness
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.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 14 Aug 2026, 13:08 UTC

WeaknessesCWE-1289

Technical details

References

Community reviews

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