]> CryptoNext Security

daniel.vangeest@cryptonext-security.com

MTG AG

falko.strenzke@mtg.de

Security Limited Additional Mechanisms for PKIX and SMIME Cryptographic Message Syntax CMS Signed Attributes signedAttrs SignedData The Cryptographic Message Syntax (CMS) has different signature verification behaviour based on whether signed attributes are present or not. This results in a potential existential forgery vulnerability in CMS and protocols which use CMS. This document describes the vulnerability and lists mitigations and best practices to avoid it. This document updates RFC 5652 by prohibiting the use of the id-data content type for new uses of the CMS SignedData type. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Limited Additional Mechanisms for PKIX and SMIME Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

Introduction The Cryptographic Message Syntax (CMS) signed-data content type allows any number of signers in parallel to sign any type of content. CMS gives a signer two options when generating a signature on some content:

• Generate a signature on the whole content; or
• Compute a hash over the content, place this hash in the message-digest attribute in the SignedAttributes type, and generate a signature on the SignedAttributes. The SignedAttributes type is placed in the signedAttrs field of the SignedData type.

The resulting signature does not commit to the presence of the SignedAttributes type, allowing an attacker to influence verification behaviour. An attacker can perform two different types of attacks:

1. Take an arbitrary CMS signed message M which was originally signed with SignedAttributes present and rearrange the structure such that the SignedAttributes field is absent and the original DER-encoded SignedAttributes appears as an encapsulated or detached content of type id-data, thereby crafting a new structure M' that was never explicitly signed by the signer. M' has the DER-encoded SignedAttributes of the original message as its content and verifies correctly against the original signature of M.
2. Let the signer sign a message of the attacker's choice without SignedAttributes. The attacker chooses this message to be a valid DER-encoding of a SignedAttributes object. The attacker can then add this encoded SignedAttributes object to the signed message and change the signed message to the one that was used to create the messageDigest attribute within the SignedAttributes. The signature created by the signer is valid for this arbitrary attacker-chosen message.

This vulnerability was presented by Falko Strenzke to the LAMPS working group at IETF 121 and is detailed in . states:

• signedAttrs is a collection of attributes that are signed. The field is optional, but it MUST be present if the content type of the EncapsulatedContentInfo value being signed is not id-data.

Thus, if a verifier accepts a content type of id-data in the EncapsulatedContentInfo type when used in SignedData, then a SignerInfo within the SignedData may or may not contain a signedAttrs field and the verifier is vulnerable to this attack. On the other hand, if the verifier doesn't accept a content type of id-data, the sender always adds the signedAttrs field, and the recipient verifies that signedAttrs is present, the attack will not succeed. The limited flexibility of either the signed or the forged message in either attack variant may mean the attacks are only narrowly applicable. Nevertheless, due to the wide deployment of the affected protocols and the use of CMS in many proprietary systems, the attacks cannot be entirely disregarded. As a mitigation, this document defines the new mimeData content type to be used in new uses of the CMS SignedData type when the encapsulated content is MIME encoded and thus avoid the use of the id-data content type. This document further describes best practices and mitigations that can also be applied to those protocols or systems that continue to use the content type id-data. This document's prohibition of the use of the id-data content type for new uses of SignedData does not retroactively affect the conformance of already-deployed implementations, which are instead addressed via the migration guidance in .

Conventions and Definitions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. For the purposes of this document, a new use of CMS SignedData is any specification that, from the date of publication of this document onward, first defines how SignedData is to be constructed or processed for a given application. This is regardless of whether that specification is published as an RFC or registers a content type identifier with IANA. An existing use is one that predates this document's publication; see for guidance applicable to existing uses.

mimeData Content Type The following object identifier identifies the mimeData content type: The mimeData content type is intended as a replacement for the data content type (id-data object identifier) in new uses of the CMS SignedData type where the content is MIME encoded. Like the data content type, the mimeData content type is encoded as an octet string. Unlike the data content type, the octet string MUST contain a MIME entity as defined in , i.e., MIME header fields followed by the body. The interpretation of the MIME entity is governed by its MIME headers.

Best Practices This section describes the best practices to avoid the vulnerability at the time of writing.

New Uses of the CMS SignedData type New uses of the CMS SignedData type MUST NOT use the id-data EncapsulatedContentInfo content type. If the new content is MIME encoded, the mimeData content type SHOULD be used unless the new use has reason to bind the signature to a more specific, purpose-built content type identifier (for example to avoid content-type confusion with unrelated applications using mimeData). If a new content type is defined, it might be appropriate to register it in the "CMS Inner Content Types" IANA subregistry within the "Media Type Sub-Parameter Registries" registry group. See for a related consideration regarding the key pair used to sign under the new protocol.

Existing Uses of id-data in CMS SignedData When a protocol which uses the id-data EncapsulatedContentInfo content type within SignedData is updated, it SHOULD deprecate the use of id-data and use a different (new or existing) identifier. A partial list of such identifiers is found in the "CMS Inner Content Types" IANA subregistry within the "Media Type Sub-Parameter Registries" registry group. If the existing content is MIME encoded, the mimeData content type SHOULD be used, though there may be reasons to use other identifiers as mentioned in . Updated protocols that do not deprecate the use of id-data should provide a rationale for not doing so, so that reviewers can assess whether the trade-off against the risk described in was adequately considered. For example, if new backwards-compatible extensions are added to a protocol it might not be appropriate to move to a new identifier at that time because doing so will result in a backwards-compatibility breaking change and the extensions will be unlikely to be deployed. On the other hand, if a protocol has a major version update or otherwise backwards-compatibility breaking change it would be appropriate to deprecate id-data in favour of a different identifier at the same time. When an updated protocol specification uses the id-data EncapsulatedContentInfo content type within SignedData, it SHOULD specify that the signedAttrs field is either always required or always forbidden. If a protocol makes such a requirement, a recipient implementing the specification MUST check whether the signedAttrs field is present or absent as specified by the protocol, and fail processing if the appropriate condition is not met. See for a related consideration regarding the key pair used across old and new protocol versions.

Key Separation Mandating signedAttrs within a protocol, whether through a new use of SignedData () or updating an existing use (), does not by itself prevent the attack described in if the signing key is also used in some other protocol or content type where signedAttrs is not mandated. Specifications MUST draw implementers' attention to this risk. This can be addressed by requiring a distinct key pair for this protocol's use of SignedData, separate from any other key pair used where signedAttrs is not mandated (whether that is an older version of this same protocol, or an unrelated protocol or content type). This can also be addressed by requiring that the mitigation from be applied uniformly across every signing operation performed with the key, not only to this protocol's messages, and even in older versions of the protocol.

Recipient Verification This section applies to all uses of the CMS SignedData type, whether a new use of SignedData, an existing use of id-data in SignedData, or the existing use of a different content type within SignedData. The entity verifying a CMS SignedData for a specific protocol SHOULD (MUST for new uses of SignedData) verify that the EncapsulatedContentInfo content type matches the value that the protocol expects, and SHOULD (MUST for new uses of SignedData) fail processing if it does not. A general-purpose CMS implementation that lacks protocol-specific enforcement of the above defined checks MUST expose the received content type to the application layer, so that the checks can be performed by the application. As specified in , a SignerInfo signedAttrs field MUST be present if the content type of the EncapsulatedContentInfo value being signed is not id-data. To avoid the attack described in , a recipient SHOULD (MUST for new uses of SignedData) verify, for each SignerInfo, that the signedAttrs field is present whenever the EncapsulatedContentInfo content type is not id-data, and SHOULD (MUST for new uses of SignedData) fail processing if it is not. Unlike the content type check above, this verification requires no protocol-specific context.

Mitigations This section describes mitigations for cases where the best practices given above cannot be applied. When the id-data EncapsulatedContentInfo content type is used, the following mitigations MAY be applied to protect against the vulnerability described in .

Recipient Detection This mitigation is performed by a recipient when processing SignedData. If signedAttrs is not present, check if the encapsulated or detached content is a valid DER-encoded SignedAttributes structure and fail if it is. Because a valid DER-encoded SignedAttributes structure necessarily includes the mandatory contentType and messageDigest attributes with their respective OIDs, a legitimate message is unlikely to coincidentally match this structure and be misidentified as an attack. However, a malicious party could intentionally present messages for signing that are detected by the countermeasure and thus introduce errors into the application processing that might be hard to trace for a non-expert.

Sender Detection This mitigation is performed by a sender who signs data received from a third party (potentially an attacker). If the sender is signing third party content and will not be setting the signedAttrs field, check that the content is not a DER-encoded SignedAttributes structure, and fail if it is. Note that also in this case, a malicious party could intentionally present messages that trigger this countermeasure and thereby trigger hard-to-trace errors during the signing process.

Security Considerations

On the Applicability of the Vulnerability

General Considerations of Applicability The vulnerability is not present in systems where the use of signedAttrs is mandatory, as long as recipients enforce the use of signedAttrs. Some examples where the use of signedAttrs is mandatory are SCEP , Certificate Transparency precertificates , firmware update , and the German Smart Metering CMS data format . Any protocol that uses an EncapsulatedContentInfo content type other than id-data is required to use signed attributes. However, this security relies on a correct implementation of the verification routine that ensures the correct content type and presence of signedAttrs. When the message is signed and then encrypted, it will be difficult for the attacker to learn the signature. However, the vulnerability might still be present if mitigations are not applied. For example:

• Signing and encryption might not be done on the same endpoints, in which case an attacker between the endpoints might be able to learn the signature for which it could remove or add the signedAttrs.
• IND-CPA (indistinguishability under chosen-plaintext attack) encryption does not give theoretical guarantees against an active attacker and thus does not guarantee that an attacker cannot rearrange the structure.

Conceivably vulnerable systems:

• Unencrypted firmware update denial of service
  • Secure firmware updates often use signatures without encryption. If the forged message can bring a device, due to lack of robustness in the parser implementation, into an error state, this may lead to a denial of service vulnerability. The possibility of creating a targeted exploit can be excluded with great certainty in this case due to the lack of control the attacker has over the forged message.
• Dense message space
  • If a protocol has a dense message space, i.e. a high probability that the forged message represents a valid command or the beginning of a valid command, then, especially if the parser is permissive with respect to trailing data, there is a risk that the message is accepted as valid. This requires a protocol where messages are signed but not encrypted.
• Signing unstructured data
  • Protocols that sign unencrypted unstructured messages, e.g. tokens, might be affected in that the signature of one token might result in the corresponding forged message being another valid token.
• External signatures over unstructured data
  • Probably the most strongly affected class of systems would be one that uses external signatures, i.e. CMS signatures with absent content (that may be transmitted encrypted separately) over unstructured data, e.g. a token of variable length. In that case the attacker could create a signed data object for a known secret message.
• Systems with permissive parsers
  • In addition to potential issues where the protocol parser is permissive (e.g. with respect to trailing space), if the CMS parser is permissive (e.g. allows non-protocol content types, or allows missing signedAttrs with content types other than id-data) then this could result in accepting invalid messages.

Further note that it is generally not good security behaviour to sign data received from a third party without first verifying that data. describes just one verification step that can be performed, specific to the vulnerability described in .

Cross-Protocol and Cross-Protocol-Version Attacks The following explains how the use of the same signing key in a protocol that adheres to this specification and at the same time in a different protocol or protocol version can still lead to vulnerabilities. One observation is that the claim made in that the vulnerability is not present when signedAttrs is mandatory and enforced holds only if the signing key is not also used to sign id-data content without signedAttrs in some other context. A signer who can be induced to sign attacker-chosen id-data content without signedAttrs (see the second attack described in ) becomes a forgery oracle for any other protocol or content type that relies on the same key pair and mandates the presence of signedAttrs. This holds even for a protocol designed correctly per or . Mandating and enforcing signedAttrs within one protocol gives no protection if the same signing key is used without such enforcement in some unrelated context, e.g. an implementation that reuses an existing signing certificate to sign under a new protocol. Vulnerabilities arising in such scenarios would classify as vulnerabilities to cross-protocol attacks. The risk also arises when an existing protocol is updated to mandate signedAttrs (). The same key may remain exposed to both the old and new behaviour, e.g. an implementation must support both during a transition period, or a user signs with the same key pair from multiple independent applications (e.g. separate mobile and desktop clients) that adopt the new behaviour at different times. This scenario falls into the category of cross-protocol-version attacks.

Degradation of Security Guarantees Through the Use of Signed Attributes The use of signed attributes in CMS signatures effectively reverts any signature scheme to a scheme based on the hash-then-sign paradigm. Modern signature schemes diverge from the hash-then-sign paradigm which allows them to reach better security reductions. Specifically, some signature schemes like SLH-DSA , LMS/HSS , and XMSS prefix a randomization string to the internal hash operation of the scheme's signature generation function and thus achieve independence from the assumption of collision resistance of the underlying hash-function in their security reduction. It should be noted that by employing signed attributes in CMS signatures, the modern signature schemes lose this security property.

IANA Considerations In the "SMI Security for S/MIME Module Identifier" (1.2.840.113549.1.9.16.0) registry within the "Structure of Management Information (SMI) Numbers (MIB Module Registrations)" registry group, create a new entry to point to this document.

Decimal	Description	Reference
TBD1	id-mod-mime-data-2026	[[This Document]]

In the "SMI Security for S/MIME CMS Content Type" (1.2.840.113549.1.9.16.1) registry within the "Structure of Management Information (SMI) Numbers (MIB Module Registrations)" registry group, add a new entry for id-ct-mimeData that points to this document.

Decimal	Description	Reference
TBD2	id-ct-mimeData	[[This Document]]

In the "CMS Inner Content Types" registry within "Media Type Sub-Parameter Registries" registry group, add a new entry:

Name	Object Identifier	Reference
mimeData	1.2.840.113549.1.9.16.1.TBD2	[[This Document]]

References Normative References ITU-T ITU-T This document describes the Cryptographic Message Syntax (CMS). This syntax is used to digitally sign, digest, authenticate, or encrypt arbitrary message content. [STANDARDS-TRACK] In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. This initial document specifies the various headers used to describe the structure of MIME messages. [STANDARDS-TRACK] Informative References National Institute of Standards and Technology (U.S.) This note describes the eXtended Merkle Signature Scheme (XMSS), a hash-based digital signature system that is based on existing descriptions in scientific literature. This note specifies Winternitz One-Time Signature Plus (WOTS+), a one-time signature scheme; XMSS, a single-tree scheme; and XMSS^MT, a multi-tree variant of XMSS. Both XMSS and XMSS^MT use WOTS+ as a main building block. XMSS provides cryptographic digital signatures without relying on the conjectured hardness of mathematical problems. Instead, it is proven that it only relies on the properties of cryptographic hash functions. XMSS provides strong security guarantees and is even secure when the collision resistance of the underlying hash function is broken. It is suitable for compact implementations, is relatively simple to implement, and naturally resists side-channel attacks. Unlike most other signature systems, hash-based signatures can so far withstand known attacks using quantum computers. This note describes a digital-signature system based on cryptographic hash functions, following the seminal work in this area of Lamport, Diffie, Winternitz, and Merkle, as adapted by Leighton and Micali in 1995. It specifies a one-time signature scheme and a general signature scheme. These systems provide asymmetric authentication without using large integer mathematics and can achieve a high security level. They are suitable for compact implementations, are relatively simple to implement, and are naturally resistant to side-channel attacks. Unlike many other signature systems, hash-based signatures would still be secure even if it proves feasible for an attacker to build a quantum computer. This document is a product of the Crypto Forum Research Group (CFRG) in the IRTF. This has been reviewed by many researchers, both in the research group and outside of it. The Acknowledgements section lists many of them. Bundesamt für Sicherheit in der Informationstechnik (BSI) This document specifies the Simple Certificate Enrolment Protocol (SCEP), a PKI protocol that leverages existing technology by using Cryptographic Message Syntax (CMS, formerly known as PKCS #7) and PKCS #10 over HTTP. SCEP is the evolution of the enrolment protocol sponsored by Cisco Systems, which enjoys wide support in both client and server implementations, as well as being relied upon by numerous other industry standards that work with certificates. This document describes version 2.0 of the Certificate Transparency (CT) protocol for publicly logging the existence of Transport Layer Security (TLS) server certificates as they are issued or observed, in a manner that allows anyone to audit certification authority (CA) activity and notice the issuance of suspect certificates as well as to audit the certificate logs themselves. The intent is that eventually clients would refuse to honor certificates that do not appear in a log, effectively forcing CAs to add all issued certificates to the logs. This document obsoletes RFC 6962. It also specifies a new TLS extension that is used to send various CT log artifacts. Logs are network services that implement the protocol operations for submissions and queries that are defined in this document. This document describes the use of the Cryptographic Message Syntax (CMS) to protect firmware packages, which provide object code for one or more hardware module components. CMS is specified in RFC 3852. A digital signature is used to protect the firmware package from undetected modification and to provide data origin authentication. Encryption is optionally used to protect the firmware package from disclosure, and compression is optionally used to reduce the size of the protected firmware package. A firmware package loading receipt can optionally be generated to acknowledge the successful loading of a firmware package. Similarly, a firmware package load error report can optionally be generated to convey the failure to load a firmware package. [STANDARDS-TRACK] This document presents a technique to securely provision a networking device when it is booting in a factory-default state. Variations in the solution enable it to be used on both public and private networks. The provisioning steps are able to update the boot image, commit an initial configuration, and execute arbitrary scripts to address auxiliary needs. The updated device is subsequently able to establish secure connections with other systems. For instance, a device may establish NETCONF (RFC 6241) and/or RESTCONF (RFC 8040) connections with deployment-specific network management systems. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 4.0. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 5751. This document defines the bundle security protocol, which provides data integrity and confidentiality services for the Bundle Protocol. Separate capabilities are provided to protect the bundle payload and additional data that may be included within the bundle. We also describe various security considerations including some policy options. This document is a product of the Delay-Tolerant Networking Research Group and has been reviewed by that group. No objections to its publication as an RFC were raised. This document defines an Experimental Protocol for the Internet community. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.2. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 3851. [STANDARDS-TRACK] This document describes a file format for the storage of flow data based upon the IP Flow Information Export (IPFIX) protocol. It proposes a set of requirements for flat-file, binary flow data file formats, then specifies the IPFIX File format to meet these requirements based upon IPFIX Messages. This IPFIX File format is designed to facilitate interoperability and reusability among a wide variety of flow storage, processing, and analysis tools. [STANDARDS TRACK] This document defines a practical architecture and protocols for offering privacy for a user who requests and uses an X.509 certificate containing a pseudonym, while still retaining the ability to map such a certificate to the real user who requested it. The architecture is compatible with IETF certificate request formats such as PKCS10 (RFC 2986) and CMC (RFC 5272). The architecture separates the authorities involved in issuing a certificate: one for verifying ownership of a private key (Blind Issuer) and the other for validating the contents of a certificate (Anonymity Issuer). The end entity (EE) certificates issued under this model are called Traceable Anonymous Certificates (TACs). This memo defines an Experimental Protocol for the Internet community. This document defines the format of an electronic signature that can remain valid over long periods. This includes evidence as to its validity even if the signer or verifying party later attempts to deny (i.e., repudiates) the validity of the signature. The format can be considered as an extension to RFC 3852 and RFC 2634, where, when appropriate, additional signed and unsigned attributes have been defined. The contents of this Informational RFC amount to a transposition of the ETSI Technical Specification (TS) 101 733 V.1.7.4 (CMS Advanced Electronic Signatures -- CAdES) and is technically equivalent to it. The technical contents of this specification are maintained by ETSI. The ETSI TS and further updates are available free of charge at: http://www.etsi.org/WebSite/Standards/StandardsDownload.aspx This memo provides information for the Internet community. This memo updates the ODETTE File Transfer Protocol, an established file transfer protocol facilitating electronic data interchange of business data between trading partners, to version 2. The protocol now supports secure and authenticated communication over the Internet using Transport Layer Security, provides file encryption, signing, and compression using Cryptographic Message Syntax, and provides signed receipts for the acknowledgement of received files. The protocol supports both direct peer-to-peer communication and indirect communication via a Value Added Network and may be used with TCP/IP, X.25, and ISDN-based networks. This memo provides information for the Internet community. This document defines Secure/Multipurpose Internet Mail Extensions (S/MIME) version 3.1. S/MIME provides a consistent way to send and receive secure MIME data. Digital signatures provide authentication, message integrity, and non-repudiation with proof of origin. Encryption provides data confidentiality. Compression can be used to reduce data size. This document obsoletes RFC 2633. [STANDARDS-TRACK] This document defines the format of an electronic signature that can remain valid over long periods. This includes evidence as to its validity even if the signer or verifying party later attempts to deny (i.e., repudiates the validity of the signature). This memo provides information for the Internet community. This document describes a protocol for adding cryptographic signature and encryption services to MIME data. [STANDARDS-TRACK]

ASN.1 Module The following module adheres to ASN.1 specifications and . MimeData-2026 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) modules(0) id-mod-mime-data-2026(TBD1) } DEFINITIONS EXPLICIT TAGS ::= BEGIN IMPORTS CONTENT-TYPE FROM CryptographicMessageSyntax-2010 { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) } ; id-ct-mimeData OBJECT IDENTIFIER ::= { iso(1) member-body(2) us(840) rsadsi(113549) pkcs(1) pkcs9(9) smime(16) ct(1) TBD2 } ct-mimeData CONTENT-TYPE ::= { IDENTIFIED BY id-ct-mimeData } END ]]>

RFCs Using the id-data EncapsulatedContentInfo Content Type This appendix lists RFCs which use the id-data content type in EncapsulatedContentInfo. These are all existing uses, as defined in ; new uses are subject to . It is a best-effort list by the authors at time of authorship. The list can be used as a starting point to determine if any of BCPs in this document can be applied. The following table summarizes the RFCs' usages of signed attributes. RFCs using id-data

RFC	Signed Attributes Usage
	Requires the use of signed attributes
	Says nothing about signed attributes
	RECOMMENDS signed attributes
	SHOULD NOT include signed attributes
	RECOMMENDS signed attributes
	Says nothing about signed attributes
	Forbids signed attributes
	Requires signed attributes
	Says nothing about signed attributes
	RECOMMENDS signed attributes
	Requires signed attributes
	RECOMMENDS signed attributes

An RFC requiring or forbidding signed attributes does not necessarily mean that a recipient will enforce this requirement when verifying, their CMS implementation may simply process the message whether or not signed attributes are present. If one of the signed attributes is necessary for the recipient to successfully verify the signature or to successfully process the CMS data then the vulnerability will not apply; at least not when assuming the signer is well-behaved and always signs with signed attributes present in accordance with the applicable specification.

RFC 8894 Simple Certificate Enrolment Protocol Figure 6 in specifies id-data as the EncapsulatedContentInfo content type, and shows the use of signedAttrs. The document itself never refers to signed attributes, but instead to authenticated attributes and an authenticatedAttributes type. Erratum ID 8247 clarifies that it should be "signed attributes" and "signedAttrs". Since SCEP requires the use of signedAttrs with the id-data EncapsulatedContentInfo content type, and the recipient must process at least some of the signed attributes, it is not affected by the vulnerability.

RFC 8572 Secure Zero Touch Provisioning (SZTP) allows the use of the id-data content type, although it also defines more specific content types. It does not say anything about signed attributes.

S/MIME RFCs , , , and require the use of the id-data EncapsulatedContentInfo content type. says:

• Receiving agents MUST be able to handle zero or one instance of each of the signed attributes listed here. Sending agents SHOULD generate one instance of each of the following signed attributes in each S/MIME message:

and

• Sending agents SHOULD generate one instance of the signingCertificate or signingCertificateV2 signed attribute in each SignerInfo structure.

So the use of signed attributes is not an absolute requirement.

RFC 6257 Bundle Security Protocol Specification says:

• In all cases where we use CMS, implementations SHOULD NOT include additional attributes whether signed or unsigned, authenticated or unauthenticated.

It does not specify what the behaviour should be if signed attributes are found by the receiver.

RFC 5655 IP Flow Information Export (IPFIX) is a file format that uses CMS for detached signatures. It says nothing about the use of signed attributes.

RFC 5636 Traceable Anonymous Certificate says:

• The signedAttr element MUST be omitted.

It does not specify what the behaviour should be if signed attributes are found by the receiver.

RFC 5126 CMS Advanced Electronic Signatures (CAdES) specifies mandatory signed attributes. One of the signed attributes is used to determine which certificate is used to verify the signature, so CAdES is not affected by the vulnerability.

RFC 5024 ODETTE File Transfer Protocol 2 uses the id-data EncapsulatedContentInfo content type and says nothing about signed attributes.

RFC 3126 Electronic Signature Formats for long term electronic signatures requires the message-digest attribute, which is a signed attribute.

Acknowledgments The authors would like to thank Russ Housley, Carl Wallace, and John Preuß Mattsson for their valuable feedback on this document.