CVE-2025-33042 — Apache Avro Java SDK: Schema-Based Code Injection
CVE Number: CVE-2025-33042
Vulnerability Name: Apache Avro Java SDK — Code Injection via Schema
Severity Score: Not yet published in official scoring systems
Exploitability Rating: Requires specific attack path but delivers high impact
Exploit Code Status: No widely accepted proof-of-concept published, but exploitation paths exist
Summary
CVE-2025-33042 is a vulnerability in the Java implementation of Apache Avro that affects how the library turns schema definitions into executable code. The flaw exists where Avro generates Java source code from schema descriptions and then compiles that code at runtime.
When applications accept schema input from untrusted or external sources and process them through Avro’s code generation path, an attacker can inject crafted content that subtly changes the structure of the generated Java class. Because this generated code is compiled and loaded within the running application, those injected instructions can run in the context of the hosting JVM — leading to unwanted behavior, including remote execution of arbitrary operations.
This vulnerability does not affect simple deserialization of Avro data alone; it becomes a risk only when untrusted schemas are used to derive and compile Java classes.
Impact
- Remote Code Execution: If exploited, arbitrary Java code can be executed in the process that performs schema generation and compilation, effectively giving the attacker control over that process.
- Data Compromise: An attacker able to run arbitrary code can read, alter, or exfiltrate sensitive in-memory data.
- Service Disruption: Injected behaviors may crash the host process or corrupt application state.
- Privilege Escalation: In environments where the JVM runs with elevated privileges, a successful injection can provide unauthorized access to system resources.
Vulnerability Mechanism
At a high level, Avro supports generating Java classes at runtime based on schema definitions. These definitions are normally trusted internal artifacts or vetted schemas committed in application repositories. Internally, Avro’s code generator composes Java source text by interpreting names, defaults, and metadata from the schema. If parts of that schema can be influenced by an attacker, those values may inadvertently become part of the generated Java source.
If those values are not sufficiently constrained or sanitized, they can produce Java constructs that were never intended by the original application developer. For example, an attacker could supply a field or namespace that contains code-like text. When Avro emits the Java source file, that text becomes part of a method body or field initializer, and once compiled, executes accordingly.
Conditions Required for Exploitation
An exploit of this vulnerability requires:
- Schema Generation at Runtime: The application must take schema definitions and generate Java code during execution. This is common in dynamic ingestion systems or tools that introspect arbitrary schemas to generate optimized Java classes.
- Untrusted Schema Input: The schema must be controllable or influenced by an attacker. This typically requires the existence of a schema upload endpoint or ingestion flow that does not strictly validate schema content.
If an organization uses Avro strictly for data exchange using pre-defined schemas (compiled and vetted at build time), the risk is negligible.
High-Risk Scenarios
The vulnerability becomes particularly dangerous in environments such as:
- Multi-tenant schema registries servicing external partners
- Data ingestion APIs that accept user-provided schemas
- Streaming systems that generate schemas on demand
- Continuous integration builds that generate code from inbound artifacts
In these contexts, an attacker could craft a malicious schema and trigger the generation of Java classes that execute injected content.
Proof of Concept (POC)
As of now, there is no widely published or standardized proof-of-concept code circulating in public exploit repositories. However, security researchers and practitioners have demonstrated that once an attacker can influence schema text that feeds into the Avro generator, the resulting Java source can be shaped to perform arbitrary operations.
Finding Signs of Exploitation
Detecting exploitation requires attention to both input events (schema submissions) and runtime behavior (Java process actions). Monitoring must span application logs, host audit data, and telemetry from runtime environments.
Below are key indicators and detection strategies:
Schema Submission Monitoring
Look for abnormal patterns in requests that submit or update schemas. Suspicious signs include:
- Schemas with unusual names or unexpected characters
- Large or deeply nested schemas
- Repeated submissions from the same source
Example query (Elasticsearch):
event.kind: "schema_upload" AND schema.content: /[^\w\s]/
| stats count() by client_ip, schema.name
| where count_ > 10
Runtime Generation Detection
Track when the Java process emits code or invokes compilation routines that are not part of regular application flow.
Example query (Splunk):
index=runtime_logs (generating OR generated) "SpecificCompiler"
| table _time host message
This flags events where Avro’s specific compiler APIs are active unexpectedly.
Host Process Behavior
Host execution logs often reveal unexpected process trees. Watch for:
- The Java process spawning shell interpreters
- Rapid invocation of the Java compiler from runtime contexts
- Executables triggering outside the known build path
Host audit query (Linux audit):
ausearch -m EXECVE -si java
| grep -E "(sh|bash|javac)"
This highlights any java process execution tree that invokes shell or compilation tools.
Network Telemetry
New outbound connections from application hosts shortly after schema uploads or code generation events may signal exploitation.
Track outbound flows from services that don’t normally initiate external communications.
Detection Queries
Below are practical detection queries usable in most observability platforms (generic format):
Application Log Detection
search log_type="application" AND "avro"
| stats count() BY source_file, message
| where count_ > 5
Use this to identify repetitive Avro generation events that correlate with unusual schema inputs.
JVM Compilation Events
search process_name="java" AND (compile OR compiler)
| stats count() BY process_id, user
| where count_ > 1
This flags repeated runtime compilation activities.
Host Level Suspicious Child Processes
process_name="java" AND child_process CONTAINS ("sh" OR "bash" OR "powershell")
| stats count() BY host
This evaluates cases where a Java process unexpectedly spawns system shells.
Mitigation and Remediation
The most effective remediation is to upgrade the Apache Avro library to a version where this issue is resolved. Every service, build environment, and container image that uses the affected Avro Java SDK should be updated to a patched release.
Official upgrade artifacts and checksums are available on the Apache Avro download page: https://avro.apache.org/project/download/
In addition to upgrading:
- Do not accept schema input from untrusted sources unless it is validated and sanitized.
- If dynamic code generation cannot be avoided, enforce strong schema validation at boundary layers.
- Deploy application monitoring that flags unexpected compilation and execution behavior.
Mapping to Weakness Taxonomy
This vulnerability is an example of a code generation injection issue where untrusted input influences generated source. It is similar in nature to vulnerabilities classified under improper generation of code.
Recognizing this pattern helps security teams prioritize review of other tools that incorporate runtime code assembly.
Final Takeaway
CVE-2025-33042 represents a serious but avoidable vulnerability that crops up when an application uses dynamic schema-driven code generation without validating who can supply schemas. Though a public exploit is not widely known today, the mechanics of the vulnerability mean that in the wrong hands, a crafted schema could become an execution vector.
To address it:
- Upgrade all affected Avro Java SDK usage via the official link: https://avro.apache.org/project/download/
- Monitor schema submissions and Java runtime behavior
- Raise alerts on unexpected compilation events and shell invocations
This approach minimizes risk and places detection controls where they are most effective.
