CVE-2026-77900: Azure App Service Auth Bypass Vulnerability

CVE-2026-77900 Overview

CVE-2026-77900 is a missing authentication vulnerability in Microsoft Azure App Service. The flaw allows an unauthenticated remote attacker to execute arbitrary code over the network against the affected service. The weakness is classified under CWE-306: Missing Authentication for Critical Function.

Microsoft published the advisory through the Microsoft Security Response Center. The vulnerability requires no user interaction and no prior privileges, which places it in the highest exposure class for internet-facing cloud workloads.

Critical Impact
An unauthenticated network attacker can execute code against Azure App Service, enabling full compromise of hosted applications and workloads.

Affected Products

Microsoft Azure App Service

Discovery Timeline

2026-10-08 - CVE-2026-77900 published to the National Vulnerability Database
2026-10-10 - Last updated in NVD database

Technical Details for CVE-2026-77900

Vulnerability Analysis

Azure App Service exposes a critical function that does not enforce authentication on incoming requests. An attacker reaching the service over the network can invoke this function without presenting credentials. The result is remote code execution within the context of the affected application tier.

The weakness aligns with [CWE-306], where a security-sensitive operation is reachable without identity verification. In a Platform-as-a-Service (PaaS) context, this exposure extends beyond a single host because tenant applications, deployment pipelines, and bound service identities may all be reachable through the compromised component.

The current Exploit Prediction Scoring System (EPSS) probability is 0.494%. No public proof-of-concept, exploit code, or evidence of in-the-wild exploitation is available at the time of publication, and the vulnerability is not listed in the CISA Known Exploited Vulnerabilities catalog.

Root Cause

The root cause is a missing authentication check on a critical function exposed by Azure App Service. The service accepts and processes requests that should require authenticated, authorized identities. Because the check is absent rather than weak, bypass does not require credential theft, token forgery, or protocol downgrade.

Attack Vector

Exploitation occurs over the network against the Azure App Service endpoint. The attacker does not need user interaction or local access. A single crafted request to the vulnerable function is sufficient to trigger code execution, which an attacker can use to pivot into application data, environment variables, managed identities, and connected Azure resources.

No verified exploit code is public. See the Microsoft Security Update CVE-2026-77900 advisory for vendor technical detail.

Detection Methods for CVE-2026-77900

Indicators of Compromise

Unexpected outbound connections from App Service workers to unknown hosts or newly registered domains.
Process execution within App Service containers that deviates from the application's normal runtime baseline.
Unauthenticated requests to administrative or management endpoints recorded in App Service HTTP logs.
New or modified deployment artifacts, web shells, or scheduled WebJobs introduced outside the CI/CD pipeline.

Detection Strategies

Review Azure App Service diagnostic logs and Kudu activity for anonymous requests to privileged routes.
Correlate Microsoft Entra ID sign-ins for managed identities against the App Service plan to detect anomalous token issuance.
Hunt for child processes spawned by w3wp.exe or Linux worker processes that are inconsistent with the application stack.

Monitoring Recommendations

Enable Microsoft Defender for App Service and forward alerts to the SOC pipeline.
Stream App Service, Front Door, and Application Gateway logs to a central analytics backend for retention and correlation.
Alert on changes to App Service configuration, deployment slots, and application settings outside approved change windows.

How to Mitigate CVE-2026-77900

Immediate Actions Required

Apply the Microsoft fix referenced in the Microsoft Security Update CVE-2026-77900 advisory as soon as it is available for your region and SKU.
Inventory all Azure App Service instances across subscriptions, including Function Apps and Logic Apps on the App Service plan.
Rotate secrets, connection strings, and managed identity credentials associated with any App Service that may have been exposed.

Patch Information

Microsoft addresses CVE-2026-77900 through platform updates to Azure App Service. For PaaS components, remediation is applied by Microsoft at the service layer and does not typically require customer action beyond verifying runtime stack versions. Confirm status in the Azure portal and the Microsoft Security Response Center advisory.

Workarounds

Restrict inbound access to App Service using Access Restrictions, Private Endpoints, or Azure Front Door with Web Application Firewall rules.
Disable unused site extensions, SCM/Kudu access, and FTP/FTPS deployment endpoints.
Enforce authentication at the Easy Auth layer and require Microsoft Entra ID tokens for all application routes.

bash

# Configuration example: restrict App Service inbound access and disable SCM public access
az webapp config access-restriction add \
  --resource-group <rg> --name <app> \
  --rule-name allow-corp --action Allow \
  --ip-address <corp-cidr> --priority 100

az resource update \
  --resource-group <rg> \
  --name <app>/config/web \
  --resource-type Microsoft.Web/sites/config \
  --set properties.scmIpSecurityRestrictionsUseMain=false \
        properties.publicNetworkAccess=Disabled