CVE-2026-39821— Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna
Possible ATT&CK Techniques 1AI
T1078 · Valid AccountsAffected Version Matrix 7
| Vendor | Product | Version Range | Status |
|---|---|---|---|
| Go standard library | net/http | < 1.25.13 | affected |
1.26.0-0< 1.26.6 | affected | ||
1.27.0-0< 1.27.0-rc.3 | affected | ||
| Go standard library | net/http/internal/http2 | < 1.25.13 | affected |
1.26.0-0< 1.26.6 | affected | ||
1.27.0-0< 1.27.0-rc.3 | affected | ||
| golang.org/x/net | golang.org/x/net/idna | < 0.55.0 | affected |
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I. Basic Information for CVE-2026-39821
Vulnerability Information
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Vulnerability Title
Invoking failure to reject ASCII-only Punycode-encoded labels in golang.org/x/net/idna
Source: CVE Program / CVE List V5
Vulnerability 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: CVE Program / CVE List V5
CVSS Information
N/A
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5
Vulnerability Title
Google Go 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Google Go是美国谷歌（Google）公司的一种静态强类型、编译型、并发型，并具有垃圾回收功能的编程语言。 Google Go存在安全漏洞，该漏洞源于ToASCII和ToUnicode函数错误接受解码为纯ASCII标签的Punycode编码标签，可能导致权限提升。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)
Affected Products
| Vendor | Product | Affected Versions | CPE | Subscribe |
|---|---|---|---|---|
| Go standard library | net/http | 0 ~ 1.25.13 | - | |
| Go standard library | net/http/internal/http2 | 0 ~ 1.25.13 | - | |
| golang.org/x/net | golang.org/x/net/idna | 0 ~ 0.55.0 | - |
II. Public POCs for CVE-2026-39821
| # | POC Description | Source Link | Shenlong Link |
|---|
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No public POC found.
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Patches & Fixes for CVE-2026-39821 (1)
Vendor Advisories for CVE-2026-39821 (4)
IV. Related Vulnerabilities
Same product: net/http
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Apply ReadHeaderTimeout when doing unencrypted HTTP/2 check - CVE-2025-58186
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CrossOriginProtection insecure bypass patterns not limited t - CVE-2025-4673
Sensitive headers not cleared on cross-origin redirect in ne - CVE-2025-22870
HTTP Proxy bypass using IPv6 Zone IDs in golang.org/x/net
Same vendor: Go standard library
- CVE-2026-56860
Avoid quadratic complexity in resolvePath in net/url - CVE-2026-56853
Apply ReadHeaderTimeout when doing unencrypted HTTP/2 check - CVE-2026-56859
Add recursion depth guard during decode in encoding/xml - CVE-2026-56862
Limit handshake messages we are willing to accept post-hands - CVE-2026-56858
Fix Javascript regexp context tracking in html/template
V. Comments for CVE-2026-39821
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