CVE-2013-2566: RC4 in TLS/SSL has single-byte biases enabling plaintext recovery

Automated analysis

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

medium priorityThe flaw is a protocol-level cryptographic weakness with high confidentiality impact and very high EPSS, but exploitation requires large volumes of same-plaintext sessions and no KEV or in-the-wild confirmation is present.

What it is

The RC4 stream cipher as used in TLS and SSL exhibits many single-byte biases, allowing statistical analysis of ciphertext across many sessions that encrypt the same plaintext to recover that plaintext. Because RC4 was widely deployed as a TLS cipher, any service still offering RC4 suites is exposed to this cryptographic weakness. The record does not specify affected versions beyond vendor product names.

Impact

A remote attacker who can observe a large number of TLS/SSL sessions carrying the same plaintext can statistically recover portions of that plaintext, defeating the confidentiality the encryption was meant to provide. The CVSS vector rates confidentiality impact as high with no integrity or availability impact.

Attack surface

Reachable over the network (AV:N) by a remote attacker who can passively capture or observe many sessions using the same plaintext; no authentication (PR:N) and no user interaction (UI:N) are required. The attack is against the TLS/SSL protocol's use of RC4 rather than a specific application endpoint.

Exploitation

Not listed in CISA KEV and no ransomware associations are documented, but EPSS is very high (0.844, ~99.7th percentile), indicating strong predicted likelihood of exploitation activity. All references are third-party advisories and issue tracking, with no public exploit code or in-the-wild confirmation cited in the record.

What to do

• Disable RC4 cipher suites in TLS/SSL configurations on all servers and clients, prioritizing TLS 1.2+ with AEAD ciphers such as AES-GCM or ChaCha20-Poly1305.
• Apply vendor updates for the listed products (Oracle, Fujitsu, Canonical/Ubuntu, Mozilla Firefox/Thunderbird/SeaMonkey) that remove or deprecate RC4 support.
• Audit internal and external TLS endpoints for RC4 negotiation and remediate any that still permit it, including firmware on the listed Oracle SPARC and ILOM devices.
• Where RC4 cannot be removed immediately, limit exposure by segmenting traffic and reducing the number of sessions that carry identical plaintext.

Detection

• Scan TLS endpoints for RC4 cipher suite support and flag any server that negotiates RC4 during handshakes.
• Monitor TLS handshake logs for cipher suite selections containing RC4 and alert on their continued use.
• Review configuration baselines and change records for RC4 being re-enabled or left enabled after patch cycles.

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