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SolrApplication · Apache

CVE-2021-27905

CRITICAL · 9.8 CVSS v3.1 Published 2021-04-13
Fix available
A fix is available. Upgrade to 8.8.2 or later.
See remediation →
100/100
Remediation priority · Urgent
High EPSS Remotely reachable No privileges Zero-click

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
The ReplicationHandler (normally registered at "/replication" under a Solr core) in Apache Solr has a "masterUrl" (also "leaderUrl" alias) parameter that is used to designate another ReplicationHandler on another Solr core to replicate index data into the local core. To prevent a SSRF vulnerability, Solr ought to check these parameters against a similar configuration it uses for the "shards" parameter. Prior to this bug getting fixed, it did not. This problem affects essentially all Solr versions prior to it getting fixed in 8.8.2.

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 Solr's ReplicationHandler (exposed at /replication) accepts 'masterUrl' and 'leaderUrl' parameters to designate remote replication sources. The application failed to validate these URLs against SSRF protections that exist for similar parameters like 'shards', allowing attackers to make Solr instance perform requests to arbitrary internal or external URLs. This affects all Solr versions prior to 8.8.2.

MitigationUpgrade to Solr 8.8.2 or later which implements proper URL validation for replication parameters, or restrict network access to the replication endpoint until patching is possible.

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
SolrApplication
Affected:< 8.8.2

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
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
High

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

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. Determine installed Apache Solr version
    Query the Solr admin API endpoint /solr/admin/info/system or check the version file in the Solr installation directory (such as CHANGES.txt, README.txt, or the VERSION file in the Solr root)
    Affected if The installed version is Apache Solr prior to 8.8.2 (versions 8.x.y where y < 2, or any 7.x or earlier version)
  2. Verify ReplicationHandler endpoint availability
    Make an HTTP GET request to /solr/replication on your Solr instance (the default port is 8983). This endpoint is enabled by default in standard Solr installations.
    Affected if The /solr/replication endpoint responds with replication handler configuration, indicating it is accessible without authentication restrictions
  3. Confirm masterUrl or leaderUrl parameters accepted
    Send a request to /solr/replication with a test parameter such as masterUrl=http://example.com or leaderUrl=http://example.com and observe if the handler processes the parameter without rejecting it due to URL validation
    Affected if The ReplicationHandler accepts arbitrary URLs in masterUrl or leaderUrl parameters without SSRF validation or rejection of non-whitelisted hosts
  4. Check for network-based mitigations
    Review firewall rules, reverse proxy configurations, or Solr security settings (such as rule-based authorization or network ACLs) that restrict access to the /replication endpoint
    Affected if No network-level or application-level restrictions exist to prevent unauthorized access to the replication endpoint from untrusted sources

Your environment is affected by CVE-2021-27905 if you are running Apache Solr version 8.8.1 or earlier, the /replication endpoint is externally accessible, and no URL validation or network restrictions are in place to prevent SSRF attacks via masterUrl or leaderUrl parameters.

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 8.8.2 or later
Fixed in 8.8.2
Interim mitigation

Upgrade to Solr 8.8.2 or later which implements proper URL validation for replication parameters, or restrict network access to the replication endpoint until patching is possible.

Recommended fix High confidence

8.8.2 or later

  1. 1. Download Apache Solr 8.8.2 or later from the official Apache Solr downloads page (https://solr.apache.org/downloads.html)
  2. 2. Stop the currently running Solr service
  3. 3. Create a complete backup of your Solr data directory and configuration files
  4. 4. Install the new Solr 8.8.2 (or later) version following standard upgrade procedures
  5. 5. Start the Solr service with the new version
  6. 6. Verify that the ReplicationHandler at '/replication' is functioning correctly with the SSRF protections now in place
  7. 7. Test that the masterUrl/leaderUrl parameters are properly validated against SSRF
Caveat Standard minor version upgrade - review Solr 8.8.x release notes for any behavioral changes, though this is primarily a security fix

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

Fix this in Solr Scoped from the published advisory
  • Consultation2.0 h
  • Implementation6.0 h
  • Testing4.0 h
  • Review / QA2.0 h
14.0 hours of engineering $2,440
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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-2021-27905 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