CVE-2026-78129 in strongSwan
Summary
by MITRE • 09/11/2026
strongSwan 4.6.2 through 6.0.7 has an infinite loop in PKCS#5 decryption.
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Analysis
by VulDB Data Team • 09/11/2026
The vulnerability identified in strongSwan versions ranging from 4.6.2 to 6.0.7 represents a critical implementation flaw within the cryptographic module responsible for handling PKCS#5 password-based encryption schemes. This specific defect manifests as an infinite loop during the decryption process, which fundamentally undermines the availability and reliability of the VPN gateway service. The issue is rooted in how the software processes encrypted private keys or configuration data that relies on PBKDF2 (Password-Based Key Derivation Function 2) mechanisms defined by PKCS#5 v1.5 or related standards. When a malformed or specifically crafted input triggers this code path, the internal iteration logic fails to terminate correctly under certain conditions, leading to an unbounded execution cycle that consumes system resources indefinitely.
From a technical perspective, the flaw resides in the parsing and decryption routines of the strongSwan IKE daemon. PKCS#5 defines methods for deriving cryptographic keys from passwords, often involving iterative hashing processes such as SHA-1 or MD5 combined with salt values to increase computational cost against brute-force attacks. In this vulnerable scenario, the loop control mechanism likely fails to validate input parameters correctly before initiating the key derivation process. This could involve a failure to check bounds on iteration counts, incorrect handling of ASN.1 encoded structures containing the encrypted data, or an edge case in error recovery logic where a decryption exception is not properly caught and handled, causing the execution flow to revert to the start of the loop rather than aborting safely. The result is that the thread responsible for processing the connection request becomes permanently blocked within this computational cycle.
The operational impact of this vulnerability is primarily centered on service availability, classifying it as a Denial of Service (DoS) vector. An attacker who can trigger this condition does not necessarily need to compromise the confidentiality or integrity of the data but can effectively crash or hang the strongSwan process. Since strongSwan typically runs with elevated privileges and manages critical network routing tables and IPsec tunnels, hanging one instance may lead to a complete loss of connectivity for users relying on that gateway. Furthermore, because the loop is infinite, it will continue to consume CPU cycles until the system resource limits are exhausted or the administrator manually intervenes by restarting the service. This creates a significant risk in production environments where high availability is paramount, as repeated exploitation attempts could lead to sustained outages and increased operational overhead for maintenance teams attempting to restore normal operations.
This vulnerability aligns with CWE-835, which describes loops that do not terminate correctly, often due to missing or incorrect termination conditions. It also maps to the MITRE ATT&CK framework under T1499, Endpoint Denial of Service, specifically within the context of resource exhaustion techniques. The exploitation vector typically requires network access if the vulnerable code path is triggered by incoming IKE packets containing maliciously crafted payloads designed to exploit the parsing logic. While local privilege escalation might be possible in some configurations depending on how the daemon handles untrusted input from lower-privileged processes, the primary threat model involves remote attackers sending specially constructed data that forces the cryptographic library into an unrecoverable state.
Mitigation strategies for this vulnerability are straightforward but require immediate action to ensure system stability. The most effective remediation is to upgrade strongSwan to a version newer than 6.0.7, where the developers have patched the loop termination logic and improved input validation within the PKCS#5 decryption routines. Organizations running affected versions should prioritize patching during their next maintenance window or apply emergency updates if the systems are exposed to untrusted networks. In addition to upgrading, administrators can implement network-level controls such as firewall rules to restrict access to IKE ports (UDP 500 and UDP 4500) from untrusted sources until patches are applied. Monitoring system logs for high CPU usage associated with strongSwan processes or unexpected service restarts can also serve as an early detection mechanism for potential exploitation attempts in the wild.