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Azure Application Gateway Ingress can bypass hostname checks
published 5 months ago
Summary
The ingress controller’s handling of special domain names lets an attacker use a disguised address (like a Punycode version) to appear as a trusted host. This can let a user gain higher privileges if the system only checks the plain text name. Update the controller to version 1.7.7-5 or later to fix the processing of these names.
What to do
- Update azure application-gateway-kubernetes-ingress to version 1.7.7-5.
Affected software
| Ecosystem | Vendor | Product | Affected versions |
|---|---|---|---|
| Azure Linux:3 | azure | application-gateway-kubernetes-ingress |
< 1.7.7-5 Fix: upgrade to 1.7.7-5
|
Original advisory text
CVE-2026-39821 affecting package application-gateway-kubernetes-ingress for versions less than 1.7.7-5
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".
References
Timeline
Published22 May 2026
Updated23 Sep 2026
First seen23 Sep 2026
Sources
AZL-87360 · OSV
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