External risk intelligence

Azure App Service Critical Function Missing Authentication Allows Remote Code Execution.

CVE advisorySeverity: CRITICAL (CVSS 9.8)

CVE-2026-77900

Azure App Service is a platform-as-a-service offering commonly used to host web applications and APIs. These deployments are frequently configured to be internet-facing to serve public-facing web traffic and application endpoints, making the underlying service infrastructure reachable over the public internet.

Missing Authentication

Halo Surface Signal: 4 out of 5 — likely to be public-facing.

External exposure likelihood

Horizon Alert

Summary of the vulnerability and why it matters

A critical vulnerability has been identified in Azure App Service, a widely used platform for hosting web applications and APIs. This issue, which allows unauthorized attackers to execute code remotely, poses a significant risk due to the internet-facing nature of many such deployments. The primary concern at this time is to confirm if our organization utilizes this technology and is potentially exposed.

An attacker could run code on Azure App Service.
This affects internet-facing web applications.
Verify if your services are impacted.

Attack Path

How an attacker could exploit the issue

An attacker can reach the vulnerable Azure App Service component over the network without needing any credentials. By sending a specially crafted request, they can trigger a critical function that lacks proper authentication, potentially leading to unauthorized code execution.

No authentication needed.
Trigger critical function remotely.
Remote code execution risk.

Live Threat

Current exploitation, exposure, and threat context

A critical function in Azure App Service that lacks authentication could allow an unauthorized attacker to execute code over a network. This vulnerability could impact the integrity and availability of hosted applications and services.

Hosted application code and configuration.
Network-based code execution.
Service disruption and unauthorized code execution.

Operational Fix

Recommended remediation, mitigation, and detection steps

This critical vulnerability in Azure App Service, allowing unauthenticated remote code execution, necessitates immediate action from teams managing the Azure environment. The first step is to identify all Azure App Service instances, determine their exposure to the network, assess business criticality, and locate the accountable owner. This information will inform a prioritized remediation plan.

Identify and confirm Azure App Service owners.
Verify public network exposure and criticality.
Plan and coordinate immediate remediation.

Supplementary metadata

CVSS vector

Validate whether this threat affects your internet-facing exposure.

Halo Threat Intelligence helps prioritize remediation with Halo Surface Signal and H/A/L/O context. Start exposure validation with a free external attack surface trial.

Free EASM Trial See How Signal Works

Frequently asked questions

What is Azure App Service?

Azure App Service is a cloud-based platform-as-a-service (PaaS) provided by Microsoft. It acts as a managed environment where developers host web applications, mobile backends, and RESTful APIs without needing to manage the underlying server hardware or operating system. It automatically handles scaling, load balancing, and infrastructure updates, making it a popular choice for public-facing web traffic.

What does CWE-306 mean for CVE-2026-77900?

CWE-306 refers to 'Missing Authentication for Critical Function.' In the context of CVE-2026-77900, it means the software performs a sensitive or high-impact action without verifying the identity of the user. Because this check is absent, an unauthorized actor can invoke this function directly, essentially bypassing the security controls intended to protect the service from unverified commands or code execution.

How is this CVE-2026-77900 triggered?

An attacker triggers this vulnerability by sending a specially crafted request over a network to the Azure App Service. The flaw does not require the attacker to possess valid credentials or have prior access to the system. Importantly, this is not triggered by standard, legitimate user interactions; it requires an intentional request designed to exploit the missing authentication in the critical function.

Is my deployment at risk according to Halo Surface Signal?

Halo Surface Signal indicates that because Azure App Service is commonly used for hosting web applications and APIs, these instances are frequently configured to be internet-facing. If your service is reachable over the public internet, it falls into the high-risk category for this CVE. Internally facing services that are not accessible from the public network generally have a lower immediate profile, though internal security policies still apply.

What should I do first to address CVE-2026-77900?

Your first step is to perform an inventory of your environment to locate all active Azure App Service instances. Once mapped, confirm which instances are internet-facing and determine the business criticality of each. Identify the service owners responsible for these assets so you can coordinate with them. This foundational information is necessary to build an effective plan to apply upcoming updates or security configurations provided by the vendor.

References

msrc.microsoft.com/update-guide/vulnerability/CVE-2026-77900

Written by Nicholas Merritt

Nicholas Merritt is VP of Security Solutions at Halo Security. With more than 25 years across application security, vulnerability prioritization, and offensive security, he helps teams translate threat data into practical exposure validation and remediation focus. He authors Halo Threat Intelligence CVE advisories using Halo Surface Signal and H/A/L/O context to prioritize internet-facing risk.