CVE-2026-77900

Executive Summary

CVE-2026-77900 is a critical missing-authentication vulnerability (CWE-306) in Microsoft Azure App Service that permits an unauthorized, remote attacker to execute code over the network. With a CVSS 3.1 base score of 9.8 (AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H), the flaw requires no credentials, no user interaction, and no special conditions to exploit. Any internet-exposed Azure App Service endpoint that reaches the vulnerable functionality is potentially at risk of full compromise of confidentiality, integrity, and availability.

Technical Analysis

The NVD description states: "Missing authentication for critical function in Azure App Service allows an unauthorized attacker to execute code over a network." This maps directly to CWE-306: Missing Authentication for Critical Function.

The core defect class is the absence of an authentication check on a function that performs a security-critical operation. In this vulnerability class, a privileged or sensitive function — one that should require prior authentication and authorization — is reachable by unauthenticated network requests. Because the affected operation is characterized as enabling code execution, the impact is elevated from simple information disclosure to remote code execution (RCE).

Key technical characteristics derived strictly from the available data:

• Attack vector (AV:N): The flaw is exploitable remotely over a network, consistent with Azure App Service being an internet-facing cloud platform.
• Attack complexity (AC:L): No special conditions or race windows are described; the flaw is considered straightforward to trigger once a vulnerable target is identified.
• Privileges required (PR:N): The "missing authentication" classification means an attacker needs no prior account, token, or credential.
• User interaction (UI:N): The attack does not depend on a victim performing an action.
• Scope (S:U): Impact is assumed to remain within the affected security authority.
• C/I/A (H:H:H): Full loss of confidentiality, integrity, and availability is possible, consistent with arbitrary code execution in the hosted service context.

The specific "critical function" left unauthenticated, the exact request path, and the underlying implementation are not disclosed in the NVD description or CNA title. No public advisory text, commit, or diff is available in the supplied data. Accordingly, this report analyzes the vulnerability at the class level and does not assert specific endpoints, parameters, function names, or file paths.

Because Azure App Service is a managed platform, the vulnerable component may reside in platform-level functionality rather than in customer application code. This distinction matters operationally: customer teams may not be able to remediate by changing their own application code, and remediation may depend on Microsoft-side service updates.

Exploitation Scenarios

No public proof-of-concept, Nuclei template, or exploit code is available for CVE-2026-77900. The following attack chain is inferred from the vulnerability class (CWE-306, missing authentication for a critical function reachable over a network) and from the CVSS vector. It does not rely on invented implementation details.

1) Reconnaissance / target identification An attacker enumerates internet-exposed Azure App Service endpoints. Because the vector is network-based and requires no privileges, targets are discoverable through standard internet scanning and service fingerprinting. Shodan-style queries such as http.html:"azurewebsites.net" or certificate subject CN matching *.azurewebsites.net can identify candidate hosts. The attacker's goal at this stage is to identify App Service instances that expose the unauthenticated critical function, since only those reachable instances are exploitable.

2) Triggering the flaw class Unauthenticated, the attacker sends network requests that reach the critical function. Because authentication is missing (CWE-306), the request is processed without any credential or session validation. The attacker does not need to bypass, forge, or steal any authentication token — the check simply does not exist on the vulnerable path. Per the CVSS vector (AC:L, PR:N, UI:N), no special preconditions are required.

3) Payload / execution method (class level) The description states the outcome is code execution over a network. In a missing-authentication-to-RCE class, the unauthenticated critical function accepts attacker-supplied input that is ultimately executed in the service context. The precise input structure, parameter, and execution primitive are not disclosed in the available data and are therefore not specified here.

4) Post-exploitation impact Given the CVSS impact subscores (C:H/I:H/A:H), successful exploitation can yield compromise of the confidentiality, integrity, and availability of the affected component. In a cloud App Service context, an attacker achieving code execution may be able to access data and configuration reachable by the service's execution context. The full blast radius depends on the identity and permissions under which the vulnerable function executes, which is not disclosed.

No evidence of in-the-wild exploitation, public PoC, or CISA KEV listing is present in the supplied data.

Impact Assessment

• Severity: Critical. CVSS 3.1 base score 9.8.
• Exploitability: Maximum for a network-vectored flaw — AV:N, AC:L, PR:N, UI:N. No authentication, no user interaction, low complexity.
• Impact subscores: Confidentiality High, Integrity High, Availability High. Successful exploitation can result in arbitrary code execution, which can cascade to full compromise of the affected service and any resources accessible to its execution context.
• Affected systems: Microsoft Azure App Service, a widely used managed web hosting platform. Internet-exposed instances that reach the vulnerable critical function are in scope. Because the description identifies no specific versions or configurations, the exact scope of affected instances cannot be bounded from the supplied data.
• Blast radius: Potentially large given the platform's popularity and internet exposure, but constrained by whatever conditions make the critical function reachable. Those conditions are undisclosed.
• Business impact: RCE in a public-facing hosting service can lead to data exposure, service takeover, lateral movement into connected cloud resources, and service disruption. Multi-tenant or shared-infrastructure concerns raise the stakes, though no cross-tenant impact is stated in the provided data.
• Known exploitation: None reported. EPSS not yet scored. Not in CISA KEV.

Detection Guidance

Sigma Rule

title: Potential Exploitation of CVE-2026-77900 - Unauthenticated Critical Function Access (Azure App Service)
id: 7b2f1c94-3a5e-4d61-9c0a-7790cve00001
status: experimental
description: Detects unauthenticated or anomalous requests to Azure App Service endpoints that may indicate attempts to reach an unauthenticated critical function (CWE-306).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-77900
author: valtersit.com
date: 2026/01/01
logsource:
  category: webserver
  product: azure
detection:
  selection_host:
    cs-host|endswith:
      - '.azurewebsites.net'
  selection_status:
    sc-status:
      - 200
      - 500
      - 502
  selection_missing_auth:
    cs-method:
      - 'GET'
      - 'POST'
      - 'PUT'
  filter_authenticated:
    cs-Cookie|contains: 'AppServiceAuthSession'
  condition: selection_host and selection_status and selection_missing_auth and not filter_authenticated
falsepositives:
  - Legitimate anonymous access to public web content
  - Health check or monitoring probes
level: medium
tags:
  - attack.t1190
  - cve.2026.77900

YARA Rule

rule CVE_2026_77900_Azure_AppService_Unauth_Critical_Function
{
    meta:
        description = "Detects artifacts referencing Azure App Service unauthenticated access patterns associated with CVE-2026-77900 (CWE-306)."
        author = "valtersit.com"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-77900"
        cve = "CVE-2026-77900"

    strings:
        $as1 = "azurewebsites.net" ascii wide nocase
        $as2 = "Azure App Service" ascii wide nocase
        $as3 = "AppServiceAuthSession" ascii wide nocase

    condition:
        any of ($as*)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-77900 - Possible Unauthenticated Access to Azure App Service Critical Function"; flow:established,to_server; http.host; content:"azurewebsites.net"; http.header; content:!"Cookie"; http.header; content:!"Authorization"; classtype:attempted-admin; reference:cve,2026-77900; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-77900; sid:7790001; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=30s
  [ network where http.host : "*.azurewebsites.net" and http.request.method : ("GET","POST","PUT") ]
  [ network where http.response.status_code : (200 or 500 or 502) and not http.request.headers.Cookie : "*AppServiceAuthSession*" ]

Splunk SPL Query

index=web sourcetype=access_combined host=*.azurewebsites.net
| eval auth_present=if(match(cookie,"AppServiceAuthSession") OR match(authorization_header,"(?i)bearer"),1,0)
| where auth_present=0
| stats count values(uri_path) as paths dc(clientip) as unique_sources by host
| where count > 10
| sort - count

Wazuh OSSEC Rule

<group name="web,azure,appservice,cve-2026-77900,">
  <rule id="779000" level="10" frequency="6" timeframe="120">
    <if_matched_sid>31100</if_matched_sid>
    <match>azurewebsites.net</match>
    <description>CVE-2026-77900 - Repeated unauthenticated requests to Azure App Service endpoints (possible CWE-306 exploitation)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_77900,attack_t1190,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 — UNPATCHED. No patch evidence is available from any tracked source, no vendor fix reference is present, and no workaround is documented in the supplied data. No affected-version range is enumerated because Azure App Service is a continuously updated managed cloud platform.

1) Immediate mitigations (compensating controls):

• Restrict network exposure: place App Service instances behind Azure Front Door, Application Gateway/WAF, or IP allowlisting so only intended clients can reach the service.
• Enforce authentication at the edge: require authentication (e.g., App Service Authentication / Easy Auth or an identity-aware proxy) so that unauthenticated requests never reach backend functions.
• Apply WAF rules (Azure WAF in Prevention mode) to block anomalous or unauthenticated requests to sensitive paths pending a vendor fix.
• Enable continuous monitoring and alerting on unauthenticated access patterns to App Service endpoints using the detection content above.
• Review and minimize the permissions of the App Service managed identity/execution context to reduce post-exploitation blast radius.

2) Patch / upgrade steps:

• No patch reference is available. Monitor Microsoft's official Azure App Service and Azure security advisory channels, plus the NVD entry (https://nvd.nist.gov/vuln/detail/CVE-2026-77900), for an official remediation.
• Because Azure App Service is managed by Microsoft, remediation may be applied service-side without a customer-initiated upgrade. Confirm status with Microsoft support and validate that protective platform changes have propagated to your region.
• Once a vendor fix or service-side remediation is announced, re-test exposed endpoints to verify the unauthenticated critical function is no longer reachable.

3) Long-term hardening:

• Adopt "secure by default" architecture: never expose backend critical functions directly to the internet; front all services with authentication and a WAF.
• Maintain an inventory of all internet-exposed App Service endpoints and continuously validate that each requires authentication.
• Enforce least-privilege identities for all App Service execution contexts.
• Incorporate CWE-306 into secure code and architecture reviews, and add unauthenticated-critical-function checks to pre-deployment security testing.
• Subscribe to vendor security advisories and maintain a vulnerability management process capable of rapidly responding to cloud platform CVEs.

References

• NVD — CVE-2026-77900: https://nvd.nist.gov/vuln/detail/CVE-2026-77900
• CWE-306: Missing Authentication for Critical Function: https://cwe.mitre.org/data/definitions/306.html
• MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• Microsoft Azure App Service security documentation: https://learn.microsoft.com/azure/app-service/
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-77900 not listed at time of writing): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No threat actor campaigns or exploit-in-the-wild evidence are known for CVE-2026-77900 based on the supplied data. EPSS is not yet scored, no public PoC exists, and the vulnerability is not in CISA KEV.