Description
denyhosts 2.6 uses an incorrect regular expression when analyzing authentication logs, which allows remote attackers to cause a denial of service (incorrect block of IP addresses) via crafted login names.
Fixed in Linux distributions
Distributions backport fixes, so the version that fixes this vulnerability differs per release — upgrade to at least the version shown. From the distributions' security trackers and advisories.
Debian
Security tracker ↗| Release | Package | Fixed in | Advisory |
|---|---|---|---|
| Debian 7 | denyhosts | 2.6-10+deb7u2 | DSA-2826-1 |
| Debian 6.0 | denyhosts | 2.6-7+deb6u2 | DSA-2826-1 |
Public exploits
Exploit or proof-of-concept code for this vulnerability is publicly available, which lowers the bar for attackers. Treat it as a reason to patch sooner.
Third-party code, not reviewed by us. Public PoCs are sometimes fake or contain malware. Read the code first and run it only in an isolated lab, against systems you are authorised to test.
Exploit-DB 1
- DenyHosts - 'regex.py' Remote Denial of Service EDB-38909 · dos · Dec 19, 2013 · verified
Sources: PoC-in-GitHub, Metasploit Framework, Exploit-DB — updated daily. Only links are listed; no exploit code is hosted here.
Advisories 1
- DSA-2826-1 denyhosts - Remote denial of ssh service Debian Security (DSA / DLA) · Mar 9, 2026
References
- 56239 third-party-advisory
- https://bugzilla.redhat.com/show_bug.cgi?id=1045982
- [oss-security] 20131222 Re: [SECURITY] [DSA 2826-1] denyhosts security update mailing-list
- DSA-2826 vendor-advisory