CVE-2026-58085 in WireGuard
Summary
by MITRE • 08/19/2026
After dispatching a decrypt operation to OCF and receiving the result, the wg(4) driver failed to check whether the MAC verification step succeeded. The driver thus silently accepted packets with an invalid Poly1305 authentication tag.
A remote attacker who can send UDP packets to a WireGuard endpoint, and who can guess the bounds of the receiver's replay window, can inject forged or modified transport data packets into the tunnel.
A remote attacker who can intercept WireGuard packets bound for a FreeBSD host can modify the ciphertext and authenticated data without detection by the receiver.
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Analysis
by VulDB Data Team • 08/19/2026
The vulnerability in question represents a critical failure in the integrity verification mechanisms of the wg(4) driver on FreeBSD systems, specifically within its interaction with the OpenCryptoki (OCF) framework. When processing incoming WireGuard packets, the driver initiates a decryption operation via OCF to recover plaintext data and verify authentication tags using the Poly1305 algorithm. However, following this dispatch, the implementation fails to inspect the return status or error code indicating whether the MAC verification step succeeded. This oversight means that if the cryptographic library returns an indication of failure due to tag mismatch, the driver ignores it and proceeds as though the operation was successful. Consequently, packets containing invalid Poly1305 authentication tags are silently accepted by the system, effectively bypassing one of the primary security guarantees provided by WireGuard's design.
This flaw fundamentally undermines the confidentiality and integrity assurances that WireGuard is designed to provide. In a properly implemented cryptographic protocol, any modification to ciphertext or associated data should result in immediate rejection of the packet upon detection of an invalid authentication tag. By failing to check this status, the driver allows forged or modified transport data packets to be processed as legitimate traffic. This creates a scenario where an attacker does not need to break the underlying encryption algorithms such as ChaCha20 but can instead exploit the logic error in the verification step to inject malicious content into the secure tunnel. The severity of this issue is compounded by the fact that it affects the core trust model of WireGuard, which relies heavily on lightweight cryptography and assumes correct implementation of all security primitives at every layer of the stack.
From an operational perspective, a remote attacker who possesses the ability to send UDP packets to a WireGuard endpoint can exploit this vulnerability if they can guess or determine the bounds of the receiver's replay window. The replay window is used by WireGuard to prevent replay attacks by tracking recently received nonces. If an attacker knows which sequence numbers are currently accepted, they can craft forged packets with valid-looking but incorrect authentication tags that fall within this active range. Because the driver does not verify the MAC success status, these injected packets will be decrypted and passed up the network stack as if they originated from a legitimate peer. This enables unauthorized data injection into the tunnel, potentially allowing for command execution on remote systems connected via the VPN, interception of sensitive information through man-in-the-middle modifications, or disruption of services by injecting malformed payloads that cause downstream application failures.
Furthermore, attackers capable of intercepting WireGuard traffic bound for FreeBSD hosts can modify existing ciphertext and authenticated data without detection. Since the receiver does not validate whether the Poly1305 tag matches the decrypted content, any alteration to the encrypted payload will be accepted as valid provided it falls within the replay window constraints. This capability allows for sophisticated man-in-the-middle attacks where an adversary can alter sensitive communications in transit while maintaining the appearance of a secure connection from the perspective of higher-layer protocols that rely on WireGuard's integrity guarantees. The lack of immediate feedback or logging regarding failed authentication attempts also hinders incident detection, as these successful injections may not trigger standard security alerts associated with dropped packets due to cryptographic errors.
To mitigate this risk, system administrators must apply vendor-provided patches immediately that correct the error handling logic in the wg(4) driver to ensure it properly checks and rejects packets when MAC verification fails. Until patching is possible, limiting exposure by restricting access to WireGuard endpoints via strict firewall rules can reduce the attack surface for remote exploitation. Additionally, monitoring network traffic for anomalies such as unexpected data patterns or connection disruptions may help identify ongoing attempts to exploit this flaw. Long-term remediation should involve rigorous code audits of cryptographic implementations in kernel drivers and adherence to secure coding standards that mandate explicit verification of all security-critical function return codes. This vulnerability aligns with CWE-354, Improper Validation of Integrity Check Value, as the system fails to properly validate data integrity before processing it. In terms of offensive tactics, this flaw facilitates MITM6 or similar techniques where an attacker modifies network traffic undetected, corresponding to ATT&CK technique T1072 which involves software injection into existing processes or communication channels via manipulation of unverified inputs.