CVE-2026-76504: Cisco Catalyst SD-WAN Manager Authentication Bypass

Overview

CVE-2026-76504 is a critical authentication bypass vulnerability affecting Cisco Catalyst SD-WAN Manager. This flaw allows an unauthenticated, remote attacker to gain administrative access to an affected system. The vulnerability is actively tracked in CISA’s Known Exploited Vulnerabilities Catalog, indicating a high likelihood of exploitation in the wild.

Technical Analysis

The vulnerability resides in the API session-based authentication management of Cisco Catalyst SD-WAN Manager. It is caused by improper handling of URI encoding within HTTP requests (CWE-177). An attacker can exploit this by sending a specially crafted HTTP request containing specific URI encoding patterns. This crafted request bypasses an authentication rule designed to restrict access to a particular API endpoint.

• Vulnerability Type: Authentication Bypass (CWE-177: Improper Handling of URI Encoding)
• Affected Product: Cisco Catalyst SD-WAN Manager
• Attack Vector: Network (AV:N)
• Complexity: Low (AC:L)
• Privileges Required: None (PR:N)
• User Interaction: None (UI:N)
• Scope: Unchanged (S:U)
• Impact: High confidentiality, integrity, and availability (C:H/I:H/A:H)
• CVSS 3.1 Score: 9.8 (CRITICAL)
• Exploitation: A successful exploit grants the attacker access to the API with the privileges of the admin user.

Detection

Detection efforts should focus on monitoring web server access logs and network traffic for unusual patterns indicative of URI encoding manipulation targeting API endpoints.

• Web Server Logs: Monitor cs-uri-stem or c-uri fields in web access logs for the presence of double-encoded characters (e.g., %252f, %252e) or other unusual hex encoding (%2f, %5c, %00) within paths typically associated with API access (e.g., /api/, /rest/).
• Network Traffic Analysis: Look for HTTP requests to Cisco Catalyst SD-WAN Manager that contain highly encoded URIs, especially those targeting administrative or sensitive API endpoints, originating from unauthenticated sources.
• Authentication Logs: Review authentication logs for successful admin logins that do not correspond to expected user activity or standard authentication flows.

Sigma Detection Rules

Cisco Catalyst SD-WAN Manager URI Encoding Bypass Attempt

title: Cisco Catalyst SD-WAN Manager URI Encoding Bypass Attempt
id: 9283745a-1234-4567-8901-abcdef123456
status: experimental
description: Detects suspicious URI encoding patterns in HTTP requests to API endpoints, potentially indicating an attempt to exploit CVE-2026-76504.
logsource:
  product: webserver
  service: access_log
detection:
  selection_api_path:
    cs-uri-stem|contains:
      - '/api/'
      - '/rest/'
  selection_encoding:
    cs-uri-stem|contains:
      - '%252f' # Double-encoded slash
      - '%252e' # Double-encoded dot
      - '%2f'   # Single-encoded slash
      - '%5c'   # Single-encoded backslash
      - '%00'   # Null byte encoding
  condition: selection_api_path and selection_encoding
level: high

Mitigations

1. Apply Vendor Patches: Immediately apply all available security patches from Cisco for Catalyst SD-WAN Manager. Refer to the Cisco Security Advisory for specific version updates.
2. Network Segmentation: Isolate Cisco Catalyst SD-WAN Manager instances on a dedicated network segment, restricting access to only necessary administrative interfaces and trusted sources.
3. Access Control: Implement strict network access controls (e.g., firewalls, ACLs) to limit inbound connections to the SD-WAN Manager’s web interface from untrusted networks.
4. Monitor Logs: Ensure comprehensive logging is enabled for the SD-WAN Manager and associated web services, and regularly review logs for suspicious activity as described in the Detection section.

References

• https://nvd.nist.gov/vuln/detail/CVE-2026-76504
• https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-sdwan-webauth-xr8beuuU
• https://www.cisa.gov/known-exploited-vulnerabilities-catalog?field_cve=CVE-2026-76504

Indicators of Compromise

No public IOCs available at time of writing.

MITRE ATT&CK

• T1078.004 — Cloud Accounts