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CVE-2013-2566 Mozilla CVE debrief

CVE-2013-2566 describes a weakness in RC4 as used by TLS and SSL: the cipher has many single-byte biases that can make plaintext recovery easier through statistical analysis across a large number of sessions using the same plaintext. The issue is not a crash or code-execution flaw; it is a cryptographic weakness that can expose confidential data when RC4 remains available. NVD lists the issue as medium severity with network attack vector, high attack complexity, no privileges or user interaction, and confidentiality impact. The affected scope in the NVD record is broad, spanning multiple vendors and products, including Fujitsu firmware entries as well as Oracle, Mozilla, and Ubuntu products. The practical takeaway is simple: if RC4 is still allowed anywhere in your TLS/SSL stack, it should be treated as a priority to remove.

Vendor
Mozilla
Product
Firefox
CVSS
MEDIUM 5.9
CISA KEV
Not listed in stored evidence
Original CVE published
2013-03-15
Original CVE updated
2026-04-29
Advisory published
2013-03-15
Advisory updated
2026-04-29

Who should care

Security teams that still allow RC4 in TLS/SSL, plus administrators of affected Fujitsu firmware and other NVD-listed products such as Mozilla, Oracle, and Ubuntu releases.

Technical summary

RC4 is a stream cipher with known single-byte biases. In TLS/SSL, those biases can be exploited by a remote attacker who can observe many sessions carrying the same plaintext, allowing statistical plaintext-recovery attacks. NVD characterizes the issue as CVSS 3.0 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N, so the main concern is confidentiality loss rather than integrity or availability.

Defensive priority

Medium overall, but high priority wherever RC4 can still be negotiated in production TLS/SSL. The risk is concentrated in legacy configurations that have not removed RC4 cipher suites.

Recommended defensive actions

  • Disable RC4 cipher suites on all TLS/SSL endpoints and clients where possible.
  • Verify that servers no longer negotiate RC4 during handshakes.
  • Audit affected products and apply vendor updates or configuration guidance from the listed advisories.
  • Review legacy exceptions, especially externally exposed services that may still support older clients.
  • Prefer non-RC4 cipher suites and modern TLS configurations across your environment.
  • Retest representative connections after changes to confirm RC4 is no longer offered or accepted.

Evidence notes

The CVE description states that RC4 in TLS/SSL has single-byte biases enabling plaintext-recovery attacks via statistical analysis across many sessions using the same plaintext. The NVD record assigns CVSS 3.0 AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N and CWE-326. The NVD CPE list includes multiple affected products across vendors, including Fujitsu firmware entries, Mozilla Firefox/Thunderbird/SeaMonkey, Oracle HTTP Server/ILOM/Application Session Controller, and Ubuntu releases. References in the record include the CVE and NVD records plus third-party advisories and research pages focused on RC4/TLS.

Official resources

This debrief is based on the official CVE/NVD record published on 2013-03-15 and the references listed there. The later modified date on the record is not the disclosure date of the vulnerability.