CVE-2025-54870: VTun-ng Information Disclosure Flaw

CVE-2025-54870 Overview

CVE-2025-54870 affects VTun-ng, a Virtual Tunnel over TCP/IP network implementation written in Rust. The vulnerability allows tunnel traffic to revert to plaintext when encryption module initialization fails. Insufficient error handling in the link-fail-detect (lfd) encryption initialization path causes the failure to be silently ignored. Versions 3.0.12 through 3.0.17 are affected, with the issue resolved in version 3.0.18. The flaw is tracked under CWE-636: Not Failing Securely (Failing Open).

Critical Impact
Network-reachable attackers can intercept and read tunnel traffic intended to be encrypted when blowfish-256 initialization fails, breaking the confidentiality guarantee of the VPN tunnel.

Affected Products

VTun-ng versions 3.0.12 through 3.0.17
Deployments configured to use the blowfish-256 encryption module
Fixed in VTun-ng version 3.0.18

Discovery Timeline

2025-08-05 - CVE-2025-54870 published to NVD
2026-04-15 - Last updated in NVD database

Technical Details for CVE-2025-54870

Vulnerability Analysis

VTun-ng establishes encrypted tunnels using configurable cipher modules invoked through the lfd (link-fail-detect) abstraction. During initialization, the constructor for the encryption layer returned an Option<LfdEncrypt> type. When initialization failed, the function returned None without propagating a meaningful error to the caller.

The calling code did not consistently treat a missing encryption module as fatal. As a result, the tunnel pipeline continued operating without the encryption stage installed, transmitting packets in plaintext over the network. The blowfish-256 initialization path is the documented trigger for this failure mode.

Root Cause

The defect is a fail-open error handling pattern in src/main/lfd_encrypt.rs and src/main/lfd_legacy_encrypt.rs. Using Option as the return type discarded the cause of the failure and allowed downstream logic to interpret absence of the cipher as an acceptable state rather than a hard error.

Attack Vector

An attacker positioned on the network path between VTun-ng peers can observe traffic whenever encryption initialization fails on either endpoint. The attack requires no authentication or user interaction. Because the failure can be triggered by configuration combinations involving blowfish-256, an adversary who can influence configuration or rely on environmental conditions causing initialization failure can passively recover tunneled data.

The upstream patch changes the constructor signatures from Option to Result<_, i32> so that initialization failures propagate as errors rather than silent None returns:

rust

// Before (vulnerable): silent failure via Option
// pub fn new(host: &mut vtun_host::VtunHost) -> Option<LfdEncrypt>

// After (fixed): explicit error propagation via Result
pub fn new(host: &mut vtun_host::VtunHost) -> Result<LfdEncrypt, i32> {
    let mut lfd_encrypt: LfdEncrypt = LfdEncrypt {
        sequence_num: 0,
        gibberish: 0,
        // ...
    };
    // ...
}

// Legacy encryption module fix
impl LfdLegacyEncrypt {
    pub fn new(host: &mut vtun_host::VtunHost) -> Result<LfdLegacyEncrypt, i32> {
        let passwd = match host.passwd {
            Some(ref passwd) => passwd.as_str(),
            None => return Err(0),
        };
        let k = md5::compute(passwd.as_bytes());
        let mut key: [u8; 16] = [0u8; 16];
        // ...
    }
}

Source: GitHub commit 8c63982

Detection Methods for CVE-2025-54870

Indicators of Compromise

VTun-ng peer logs showing successful tunnel establishment without a corresponding encryption module initialization message.
Network captures on the tunnel transport port revealing recognizable plaintext payloads or protocol headers that should be encapsulated.
Presence of VTun-ng binaries at versions 3.0.12 through 3.0.17 on tunnel endpoints.

Detection Strategies

Inventory all hosts running VTun-ng and compare installed versions against the fixed 3.0.18 release.
Inspect VTun-ng host configurations for blowfish-256 cipher entries, which are the documented trigger for the failure path.
Perform passive packet inspection on the tunnel link to confirm payloads exhibit expected ciphertext entropy.

Monitoring Recommendations

Alert on VTun-ng process restarts that are not followed by an encryption initialization success message in stderr or syslog.
Baseline tunnel traffic entropy and flag drops in randomness that indicate plaintext fallback.
Track configuration file changes referencing cipher selection to detect drift toward affected settings.

How to Mitigate CVE-2025-54870

Immediate Actions Required

Upgrade all VTun-ng deployments to version 3.0.18 or later, which converts encryption initialization to a Result type that fails closed.
Audit running configurations and remove blowfish-256 cipher usage until upgrades are confirmed.
Rotate any shared secrets and credentials that may have transited tunnels while the affected versions were in use.

Patch Information

The fix is published in VTun-ng 3.0.18. The upstream change is recorded in commit 8c63982b6c487c52db1d56ab94c266f0bc857140 and described in the GitHub Security Advisory GHSA-m3jc-27c6-2wrf. The patch converts the new constructors in lfd_encrypt.rs and lfd_legacy_encrypt.rs from returning Option to returning Result<_, i32>, ensuring initialization failures abort tunnel setup.

Workarounds

Avoid configuring blowfish-256 as the tunnel cipher on affected versions per the vendor advisory.
Restrict tunnel transport to network paths under organizational control until endpoints are patched.
Layer an independent transport encryption (such as IPsec or WireGuard) beneath VTun-ng to prevent plaintext exposure during fallback.

bash

# Verify installed VTun-ng version
vtun-ng --version

# Confirm cipher selection in vtund.conf does not reference blowfish-256
grep -in 'blowfish' /etc/vtund.conf

# Upgrade from source to the fixed release
git clone https://github.com/leakingmemory/vtun-ng.git
cd vtun-ng
git checkout v3.0.18
cargo build --release