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

What is CVE-2026-18401?

CVE-2026-18401 is a medium-severity denial of service vulnerability in FasterXML jackson-core, published on 4 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official 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.

Weakness: CWE-770

Which versions are affected by CVE-2026-18401?

6 open source packages ship an affected version, including com.fasterxml.jackson.core:jackson-core and tools.jackson.core:jackson-core.

How do I fix CVE-2026-18401?

Upgrade com.fasterxml.jackson.core:jackson-core to a fixed version for your release line: 2.18.6 or 2.21.1.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-18401 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-18401 exists.

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as CLEANSTART-2026-AU08981, CLEANSTART-2026-BR90501, CLEANSTART-2026-FD40389, CLEANSTART-2026-GT01967, CLEANSTART-2026-HK45779, CLEANSTART-2026-HQ66440 and 136 more.