CVE-2016-6329

What it actually does

Enables recovery of plaintext from long-lived encrypted sessions when OpenVPN uses a 64-bit block cipher such as Blowfish in CBC mode by exploiting the birthday bound on block collisions.

Why it matters to the business

Organizations relying on older OpenVPN deployments face gradual exposure of session data over time, increasing risk of sensitive information leakage in persistent VPN tunnels without immediate operational disruption.

How an attack unfolds

An attacker positions themselves to capture traffic from a long-running OpenVPN session using a vulnerable 64-bit cipher, accumulates sufficient ciphertext blocks, and applies a birthday attack to recover portions of the plaintext, as demonstrated against HTTP-over-OpenVPN flows.

How to fix it

Upgrade affected packages to the fixed versions: ferretdb (Chainguard) 1.21.0-r1 and ferretdb (Wolfi) 1.21.0-r1. Additionally configure OpenVPN to prefer 128-bit or larger block ciphers and enforce session key renegotiation at shorter intervals.

How to confirm the fix worked

Verify package versions match or exceed the listed fixed releases, review OpenVPN configuration for cipher selection and renegotiation settings, and test that new sessions no longer permit observable 64-bit block cipher usage in long-lived connections.

Typical attack path

1. Locate or establish a long-duration OpenVPN session employing a 64-bit block cipher
2. Capture large volumes of ciphertext from the encrypted tunnel
3. Apply birthday attack analysis to identify block collisions and reconstruct plaintext segments
4. Extract usable cleartext data such as HTTP requests or responses

How to detect it

Review historical VPN logs and packet captures for extended sessions using Blowfish or other 64-bit ciphers; look for patterns of sustained high-volume ciphertext without key changes, though specific exploitation artifacts are rare given the attack's passive nature.

What your security team should do

• Prioritize upgrade of ferretdb packages to the specified fixed versions across all environments
• Audit OpenVPN deployments for use of 64-bit block ciphers and replace with AES-GCM or equivalent
• Enable aggressive key renegotiation timers to limit session duration
• Monitor EPSS and threat intelligence feeds for any future increase in exploitation activity