CVE-2026-89425 in jackson-core
Summary
by MITRE • 09/23/2026
UTF8DataInputJsonParser._reportInvalidToken() in FasterXML jackson-core builds the offending-token text for its error message by appending Java identifier characters to a StringBuilder in a loop that has no upper bound. Unlike the three sibling parser implementations, including UTF8StreamJsonParser, it never consults ErrorReportConfiguration.getMaxErrorTokenLength() (default 256). A malformed token supplied to a parser created through JsonFactory.createParser(DataInput) is therefore accumulated in full. No StreamReadConstraints setting mitigates this: maxDocumentLength cannot be applied to DataInput sources at all, and maxStringLength does not cover this path because the accumulation bypasses ReadConstrainedTextBuffer. The reporter measured a 20,000,109-character exception message from a 20-million-character malformed token on the DataInput path, against 367 characters for identical input on the InputStream path. Scaling the payload drives the StringBuilder, which also incurs byte-to-char expansion and internal array doubling, to many times the raw payload size and can trigger OutOfMemoryError for the whole JVM. UTF8DataInputJsonParser was introduced in 2.8.0 together with createParser(DataInput); releases before 2.8.0 do not contain the affected class.
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Analysis
by VulDB Data Team • 09/23/2026
The vulnerability identified in FasterXML jackson-core centers on a critical resource exhaustion flaw within the UTF8DataInputJsonParser implementation, specifically triggered by the _reportInvalidToken method introduced in version 2.8.0. This component is responsible for parsing JSON data from Java DataInput streams and serves as an entry point for applications that rely on this specific input mechanism rather than standard InputStream or Reader sources. The core technical flaw lies in how error messages are constructed when a malformed token is encountered during the parsing process. Unlike its sibling parser implementations such as UTF8StreamJsonParser, which adhere to configured limits via ErrorReportConfiguration.getMaxErrorTokenLength(), the DataInput variant fails to consult these constraints. Consequently, when an invalid or excessively long token is detected, the system attempts to build a complete representation of that offending token for inclusion in the error message without any upper bound on length. This oversight means that every character of a maliciously crafted input string is accumulated into memory regardless of its size, creating a direct pathway for resource abuse.
The operational impact of this flaw is severe and manifests primarily as an OutOfMemoryError capable of crashing the entire Java Virtual Machine rather than just terminating the current thread or request. The reporter demonstrated that supplying a malformed token of approximately twenty million characters results in an exception message containing over twenty million one hundred thousand nine characters, whereas identical input processed through the InputStream path is correctly truncated to three hundred sixty-seven characters as per default configuration limits. This discrepancy highlights a significant inconsistency in how different parser implementations handle error reporting and resource management. The situation is exacerbated by the fact that standard StreamReadConstraints settings do not mitigate this specific attack vector. Specifically, maxDocumentLength cannot be applied to DataInput sources due to architectural limitations within the library, and maxStringLength fails to protect against this path because the token accumulation logic bypasses the ReadConstrainedTextBuffer mechanism entirely. This leaves a gap in defense-in-depth strategies that rely on these global constraints for protection against oversized inputs.
From a technical perspective, the vulnerability exploits the internal mechanics of Java's StringBuilder class during error handling. As the parser accumulates characters from the malformed token to construct the error message, it incurs byte-to-character expansion and triggers repeated internal array doubling operations within the StringBuilder implementation. These memory allocation patterns multiply the actual payload size significantly in RAM, accelerating the exhaustion of heap space. This behavior aligns with Common Weakness Enumeration CWE-400, which describes uncontrolled resource consumption leading to denial of service conditions. Furthermore, this attack vector can be mapped to MITRE ATT&CK technique T1496, Resource Hijacking, where an attacker consumes system resources such as memory or CPU cycles to degrade the availability of services for legitimate users. The lack of input validation during error message generation transforms a standard parsing exception into a potent denial-of-service weapon against any application utilizing jackson-core with DataInput-based parsers and relying on default configurations without explicit overrides for this specific edge case.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. For organizations currently using affected versions of jackson-core, the most effective solution is to upgrade to a patched release where this inconsistency has been resolved by enforcing token length limits across all parser implementations including those based on DataInput. In environments where upgrading is not immediately feasible, developers should avoid using JsonFactory.createParser(DataInput) for untrusted or externally supplied JSON data and instead utilize InputStream-based parsers which correctly enforce StreamReadConstraints. Additionally, implementing custom error handling wrappers that catch exceptions from the jackson library can prevent the propagation of excessively large exception messages to logs or client responses, thereby reducing the memory footprint during failure scenarios. It is also advisable to configure JVM heap sizes with appropriate limits and monitoring to detect rapid memory consumption indicative of such attacks early in their lifecycle. Since this vulnerability only affects releases starting from version 2.8.0, older versions are not susceptible but may lack other security improvements present in newer releases, making a comprehensive upgrade the recommended path forward for maintaining robust application security posture against resource exhaustion threats.