CVE-2026-13734

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WireGuard VPN Replay Attack in Zephyr OS

Publication date: 2026-08-28

Last updated on: 2026-09-01

Assigner: Zephyr Project

Description

Zephyr's WireGuard VPN data-plane receive handler wg_process_data_message() in subsys/net/lib/wireguard/wg_crypto.c validated the anti-replay counter too late. After AEAD decryption of a MESSAGE_TRANSPORT_DATA packet succeeded, the code committed several peer-state changes — update_peer_addr() (endpoint roaming update), the keypair->last_rx/peer->last_rx liveness timers, and keypair_update() (promote next→current and destroy the previous keypair) — and only afterward called wg_check_replay(). On a replayed packet the replay check returned -EINVAL, but none of the preceding mutations were rolled back. The AEAD tag authenticates content but not freshness, so a replayed-but-authentic transport packet decrypts correctly. An attacker who captures one valid ciphertext off the wire (an on-path or shared-medium observer) can re-inject it from an arbitrary spoofed source address. Reaching the handler requires no credentials: it is driven directly from inbound UDP datagrams via the dispatch in subsys/net/lib/wireguard/wg.c. Because the state mutations committed before the replay check, the replay repoints the peer endpoint to the attacker-chosen source address (roaming hijack), redirecting the victim's subsequent outbound tunnel traffic until the legitimate peer's next packet re-corrects it; it also prematurely destroys the previous keypair and refreshes the RX liveness timer. The tunnel payload stays encrypted under the session keypair, so this is an integrity/availability impact (traffic redirection and session disruption), not payload disclosure. The fix moves wg_check_replay() to immediately after a successful decrypt, before any peer-state mutation, matching the WireGuard specification and the Linux reference implementation.

CVSS Scores

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Meta Information

Published

2026-08-28

Last Modified

2026-09-01

Generated

2026-09-18

AI Q&A

2026-08-29

EPSS Evaluated

2026-09-15

NVD

CVE-2026-13734

Affected Vendors & Products

Vendor	Product	Version / Range
zephyr	wireguard	*

Helpful Resources

Exploitability

CWE

KEV

CWE ID	Description
CWE-294	A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes).

Attack-Flow Graph

Executive Summary

This vulnerability in Zephyr's WireGuard VPN involves a delayed anti-replay counter validation in the data-plane receive handler. After successfully decrypting a packet, the code updates peer state like endpoint address and session keys before checking if the packet is a replay. This allows attackers to replay captured packets, redirecting traffic or disrupting sessions without decrypting payloads.

Detection Guidance

This vulnerability involves WireGuard's anti-replay counter being validated too late, allowing replayed packets to manipulate peer state. Detection requires monitoring for unexpected peer endpoint changes or replayed packets. Check WireGuard logs for repeated packets from the same source or sudden endpoint address changes. Use tcpdump or Wireshark to capture and analyze WireGuard traffic for duplicate packets or unexpected source addresses.

Impact Analysis

An attacker could capture a valid encrypted packet and replay it from a different address. This could hijack your VPN peer endpoint, redirecting your outbound traffic through the attacker's chosen path. It may also disrupt your session by prematurely refreshing session keys.

Compliance Impact

This vulnerability primarily impacts integrity and availability of VPN traffic rather than direct data disclosure. It could lead to unauthorized traffic redirection or session disruption, which may violate requirements for secure data transmission in GDPR (data integrity) and HIPAA (secure communication channels). However, no specific compliance implications are detailed in the provided CVE data.

Mitigation Strategies

Apply the official patch from Zephyr Project that moves the anti-replay check before state mutations. Update WireGuard to the latest version in your Zephyr OS build. If patching is not immediately possible, restrict WireGuard traffic to trusted networks or use additional network-layer protections to prevent replayed packets from reaching the vulnerable handler.

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