CVE-2015-7575

What it actually does

The library accepts MD5-based signatures inside TLS 1.2 Server Key Exchange messages instead of rejecting them, allowing an attacker who can trigger an MD5 collision to produce a forged signature that the client will accept.

Why it matters to the business

Attackers positioned on the network can impersonate legitimate servers during encrypted sessions, exposing user data in transit and undermining trust in web and application connections across many older systems still in use.

How an attack unfolds

An attacker places themselves in the network path, intercepts the TLS handshake, crafts a Server Key Exchange message containing an MD5 signature that collides with a legitimate one, and supplies it to the vulnerable client which accepts the forged credentials and proceeds with the session under attacker control.

How to fix it

Upgrade Mozilla NSS to version 3.20.2 or later, Firefox to 43.0.2 or later, and Firefox ESR to 38.5.2 or later. Disable TLS 1.2 or MD5 support where upgrades are not immediately possible and enforce stronger signature algorithms via configuration.

How to confirm the fix worked

Confirm NSS reports version 3.20.2+, test that clients reject MD5 signatures during TLS 1.2 handshakes using a test server configured to offer only MD5-based key exchange, and review update logs for the affected packages.

Typical attack path

1. Position on network path between client and target server
2. Observe or force a TLS 1.2 handshake
3. Generate MD5 collision for chosen Server Key Exchange parameters
4. Inject forged signature in the handshake message
5. Complete handshake and decrypt or modify subsequent traffic

How to detect it

Review TLS handshake logs and packet captures for Server Key Exchange messages that contain MD5 signatures; correlate with client version strings indicating vulnerable NSS builds and look for unexpected certificate or key material presented during sessions.

What your security team should do

• Prioritize patching all instances of NSS and Firefox to the fixed releases immediately given the age of the flaw and continued presence in legacy environments
• Scan inventories for any remaining affected Mozilla or NSS packages and schedule replacement or isolation
• Add network monitoring for anomalous TLS handshakes that advertise MD5 signatures