peass-ng/PEASS-ng · error · ArgumentException

illegal object in GetInstance:

Error message

illegal object in GetInstance: 

What it means

DerEnumerated.GetInstance mirrors DerBoolean.GetInstance: it accepts only null or an existing DerEnumerated instance. Any other object type produces this ArgumentException, with the runtime type name appended, because ASN.1 ENUMERATED cannot be represented by other Der* classes.

Source

Thrown at winPEAS/winPEASexe/winPEAS/3rdParty/BouncyCastle/asn1/DerEnumerated.cs:26

       : Asn1Object
    {
        private readonly byte[] bytes;
        private readonly int start;

        /**
         * return an integer from the passed in object
         *
         * @exception ArgumentException if the object cannot be converted.
         */
        public static DerEnumerated GetInstance(
            object obj)
        {
            if (obj == null || obj is DerEnumerated)
            {
                return (DerEnumerated)obj;
            }

            throw new ArgumentException("illegal object in GetInstance: " + Platform.GetTypeName(obj));
        }

        /**
         * return an Enumerated from a tagged object.
         *
         * @param obj the tagged object holding the object we want
         * @param explicitly true if the object is meant to be explicitly
         *              tagged false otherwise.
         * @exception ArgumentException if the tagged object cannot
         *               be converted.
         */
        public static DerEnumerated GetInstance(
            Asn1TaggedObject obj,
            bool isExplicit)
        {
            Asn1Object o = obj.GetObject();

            if (isExplicit || o is DerEnumerated)

View on GitHub (pinned to 53fb989abc)

Solutions

1. Use the type named in the exception message, e.g. DerInteger.GetInstance(obj), then convert with new DerEnumerated(intVal) if semantics allow
2. For tagged objects call GetInstance((Asn1TaggedObject)obj, true)
3. Confirm the ASN.1 schema marks the field ENUMERATED and the encoder complies
4. Catch ArgumentException and treat the field as invalid

Example fix

// before
var e = DerEnumerated.GetInstance(asn1Obj);
// after
var e = asn1Obj is DerEnumerated en ? en
      : asn1Obj is DerInteger i ? new DerEnumerated(i.PositiveValue)
      : throw new FormatException("not an ENUMERATED");

bool ok = obj == null || obj is DerEnumerated;

bool IsDerEnumerated(object o) => o is DerEnumerated;

try { var e = DerEnumerated.GetInstance(obj); }
catch (ArgumentException ex) { /* inspect type name in ex.Message, handle fallback */ }

When it happens

Trigger: Calling DerEnumerated.GetInstance(obj) with a DerInteger, DerSequence, DerOctetString, or any non-DerEnumerated object; parsing an ASN.1 field declared ENUMERATED that arrived encoded as a different type.

Common situations: Parsing X.509 extensions, CRL reason codes, or PKCS structures where an ENUMERATED field is unexpectedly an INTEGER (some encoders emit INTEGER for enumerated values); using the wrong GetInstance overload for tagged objects.

Related errors

• illegal object in GetInstance:
• unknown object encountered in constructed OCTET STRING:
• unknown tag {tagNo} encountered
• unexpected end-of-contents marker
• indefinite-length primitive encoding encountered

AI-assisted analysis of peass-ng/PEASS-ng@53fb989abc (2026-09-02). Data as JSON: /api/errors/39f78b6878e5eaa2. Report an issue: GitHub.