CVE-2026-78123: strongSwan Use-After-Free Vulnerability

CVE-2026-78123 Overview

CVE-2026-78123 is an expired pointer dereference vulnerability [CWE-825] in strongSwan, an open-source IPsec-based Virtual Private Network (VPN) solution. The flaw resides in the Public-Key Cryptography Standards #7 (PKCS#7) parsing logic inside the openssl plugin. Affected versions span strongSwan 5.0.2 through 6.0.7. A remote attacker can trigger the dereference by supplying a malformed PKCS#7 structure, causing the strongSwan daemon to crash and interrupting VPN service. The issue was disclosed by the strongSwan project and patched in release 6.1.0.

Critical Impact
Remote attackers can crash the strongSwan IKE daemon by delivering crafted PKCS#7 data to the openssl plugin, producing a denial-of-service condition against VPN gateways.

Affected Products

strongSwan 5.0.2 through 5.9.x with the openssl plugin enabled
strongSwan 6.0.0 through 6.0.7
Distributions and appliances bundling vulnerable strongSwan builds

Discovery Timeline

2026-09-07 - strongSwan project publishes vulnerability advisory
2026-09-11 - CVE-2026-78123 published to the National Vulnerability Database (NVD)
2026-09-15 - Last updated in NVD database

Technical Details for CVE-2026-78123

Vulnerability Analysis

The vulnerability lives in strongSwan's PKCS#7 container parser within the openssl plugin. PKCS#7 is a cryptographic message syntax used to sign and encrypt data, and strongSwan parses these structures when handling certificate payloads and cryptographic containers during Internet Key Exchange (IKE) negotiation.

The parser retains a pointer to memory that has already been released, meeting the definition of an expired pointer dereference [CWE-825]. When the parser later reads through that pointer, the underlying object is no longer valid. The result is undefined behavior that reliably produces a process crash in the charon daemon.

Because IKE listeners on VPN gateways typically accept unauthenticated input on UDP ports 500 and 4500, an attacker does not need credentials. Successful exploitation terminates the VPN service, which the CVSS vector reflects as availability-only impact with no confidentiality or integrity loss.

Root Cause

The root cause is object-lifetime mismanagement inside the PKCS#7 parsing routines. A structure is freed or goes out of scope while a reference to it remains in use by later parsing logic. Subsequent dereferences read from memory whose contents are no longer guaranteed, triggering the crash. The openssl plugin variant of PKCS#7 handling is the affected code path.

Attack Vector

Exploitation is remote and requires no authentication or user interaction, though the CVSS vector notes high attack complexity. An attacker sends a crafted PKCS#7 blob to a strongSwan endpoint that will parse it, for example during IKE authentication when a peer supplies a certificate container. Parsing the malformed structure triggers the expired pointer dereference and crashes the daemon.

No verified public exploit or proof-of-concept is available at publication. See the strongSwan vulnerability advisory for the vendor's technical description.

Detection Methods for CVE-2026-78123

Indicators of Compromise

Unexpected termination or repeated restarts of the charon daemon on strongSwan hosts.
Core dumps or segmentation faults referencing PKCS#7 parsing functions in the openssl plugin.
IKE negotiation failures immediately following receipt of a peer certificate payload from an untrusted source.

Detection Strategies

Inventory strongSwan installations and flag any running versions between 5.0.2 and 6.0.7 with the openssl plugin loaded.
Alert on abnormal exit codes or crash signals from the charon process in host telemetry.
Correlate IKE traffic spikes on UDP 500 and 4500 with VPN daemon restarts.

Monitoring Recommendations

Ship strongSwan logs and systemd service state to a centralized logging platform for crash pattern analysis.
Monitor VPN gateway availability metrics and tunnel establishment success rates for sudden drops.
Track inbound IKE traffic from unexpected source addresses that precede daemon failures.

How to Mitigate CVE-2026-78123

Immediate Actions Required

Upgrade strongSwan to release 6.1.0 or later on all VPN gateways and clients.
If immediate patching is not feasible, restrict IKE exposure using firewall rules that permit only trusted peer addresses.
Restart the charon daemon after applying the update to load the fixed binary.

Patch Information

The strongSwan project fixed CVE-2026-78123 in strongSwan 6.1.0. Users on the 5.x branch should plan migration to a supported release, since the vulnerable range extends across 5.0.2 through 6.0.7. Refer to the strongSwan advisory for release notes and backport guidance.

Workarounds

Disable the openssl plugin and use an alternative cryptographic backend such as gcrypt or botan where operationally feasible.
Enforce strict peer authentication policies that limit which endpoints can present PKCS#7 structures to the daemon.
Place VPN concentrators behind network access controls that filter IKE traffic to known peer ranges only.

bash

# Example: restrict IKE traffic to known peers using iptables
iptables -A INPUT -p udp --dport 500 -s <trusted_peer_cidr> -j ACCEPT
iptables -A INPUT -p udp --dport 4500 -s <trusted_peer_cidr> -j ACCEPT
iptables -A INPUT -p udp --dport 500 -j DROP
iptables -A INPUT -p udp --dport 4500 -j DROP