CVE-2011-3389: SSL/TLS CBC IV weakness enables BEAST plaintext header recovery

Automated analysis

Generated by VULONE's analysis model from the NVD record, CISA KEV and EPSS data on 18 September 2026. Confidence: high.

high priorityAlthough the CVSS base score is medium, the very high EPSS percentile and broad multi-vendor impact on HTTPS confidentiality warrant high defensive priority.

What it is

The SSL protocol's use of CBC mode with chained initialization vectors allows a man-in-the-middle attacker to recover plaintext HTTP headers from an HTTPS session via a blockwise chosen-boundary attack (BCBA), known as BEAST. It affects SSL/TLS implementations in Windows, Internet Explorer, Firefox, Chrome, Opera, curl, and various Linux distributions. Because it undermines the confidentiality of HTTPS traffic, it matters for any environment still relying on vulnerable TLS configurations.

Impact

An attacker positioned on the network path can decrypt portions of HTTPS traffic, specifically HTTP headers, exposing cookies, authentication tokens, and other sensitive session data. This can lead to session hijacking or credential theft.

Attack surface

Reached over the network by a man-in-the-middle who can inject JavaScript into the victim's browser session; no authentication is required, but the attack depends on the victim loading attacker-controlled JavaScript that uses WebSocket, Java URLConnection, or Silverlight WebClient APIs.

Exploitation

Not listed in CISA KEV and no ransomware usage documented, but EPSS is very high (0.733 probability, 99.4th percentile), indicating significant predicted exploitation activity; references are advisory and technical in nature.

What to do

• Apply vendor patches and updates for affected browsers, operating systems, and TLS libraries.
• Disable or deprioritize CBC-mode cipher suites in favor of AEAD ciphers such as AES-GCM where supported.
• Enable TLS 1.1 or higher, or apply the 1/n-1 record splitting workaround on servers and clients.
• Restrict or disable unnecessary browser plugins and APIs (Java, Silverlight) that facilitate the attack.
• Enforce HSTS and avoid mixed content to reduce injection opportunities.

Detection

• Monitor for anomalous TLS handshake patterns or repeated small-record TLS traffic consistent with 1/n-1 splitting or BCBA probing.
• Inspect network traffic for injected JavaScript or unexpected cross-origin requests targeting HTTPS sessions.
• Track use of legacy CBC cipher suites in TLS logs and alert on their presence.
• Correlate endpoint logs for Java, Silverlight, or WebSocket activity originating from untrusted pages.

This assessment is produced automatically and is not human-reviewed. Verify against the vendor advisory before acting on it.