CVE-2026-55976: 
Homebrew vulnerability analysis and mitigation

CVE-2026-55976 is a Server-Side Request Forgery (SSRF) vulnerability in the Avro SerDe schema resolution feature of Apache Hive. It affects Apache Hive versions 2.1.0 through 4.2.0 (i.e., all versions before 4.2.1), allowing an authenticated remote attacker with CREATE TABLE privilege to cause the Hive server to fetch an attacker-controlled URL via the avro.schema.url table property. The vulnerability was published on August 25, 2026, with a patch released in Apache Hive 4.2.1 on August 26, 2026. It carries a CVSS v3.1 base score of 9.1 (Critical) (GitHub Advisory, Apache Hive Commit).

The root cause is insufficient input validation on the avro.schema.url table property in the Avro SerDe (Serializer/Deserializer) component of Apache Hive, classified as CWE-918 (Server-Side Request Forgery). When an Avro table is created or altered with a malicious avro.schema.url value pointing to an attacker-controlled endpoint, the Hive server fetches that URL at query time to resolve the schema — without adequately restricting the URL scheme or destination host. The fix (HIVE-29671) introduces new configuration parameters (hive.avro.schema.url.allowed.schemes, hive.avro.schema.url.remote.http.enabled, hive.avro.schema.url.http.allowed.hosts) to enforce an allowlist of permitted URI schemes and hosts, and adds authorization checks for schema URL read entities during CREATE/ALTER TABLE and SELECT operations (GitHub Advisory, Apache Hive Commit, Apache Jira).

Successful exploitation allows an attacker to make the Hive server issue arbitrary HTTP/HTTPS or file:// requests from the server's network identity, exposing cloud instance metadata (e.g., AWS IMDSv1 at 169.254.169.254), internal network services not otherwise reachable by the attacker, and local server files accessible to the Hive process. The confidentiality and integrity impacts are rated High, as sensitive credentials, tokens, or configuration data could be exfiltrated, and the attacker could potentially influence internal service interactions. Availability is not directly impacted, but lateral movement within cloud or internal networks is a realistic secondary risk (GitHub Advisory).

There is no public proof-of-concept exploit and no evidence of in-the-wild exploitation at the time of disclosure (GitHub Advisory). The EPSS score is approximately 0.285% (0.587% per GitHub Advisory), placing it in the 45th percentile for exploitation likelihood within 30 days. The NVD SSVC assessment notes the attack is automatable with total technical impact, though no exploitation has been observed. The vulnerability is not currently listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. No threat actor attribution has been reported.

1. Authenticate to HiveServer2: Obtain valid Hive credentials for an account with CREATE TABLE (DDL) privileges on any database. No admin rights are required.
2. Create a malicious Avro table: Execute a DDL statement that sets avro.schema.url to an attacker-controlled URL, such as a cloud metadata endpoint or an internal service:

   CREATE EXTERNAL TABLE ssrf_test
   ROW FORMAT SERDE 'org.apache.hadoop.hive.serde2.avro.AvroSerDe'
   STORED AS INPUTFORMAT 'org.apache.hadoop.hive.ql.io.avro.AvroContainerInputFormat'
   OUTPUTFORMAT 'org.apache.hadoop.hive.ql.io.avro.AvroContainerOutputFormat'
   LOCATION '/tmp/ssrf_test'
   TBLPROPERTIES ('avro.schema.url'='http://169.254.169.254/latest/meta-data/');

3. Trigger schema resolution: Issue a SELECT query against the malicious table to cause the Hive server to fetch the attacker-controlled URL during schema resolution:

   SELECT * FROM ssrf_test LIMIT 1;

4. Capture the response: Monitor the attacker-controlled server (or observe error messages) to retrieve the fetched content — which may include cloud instance metadata, IAM credentials, or internal service responses.
5. Pivot or escalate: Use retrieved credentials or metadata (e.g., AWS IAM role tokens) to access additional cloud resources or internal services for lateral movement (GitHub Advisory, Apache Hive Commit).

• Metastore/DDL: Avro tables in the Hive metastore with avro.schema.url set to http://, https://, file://, or ftp:// schemes, especially pointing to link-local addresses (e.g., 169.254.169.254), internal hostnames, or unexpected external hosts.
• Logs: HiveServer2 and Metastore logs showing CREATE TABLE or ALTER TABLE statements that set avro.schema.url to non-standard or suspicious URLs; schema-resolution failures or outbound HTTP fetch attempts logged around query execution time.
• Network: Outbound HTTP/HTTPS connections from the Hive server host to cloud metadata endpoints (e.g., 169.254.169.254, fd00:ec2::254), internal service IPs, or unexpected external hosts shortly after Avro DDL or SELECT activity; VPC/instance flow logs showing Hive host identity making unusual requests.
• Correlation: CREATE TABLE or ALTER TABLE activity setting avro.schema.url followed closely by SELECT queries on the same Avro table from the same or different user accounts (GitHub Advisory).

Upgrade Apache Hive to version 4.2.1 or later, which introduces URI scheme allowlisting and authorization checks for avro.schema.url (GitHub Advisory, Apache Hive Commit). As interim mitigations: restrict CREATE TABLE and ALTER TABLE privileges to trusted users only; audit the metastore for existing Avro tables with suspicious avro.schema.url values and remove or correct them; implement network egress controls on the Hive server host to block outbound connections to cloud metadata endpoints (e.g., 169.254.169.254) and unauthorized internal services. After upgrading, configure hive.avro.schema.url.allowed.schemes to restrict permitted URI schemes, and leave hive.avro.schema.url.remote.http.enabled set to false (the secure default) unless HTTP schema URLs are explicitly required.