# CVE-2026-27136: Golang x/net/html XSS via duplicate attribute parsing

Source: https://junglewise.ai/threats/cve-2026-27136-golang-x-net-html-xss-via-duplicate-attribute-parsing  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-22. Updated: 2026-05-29  
License: CC BY 4.0 with a link back

CVE-2026-27136 · Severity: medium · CVSS 6.1 · Published 2026-05-22

A vulnerability in the Go programming language's standard networking library can allow attackers to bypass security filters on websites. By providing specially crafted HTML with duplicate attributes, an attacker can trick the system into executing malicious scripts in a user's browser. This could lead to unauthorized actions being performed on behalf of users or the theft of sensitive session information.

Technologies: [golang.org/x/net (Go)](https://junglewise.ai/threats/technologies/golang-org-x-net). Vendors: [Go](https://junglewise.ai/threats/vendors/go).

## Quick answers

**Is CVE-2026-27136 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-27136 being exploited in the wild.

**How severe is CVE-2026-27136?** Medium severity, with a CVSS score of 6.1, published 22 May 2026.

**Which products and versions are affected?** golang.org/x/net (Go) SEMVER:introduced=0,fixed=0.55.0.

**Is there a fix for CVE-2026-27136?** Yes. golang.org/x/net (Go): upgrade to 0.55.0 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-27136 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-27136?** Check whether you run golang.org/x/net (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- golang.org/x/net (Go): upgrade to 0.55.0 or later

  ```sh
  go get golang.org/x/net@v0.55.0
  ```


## Technical details

A cross-site scripting (XSS) vulnerability exists in the golang.org/x/net/html package due to improper handling of duplicate attributes during HTML parsing. When the parser encounters multiple identical attributes, it may produce an unexpected HTML tree structure that differs from how a web browser would interpret the same markup. This discrepancy allows an attacker to bypass HTML sanitizers that rely on the library's Parse or ParseFragment functions, leading to the execution of malicious scripts when the resulting tree is rendered. The issue affects versions of the package prior to v0.55.0 and is triggered when processing arbitrary, untrusted HTML input.

## Affected products

- Golang net/html before v0.55.0

## Identifiers

- CVE-2026-27136
- GO-2026-5030

## Timeline

- 2026-05-21: other: Issue opened on GitHub
- 2026-05-22: advisory: NVD and Go Project published advisory
- 2026-05-22: patched: Fix released in version 0.55.0

## References

- [https://go.dev/cl/781685](https://go.dev/cl/781685)
- [https://go.dev/issue/79575](https://go.dev/issue/79575)
- [https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8](https://groups.google.com/g/golang-announce/c/iI-mYSI0lu8)
- [https://pkg.go.dev/vuln/GO-2026-5030](https://pkg.go.dev/vuln/GO-2026-5030)

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