CVE-2007-4321
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedfail2ban 0.8 and earlier does not properly parse sshd log files, which allows remote attackers to add arbitrary hosts to the /etc/hosts.deny file and cause a denial of service by adding arbitrary IP addresses to the sshd log file, as demonstrated by logging in via ssh with a client protocol version identification containing an IP address string, a different vector than CVE-2006-6302.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · moderate confidencefail2ban versions 0.8 and earlier contain a log injection vulnerability in SSH log parsing. Attackers can include arbitrary IP addresses in the SSH client protocol version identification string during connection. When fail2ban parses these logs, it incorrectly interprets the injected IP as the attacker/ offender, causing it to add legitimate or arbitrary hosts to /etc/hosts.deny, resulting in denial of service for valid users.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data= 0.8CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- M
- Authentication
- None
- Confidentiality
- Partial
- Integrity
- Partial
- Availability
- Partial
AV:N/AC:M/Au:N/C:P/I:P/A:P
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Check installed fail2ban versionRun 'fail2ban --version' or check via package manager (rpm -q fail2ban or dpkg -l fail2ban)Affected if Version is 0.8 or earlier (including 0.8, 0.7.x, etc.)
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Verify fail2ban is configured for SSH monitoringInspect fail2ban configuration files (typically in /etc/fail2ban/) for enabled SSH-related filters or actionsAffected if SSH monitoring is enabled and fail2ban is actively parsing SSH authentication logs
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Confirm fail2ban is actively adding entries to deny listsCheck /etc/hosts.deny for recent entries added by fail2ban, and review fail2ban logs for ban actionsAffected if fail2ban is writing to /etc/hosts.deny based on SSH log parsing
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Look for anomalous IP entries in hosts.denyReview /etc/hosts.deny for any IP addresses that do not correspond to known malicious hosts or failed login attempts from your logsAffected if There are entries for IP addresses that cannot be verified as actual attackers from SSH auth logs
You are affected if fail2ban version 0.8 or earlier is installed with SSH monitoring enabled, and it has added entries to /etc/hosts.deny that may include legitimate IPs injected via SSH client version strings.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor dataUpgrade fail2ban to a version beyond 0.8 that includes proper validation of IP addresses extracted from log entries, or implement additional parsing logic to verify the source of IP addresses before adding them to deny lists.
- Consultation2.0 h
- Implementation4.0 h
- Testing3.0 h
- Review / QA2.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $3,088.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2007-4321 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2007-4321 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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