CVE-2026-119195

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, constructed string and indefinite-length ANY decoding repeatedly concatenates each fragment onto a growing bytes value. Attacker-controlled BER or CER data can use many constructed OCTET STRING or character-string fragments. DER rejects constructed OCTET STRING without a schema, but accepts its definite-length form when asn1Spec selects univ.OctetString; it also accepts constructed character strings such as UTF8String without a schema, even though those encodings are not valid DER. The separate indefinite-length ANY path is reachable in BER and CER when asn1Spec selects univ.Any; DER rejects indefinite lengths. OctetStringPayloadDecoder uses growing header concatenation and AnyPayloadDecoder.indefLenValueDecoder uses growing chunk concatenation, repeatedly copying an increasing prefix and causing quadratic work. A sufficiently fragmented value can occupy decoding workers and cause CPU exhaustion without confidentiality or integrity impact. This issue is fixed in version 0.6.5.

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, constructed string and indefinite-length ANY decoding repeatedly concatenates each fragment onto a growing bytes value. Attacker-controlled BER or CER data can use many constructed OCTET STRING or character-string fragments. DER rejects constructed OCTET STRING without a schema, but accepts its definite-length form when asn1Spec selects univ.OctetString; it also accepts constructed character strings such as UTF8String without a schema, even though those encodings are not valid DER. The separate indefinite-length ANY path is reachable in BER and CER when asn1Spec selects univ.Any; DER rejects indefinite lengths. OctetStringPayloadDecoder uses growing header concatenation and AnyPayloadDecoder.indefLenValueDecoder uses growing chunk concatenation, repeatedly copying an increasing prefix and causing quadratic work. A sufficiently fragmented value can occupy decoding workers and cause CPU exhaustion without confidentiality or integrity impact. This issue is fixed in version 0.6.5.

Summary

In pyasn1 0.6.4 and earlier, decoding a constructed OCTET STRING or character string made of many fragments can spend disproportionate CPU time repeatedly copying the accumulated value. A separate indefinite-length ANY path has the same problem while collecting the encoded components. The work grows roughly quadratically with the number of fixed-size fragments, although the input and decoded value grow linearly. An application that lets an attacker supply such ASN.1 input to an affected decoder can suffer CPU exhaustion.

The affected string paths are available in BER and CER. The DER decoder also accepts certain non-DER constructed strings: a definite-length constructed OCTET STRING when an asn1Spec selects univ.OctetString, and constructed character strings such as UTF8String even without a schema. Indefinite-length ANY is affected in BER and CER when an asn1Spec selects univ.Any; DER rejects indefinite lengths.

This is a distinct decoder path from the OBJECT IDENTIFIER/RELATIVE-OID accumulation addressed by CVE-2026-59885. It has the same complexity class, but the OID fix in 0.6.4 did not change these string or ANY loops.

Technical details

In the v0.6.4 BER decoder, OctetStringPayloadDecoder appends each decoded fragment to a growing bytes value with header += component in both its definite-length and indefinite-length constructed forms. AnyPayloadDecoder.indefLenValueDecoder similarly uses chunk += component. In the exercised paths, each concatenation copies an increasingly large prefix. For N fixed-size fragments, the total copied data grows with 1 + 2 + ... + N, or O(N²).

The OCTET STRING decoder is also the base class of the decoders for UTF8String, NumericString, PrintableString, TeletexString, VideotexString, IA5String, GraphicString, VisibleString, GeneralString, UniversalString, BMPString, ObjectDescriptor, GeneralizedTime, and UTCTime. These types inherit the same constructed-string reassembly path.

The DER decoder's OCTET STRING restriction replaces the tag-driven decoder, but its schema-driven TYPE_MAP retains the BER-derived decoder. Thus der.decode(data) rejects a constructed OCTET STRING, while der.decode(data, asn1Spec=univ.OctetString()) accepts its definite-length form and reaches the accumulation loop. DER does not replace the inherited character-string decoders, so a constructed UTF8String is also accepted without asn1Spec. These inputs violate X.690's DER string-encoding rule and should not be described as valid DER encodings.

The original short-fragment, definite-length OCTET STRING reproducer is valid BER. The CER decoder accepts it, but it is not a conforming CER string encoding: X.690 requires an indefinite-length outer constructed string and 1,000 contents octets per fragment except possibly the last. A conforming CER-shaped constructed OCTET STRING with those fragment sizes also reaches the loop and shows quadratic growth. The indefinite-length ANY reproducer below can represent a CER SEQUENCE OF OCTET STRING and reaches the separate ANY loop when decoded with an ANY schema.

The decoder's eight-octet length-field limit and nesting-depth limit do not bound the number of sibling fragments in these examples. The BER reproducer below uses only a three-octet outer length field; all fragments have shallow nesting. There is no per-message fragment-count cap in v0.6.4.

Impact

If an application passes…

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, constructed string and indefinite-length ANY decoding repeatedly concatenates each fragment onto a growing bytes value. Attacker-controlled BER or CER data can use many constructed OCTET STRING or character-string fragments. DER rejects constructed OCTET STRING without a schema, but accepts its definite-length form when asn1Spec selects univ.OctetString; it also accepts constructed character strings such as UTF8String without a schema, even though those encodings are not valid DER. The separate indefinite-length ANY path is reachable in BER and CER when asn1Spec selects univ.Any; DER rejects indefinite lengths. OctetStringPayloadDecoder uses growing header concatenation and AnyPayloadDecoder.indefLenValueDecoder uses growing chunk concatenation, repeatedly copying an increasing prefix and causing quadratic work. A sufficiently fragmented value can occupy decoding workers and cause CPU exhaustion without confidentiality or integrity impact. This issue is fixed in version 0.6.5.

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, constructed string and indefinite-length ANY decoding repeatedly concatenates each fragment onto a growing bytes value. Attacker-controlled BER or CER data can use many constructed OCTET STRING or character-string fragments. DER rejects constructed OCTET STRING without a schema, but accepts its definite-length form when asn1Spec selects univ.OctetString; it also accepts constructed character strings such as UTF8String without a schema, even though those encodings are not valid DER. The separate indefinite-length ANY path is reachable in BER and CER when asn1Spec selects univ.Any; DER rejects indefinite lengths. OctetStringPayloadDecoder uses growing header concatenation and AnyPayloadDecoder.indefLenValueDecoder uses growing chunk concatenation, repeatedly copying an increasing prefix and causing quadratic work. A sufficiently fragmented value can occupy decoding workers and cause CPU exhaustion without confidentiality or integrity impact. This issue is fixed in version 0.6.5.

pyasn1 is a generic ASN.1 library for Python. Prior to 0.6.5, constructed string and indefinite-length ANY decoding repeatedly concatenates each fragment onto a growing bytes value. Attacker-controlled BER or CER data can use many constructed OCTET STRING or character-string fragments. DER rejects constructed OCTET STRING without a schema, but accepts its definite-length form when asn1Spec selects univ.OctetString; it also accepts constructed character strings such as UTF8String without a schema, even though those encodings are not valid DER. The separate indefinite-length ANY path is reachable in BER and CER when asn1Spec selects univ.Any; DER rejects indefinite lengths. OctetStringPayloadDecoder uses growing header concatenation and AnyPayloadDecoder.indefLenValueDecoder uses growing chunk concatenation, repeatedly copying an increasing prefix and causing quadratic work. A sufficiently fragmented value can occupy decoding workers and cause CPU exhaustion without confidentiality or integrity impact. This issue is fixed in version 0.6.5.