CVE-2021-27905 Overview
CVE-2021-27905 is a critical Server-Side Request Forgery (SSRF) vulnerability affecting the ReplicationHandler component in Apache Solr. The ReplicationHandler, which is normally registered at the /replication endpoint under a Solr core, accepts a masterUrl parameter (also known as leaderUrl) that designates another ReplicationHandler on a remote Solr core to replicate index data from. Prior to the security fix, Solr failed to validate these parameters against its existing configuration safeguards—the same protections used for the shards parameter—allowing attackers to abuse this endpoint to make arbitrary server-side HTTP requests.
Critical Impact
This SSRF vulnerability allows unauthenticated remote attackers to leverage Apache Solr servers to make arbitrary HTTP requests to internal services, potentially exposing sensitive data, internal infrastructure, and enabling further attacks on backend systems.
Affected Products
- Apache Solr versions prior to 8.8.2
Discovery Timeline
- April 13, 2021 - CVE-2021-27905 published to NVD
- November 21, 2024 - Last updated in NVD database
Technical Details for CVE-2021-27905
Vulnerability Analysis
The vulnerability exists within the ReplicationHandler component of Apache Solr, which handles data replication between Solr cores. The masterUrl (or leaderUrl) parameter allows specifying a remote Solr instance from which to replicate index data. The core issue is that Solr failed to implement proper validation and allowlist checking for these URL parameters before making outbound HTTP requests.
Apache Solr already had security mechanisms in place for the shards parameter to prevent SSRF attacks, but these same protections were not applied to the replication handler's URL parameters. This oversight meant that an attacker could craft malicious requests to the /replication endpoint, causing the Solr server to make HTTP requests to arbitrary internal or external destinations.
The SSRF vulnerability can be exploited to scan internal networks, access internal services that are not exposed to the internet, retrieve sensitive information from metadata endpoints (such as cloud provider instance metadata services), or potentially pivot to attack other internal systems.
Root Cause
The root cause of CVE-2021-27905 is improper input validation (CWE-918: Server-Side Request Forgery). The ReplicationHandler component accepted user-supplied URLs via the masterUrl and leaderUrl parameters without validating them against an allowlist or applying the same security controls used elsewhere in the codebase. This lack of consistent security controls across similar functionality allowed the SSRF condition to exist.
Attack Vector
An attacker can exploit this vulnerability by sending a crafted HTTP request to the Solr /replication endpoint with a malicious masterUrl parameter pointing to an internal service or arbitrary external resource. Since this is a network-based attack requiring no authentication and no user interaction, it can be exploited remotely by any attacker with network access to the Solr instance.
The attack flow typically involves:
- Identifying an exposed Apache Solr instance
- Crafting a request to the /replication endpoint with a malicious masterUrl parameter
- The Solr server makes an HTTP request to the attacker-specified URL
- Response data may be returned to the attacker or used to probe internal services
Since no code examples are available from verified sources, readers should consult the Apache Announcement Thread for technical details on the vulnerability mechanism.
Detection Methods for CVE-2021-27905
Indicators of Compromise
- Unusual HTTP requests to the /replication endpoint containing masterUrl or leaderUrl parameters pointing to internal IP ranges (e.g., 10.x.x.x, 172.16.x.x, 192.168.x.x, 169.254.169.254)
- Outbound HTTP connections from Solr servers to unexpected internal services or cloud metadata endpoints
- Access logs showing replication requests from external sources with non-standard destination URLs
- Network traffic from Solr instances to internal infrastructure that shouldn't be accessed via replication
Detection Strategies
- Monitor Solr access logs for requests to /replication endpoints containing suspicious masterUrl or leaderUrl parameters
- Implement network-level monitoring to detect outbound connections from Solr servers to internal services or cloud metadata endpoints (e.g., 169.254.169.254)
- Deploy web application firewall (WAF) rules to inspect and block replication requests with non-allowlisted URLs
- Use SentinelOne Singularity to monitor for anomalous network behavior from Solr processes
Monitoring Recommendations
- Enable detailed access logging on all Apache Solr instances to capture full request parameters
- Configure alerts for replication endpoint access from external IP addresses
- Monitor egress traffic from Solr servers for connections to internal IP ranges or sensitive endpoints
- Implement network segmentation monitoring to detect lateral movement attempts via SSRF
How to Mitigate CVE-2021-27905
Immediate Actions Required
- Upgrade Apache Solr to version 8.8.2 or later immediately
- Restrict network access to Solr instances, limiting exposure to only trusted clients
- Implement network-level controls to prevent Solr servers from making arbitrary outbound HTTP requests
- Review Solr access logs for evidence of exploitation attempts prior to patching
Patch Information
Apache has addressed this vulnerability in Solr version 8.8.2 by implementing proper validation for the masterUrl and leaderUrl parameters, applying the same allowlist-based controls that were already in use for the shards parameter. Organizations should upgrade to version 8.8.2 or later to remediate this vulnerability. For more details, refer to the Apache Announcement Thread and the NetApp Security Advisory.
Workarounds
- Deploy a reverse proxy or web application firewall in front of Solr to filter requests containing masterUrl or leaderUrl parameters
- Implement network segmentation to isolate Solr servers and limit their ability to reach internal services
- Disable or restrict access to the /replication endpoint if replication functionality is not required
- Use firewall rules to block outbound HTTP connections from Solr servers to internal IP ranges
# Example: Block Solr from accessing internal networks via iptables
iptables -A OUTPUT -m owner --uid-owner solr -d 10.0.0.0/8 -j DROP
iptables -A OUTPUT -m owner --uid-owner solr -d 172.16.0.0/12 -j DROP
iptables -A OUTPUT -m owner --uid-owner solr -d 192.168.0.0/16 -j DROP
iptables -A OUTPUT -m owner --uid-owner solr -d 169.254.169.254 -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
