CVE-2026-71271 in Memos
Summary
by MITRE • 08/05/2026
Memos' webhook URL validation, isReservedIP() (internal/webhook/validate.go), checks a candidate IP against a reservedCIDRs list that omits 0.0.0.0/8 and never calls ip.IsUnspecified() — unlike the correctly implemented sibling function isInternalIP() in internal/httpgetter/html_meta.go, which does. Because Linux redirects connections to 0.0.0.0 to loopback (127.0.0.1), an attacker registering a webhook URL of http://0.0.0.0:PORT/ bypasses the reserved-IP check and causes the Memos server to make outbound HTTP requests to its own loopback interface, exposing internal-only services.
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Analysis
by VulDB Data Team • 08/05/2026
The vulnerability resides in the webhook URL validation mechanism within the Memos application, specifically in the isReservedIP() function located at internal/webhook/validate.go. This function performs IP address validation by checking candidate IPs against a reservedCIDRs list but fails to include the 0.0.0.0/8 CIDR block in its validation checks. The flaw becomes particularly significant because it does not invoke ip.IsUnspecified() method, which is present in the correctly implemented sibling function isInternalIP() found in internal/httpgetter/html_meta.go. This inconsistency creates a critical security gap that allows attackers to bypass intended network restrictions through a simple yet effective technique.
The technical exploitation occurs when an attacker registers a webhook URL using the specific format http://0.0.0.0:PORT/, which circumvents the reserved IP validation entirely. The 0.0.0.0/8 CIDR range represents unspecified addresses in IPv4 networking, and while Linux systems automatically redirect connections to this address to the loopback interface 127.0.0.1, the Memos server fails to account for this behavior in its validation logic. This misconfiguration allows the application to establish outbound HTTP requests to its own internal loopback interface, effectively creating an internal network traversal vector that bypasses normal security boundaries.
The operational impact of this vulnerability is substantial as it enables attackers to potentially access internal-only services that would normally be protected from external network access. When the Memos server processes the malicious webhook URL, it makes outbound requests to localhost addresses, which could expose sensitive internal applications, databases, or administrative interfaces that are not intended to be accessible from external networks. This creates a direct path for attackers to enumerate and potentially exploit services running on the same host that may have weaker security controls than externally exposed components.
This vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and CWE-284 (Improper Access Control) categories, as it represents a failure in access control mechanisms and improper handling of network entity references. The issue also maps to ATT&CK technique T1071.004 (Application Layer Protocol: DNS) and T1046 (Network Service Scanning) through the exploitation of internal network traversal capabilities. Organizations should implement immediate mitigations including updating the isReservedIP() function to include proper handling of unspecified IP addresses, ensuring consistency with the isInternalIP() implementation, and potentially implementing additional network segmentation controls to prevent such internal service exposure. The fix should ensure that all unspecified IP address ranges are properly validated and rejected during webhook URL processing to prevent similar issues in other components.