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Commons NetApplication · Apache

CVE-2021-37533

MEDIUM · 6.5 CVSS v3.1 Published 2022-12-03
Fix available
A fix is available. Upgrade to 3.9.0 or later.
See remediation →
72/100
Remediation priority · Elevated
Remotely reachable No privileges

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 · unedited
Prior to Apache Commons Net 3.9.0, Net's FTP client trusts the host from PASV response by default. A malicious server can redirect the Commons Net code to use a different host, but the user has to connect to the malicious server in the first place. This may lead to leakage of information about services running on the private network of the client. The default in version 3.9.0 is now false to ignore such hosts, as cURL does. See https://issues.apache.org/jira/browse/NET-711.

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 · high confidence

Apache Commons Net FTP client prior to version 3.9.0 trusts the host address in PASV (passive mode) responses by default. A malicious FTP server can send a crafted PASV response redirecting the client to an arbitrary host (such as internal network addresses), causing the client to leak information about services running on the client's private network.

MitigationUpgrade Apache Commons Net to version 3.9.0 or later, which changes the default behavior to ignore hosts in PASV responses (similar to cURL).

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
Commons NetApplication
Affected:< 3.9.0
Debian LinuxOperating system
Affected:= 10.0= 11.0

CVSS 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
Low
Privileges
None
User interaction
Required
Scope
Unchanged
Confidentiality
High
Integrity
None
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:N/A:N

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 checks

Work through these to decide whether this CVE applies to you.

  1. Identify Apache Commons Net version in use
    For Java applications: check the commons-net jar file name in classpath or dependencies (pom.xml, build.gradle). For Debian systems: run `dpkg -l | grep commons-net` or `apt show libcommons-net-java`
    Affected if Version is present and less than 3.9.0
  2. Confirm passive mode FTP is enabled
    Review application code or configuration for FTPClient setPassiveMode(true) calls, or FTPConnectionMode.PASSIVE setting
    Affected if Passive mode is explicitly enabled or used by default in the application
  3. Check if custom host verification is implemented
    Search codebase for implementations of FTPClient.setAuthoritativeHost(boolean) or similar host validation logic around PASV responses
    Affected if No custom host validation exists and the library version is vulnerable (<3.9.0)
  4. Inspect FTP connection behavior
    Monitor network connections from the client application during FTP passive mode connections using tcpdump or similar tools
    Affected if Connections are attempted to hosts other than the original FTP server without explicit configuration

You are affected if Apache Commons Net version is below 3.9.0 and your application uses FTP passive mode without implementing custom host validation for PASV responses.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

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.

dbcve · scoped
Upgrade available Upgrade to 3.9.0 or later
Fixed in 3.9.0
Interim mitigation

Upgrade Apache Commons Net to version 3.9.0 or later, which changes the default behavior to ignore hosts in PASV responses (similar to cURL).

Recommended fix High confidence

Apache Commons Net 3.9.0

  1. 1. Identify all applications and dependencies that use Apache Commons Net library
  2. 2. For each project, check the current version of commons-net dependency (likely in pom.xml, build.gradle, or package manager files)
  3. 3. Upgrade Apache Commons Net to version 3.9.0 or later in your dependency management
  4. 4. Rebuild and redeploy all affected applications
  5. 5. Verify the fix by testing FTP connections with PASV mode to ensure functionality is maintained
  6. 6. Alternatively, if upgrade is not possible, set the system property 'ftp.disableStrictPassiveMode' to 'true' or programmatically call FTPClientConfig.setStrictParsing(false) to disable trusting the PASV response host
Caveat Changing the default behavior may break existing deployments that rely on the FTP client following the host in PASV responses; test thoroughly after upgrade

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Commons Net Scoped from the published advisory
  • Consultation1.0 h
  • Implementation1.0 h
  • Testing2.0 h
  • Review / QA1.0 h
5.0 hours of engineering $860
Get the upgrade done

An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,376.

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-37533 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • 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
  • No spam, self-promotion, credentials, or personal data