How severe is CVE-2021-27905?

CVE-2021-27905 has a CVSS score of 9.8/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-27905?

CVE-2021-27905 affects the following packages: org.apache.solr:solr-parent (Maven). Ecosystems affected: Maven.

How do I fix CVE-2021-27905?

Update org.apache.solr:solr-parent to version 8.8.2 or later. Run your dependency manager's update command and verify no transitive dependencies pin the vulnerable version. Use O3 Security's SCA scanner to confirm the fix is applied across your entire dependency tree.

Is CVE-2021-27905 actively exploited in the wild?

Yes. There are 7 known exploit references for CVE-2021-27905, including 4 in-the-wild exploitations observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-27905?

CVE-2021-27905 has an EPSS (Exploit Prediction Scoring System) score of 93.9%, placing it in the 100th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. A score above 50% indicates significantly elevated exploitation risk — immediate patching is strongly recommended.

What type of vulnerability is CVE-2021-27905?

CVE-2021-27905 is classified as Server-Side Request Forgery (SSRF) (CWE-918). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

Is there exploit tooling available for CVE-2021-27905?

A Nuclei detection template exists for CVE-2021-27905, meaning automated scanners can fingerprint it. Public exploit tooling significantly increases real-world risk — prioritize patching and ensure detection coverage. Exploit tooling should only ever be used in isolated, authorized testing environments.

When was CVE-2021-27905 published, and has it been updated?

CVE-2021-27905 was published on April 13, 2021 and was last updated on April 10, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕ Maven

CVE-2021-27905

CRITICAL

Server-Side Request Forgery in Apache Solr

Also known asBIT-solr-2021-27905GHSA-5phw-3jrp-3vj8

Published

Apr 13, 2021

Updated

Apr 10, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

7 known

EPSS Exploitation Probability

via FIRST.org ↗

93.9%probability of exploitation in next 30 days

Very High Risk100th percentile0.00%

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

Blast Radius

1 pkg affected

☕org.apache.solr:solr-parent

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

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.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.apache.solr:solr-parent`	all versions	8.8.2

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

The ReplicationHandler (normally registered at "/replication" under a So…

github.comGithubApr 2021

The ReplicationHandler (normally registered at "/replication" under a So…

github.comGithubAug 2021

The ReplicationHandler (normally registered at "/replication" under a So…

github.comGithubNov 2022

The ReplicationHandler (normally registered at "/replication" under a So…

unit42.paloaltonetworks.comAPIApr 2021

Frequently Asked Questions

O3 Security · Impact-Aware SCA

Is CVE-2021-27905 in your stack?

O3 detects CVE-2021-27905 across Maven dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

Scan my stack How O3 SCA works

CVSS Score

9.8/10

CVSS v3 base score

Vector breakdown

AV:Attack Vector

Network

AC:Attack Complexity

Low

PR:Privileges Required

None

UI:User Interaction