peass-ng/PEASS-ng · error · ArgumentException

illegal object in GetInstance:

Error message

illegal object in GetInstance: 

What it means

DerBoolean.GetInstance is BouncyCastle's strict conversion helper: it only accepts null or an object that is already a DerBoolean. Passing any other object type (e.g. DerInteger, DerOctetString, a raw bool/byte[]) makes the parser throw this ArgumentException with the offending type name appended to the message.

Source

Thrown at winPEAS/winPEASexe/winPEAS/3rdParty/BouncyCastle/asn1/DerBoolean.cs:27

        private readonly byte value;

        public static readonly DerBoolean False = new DerBoolean(false);
        public static readonly DerBoolean True = new DerBoolean(true);

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

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

        /**
         * return a DerBoolean from the passed in bool.
         */
        public static DerBoolean GetInstance(
            bool value)
        {
            return value ? True : False;
        }

        /**
         * return a Boolean 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

View on GitHub (pinned to 53fb989abc)

Solutions

1. Inspect the actual type in the message (Platform.GetTypeName output) and use the matching Der* GetInstance, e.g. DerInteger.GetInstance
2. If the value is inside a tagged object, call GetInstance((Asn1TaggedObject)obj, true) so tagging is unwrapped first
3. Verify the ASN.1 structure you are parsing actually defines this field as BOOLEAN
4. Wrap parsing in try/catch ArgumentException and treat the element as absent/malformed

Example fix

// before
var b = DerBoolean.GetInstance(asn1Obj);
// after
if (asn1Obj is DerBoolean db) { var b = db; }
else if (asn1Obj is Asn1TaggedObject t) { var b = DerBoolean.GetInstance(t, true); }
else { /* handle wrong type */ }

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

bool IsDerBoolean(object o) => o is DerBoolean;

try { var b = DerBoolean.GetInstance(obj); }
catch (ArgumentException ex) { /* log ex.Message, treat field as invalid */ }

When it happens

Trigger: Calling DerBoolean.GetInstance(obj) with an Asn1Object that is not a DerBoolean (e.g. a DerInteger or Asn1Null parsed from an ASN.1 stream), or passing a non-null object when expecting a BOOLEAN-tagged element that was actually encoded as a different ASN.1 type.

Common situations: Parsing X.509/CRL/CMS structures where a field is misinterpreted or a different certificate profile encodes a value as INTEGER/NULL instead of BOOLEAN; mixing up GetInstance(object) vs GetInstance(Asn1TaggedObject, bool) overloads.

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/98404a128c3a0297. Report an issue: GitHub.