CVE-2026-18401 — jackson-core
Fix: FasterXML/jackson-core@b0c428eCVE-2026-18401 is a CWE-770 vulnerability in tools.jackson.core:jackson-core. A fix is available for tools.jackson.core:jackson-core — see the affected versions and patch details below.
jackson-core: Number length constraint bypass in non-blocking (async) JSON parser leads to potential denial of service
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-18401.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Real-World Exposure
tools.jackson.core:jackson-core☕com.fasterxml.jackson.core:jackson-core☕com.fasterxml.jackson.core:jackson-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| ☕Maven | tools.jackson.core:jackson-core | ≥ 3.0.0&&< 3.1.0 | 3.1.0tools.jackson.core:jackson-core:3.1.0 |
| ☕Maven | com.fasterxml.jackson.core:jackson-core | ≥ 2.19.0&&< 2.21.1 | 2.21.1com.fasterxml.jackson.core:jackson-core:2.21.1 |
| ☕Maven | com.fasterxml.jackson.core:jackson-core | ≥ 2.15.0&&< 2.18.6 | 2.18.6com.fasterxml.jackson.core:jackson-core:2.18.6 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for tools.jackson.core:jackson-core, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update tools.jackson.core:jackson-core to 3.1.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-18401 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-18401 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-18401. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-18401 in your dependencies?
O3 Security finds CVE-2026-18401 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.