CVE-2023-52557 in OpenBSD
Summary
by MITRE • 03/01/2024
In OpenBSD 7.3 before errata 016, npppd(8) could crash by a l2tp message which has an AVP (Attribute-Value Pair) with wrong length.
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Analysis
by VulDB Data Team • 10/10/2025
The vulnerability identified as CVE-2023-52557 affects OpenBSD version 7.3 prior to errata 016 and specifically targets the npppd(8) daemon which handles l2tp (Layer 2 Tunneling Protocol) connections. This issue represents a classic buffer over-read or malformed input processing flaw that can lead to system instability and potential denial of service conditions. The npppd daemon is responsible for managing point-to-point protocol over l2tp connections, making it a critical component in network infrastructure deployments that rely on tunneling protocols for secure communications.
The technical flaw manifests when the npppd daemon processes l2tp messages containing Attribute-Value Pairs with incorrect length specifications. In l2tp protocol implementations, AVPs are fundamental data structures that carry various parameters and configuration information between tunnel endpoints. When an AVP contains a malformed length field that does not correspond to the actual data payload, the daemon's parsing logic fails to properly validate the input before attempting to process the data. This mismatch between declared and actual data length creates a scenario where the parser attempts to read beyond allocated memory boundaries or processes data in an unexpected manner, leading to undefined behavior and ultimately system crash conditions.
From an operational impact perspective, this vulnerability poses significant risks to network infrastructure reliability and availability. Organizations utilizing OpenBSD systems with npppd running as part of their l2tp infrastructure face potential service disruption when malicious or malformed l2tp messages are received. The crash condition affects the daemon's ability to maintain stable connections, potentially causing network outages for users relying on l2tp tunneling services. This vulnerability is particularly concerning in environments where l2tp is used for remote access VPN services or as part of enterprise network connectivity solutions, where uninterrupted service availability is critical for business operations.
The vulnerability aligns with CWE-129, which addresses "Improper Validation of Array Index," and CWE-125, which covers "Out-of-Bounds Read." These weaknesses in input validation and memory access control demonstrate the importance of proper bounds checking in network protocol implementations. From an attack perspective, this vulnerability could be exploited by remote adversaries who send specially crafted l2tp messages to trigger the crash condition, representing a denial-of-service vector that requires minimal privileges and technical expertise to execute. The ATT&CK framework categorizes this under T1499.004, which covers "Endpoint Denial of Service," as the attack surface involves network services that can be disrupted through malformed input processing.
Mitigation strategies for CVE-2023-52557 primarily involve applying the vendor-provided errata 016 update for OpenBSD 7.3, which includes patched code that properly validates AVP length fields before processing the associated data. System administrators should also implement network-level monitoring to detect unusual l2tp traffic patterns that might indicate exploitation attempts. Additional defensive measures include restricting l2tp access to trusted networks, implementing rate limiting for l2tp connection requests, and maintaining regular system updates to ensure all known vulnerabilities are addressed. Organizations should also consider implementing intrusion detection systems that can identify malformed l2tp messages and alert security teams to potential exploitation attempts. The vulnerability underscores the importance of robust input validation in network protocol implementations and highlights the critical need for regular security updates in operating system distributions.