RFC 8725: JSON Web Token Best Current Practices | RFC Editor

JSON Web Tokens, also known as JWTs, are URL-safe JSON-based security tokens that contain a set of claims that can be signed and/or encrypted. JWTs are being widely used and deployed as a simple security token format in numerous protocols and applications, both in the area of digital identity and in other application areas. This Best Current Practices document updates RFC 7519 to provide actionable guidance leading to secure implementation and deployment of JWTs.¶
This memo documents an Internet Best Current Practice.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on BCPs is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8725.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
JSON Web Tokens, also known as JWTs [RFC7519], are URL-safe JSON-based security tokens that contain a set of claims that can be signed and/or encrypted. The JWT specification has seen rapid adoption because it encapsulates security-relevant information in one easy-to-protect location, and because it is easy to implement using widely available tools. One application area in which JWTs are commonly used is representing digital identity information, such as OpenID Connect ID Tokens [OpenID.Core] and OAuth 2.0 [RFC6749] access tokens and refresh tokens, the details of which are deployment-specific.¶
Since the JWT specification was published, there have been several widely published attacks on implementations and deployments. Such attacks are the result of under-specified security mechanisms, as well as incomplete implementations and incorrect usage by applications.¶
The goal of this document is to facilitate secure implementation and deployment of JWTs. Many of the recommendations in this document are about implementation and use of the cryptographic mechanisms underlying JWTs that are defined by JSON Web Signature (JWS) [RFC7515], JSON Web Encryption (JWE) [RFC7516], and JSON Web Algorithms (JWA) [RFC7518]. Others are about use of the JWT claims themselves.¶
These are intended to be minimum recommendations for the use of JWTs in the vast majority of implementation and deployment scenarios. Other specifications that reference this document can have stricter requirements related to one or more aspects of the format, based on their particular circumstances; when that is the case, implementers are advised to adhere to those stricter requirements. Furthermore, this document provides a floor, not a ceiling, so stronger options are always allowed (e.g., depending on differing evaluations of the importance of cryptographic strength vs. computational load).¶
Community knowledge about the strength of various algorithms and feasible attacks can change quickly, and experience shows that a Best Current Practice (BCP) document about security is a point-in-time statement. Readers are advised to seek out any errata or updates that apply to this document.¶
The intended audiences of this document are:¶
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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This section lists some known and possible problems with JWT implementations and deployments. Each problem description is followed by references to one or more mitigations to those problems.¶
Signed JSON Web Tokens carry an explicit indication of the signing algorithm, in the form of the "alg" Header Parameter, to facilitate cryptographic agility. This, in conjunction with design flaws in some libraries and applications, has led to several attacks:¶
For mitigations, see Sections 3.1 and 3.2.¶
In addition, some applications use a keyed Message Authentication Code (MAC) algorithm, such as "HS256", to sign tokens but supply a weak symmetric key with insufficient entropy (such as a human-memorable password). Such keys are vulnerable to offline brute-force or dictionary attacks once an attacker gets hold of such a token [Langkemper].¶
For mitigations, see Section 3.5.¶
Some libraries that decrypt a JWE-encrypted JWT to obtain a JWS-signed object do not always validate the internal signature.¶
For mitigations, see Section 3.3.¶
Many encryption algorithms leak information about the length of the plaintext, with a varying amount of leakage depending on the algorithm and mode of operation. This problem is exacerbated when the plaintext is initially compressed, because the length of the compressed plaintext and, thus, the ciphertext depends not only on the length of the original plaintext but also on its content. Compression attacks are particularly powerful when there is attacker-controlled data in the same compression space as secret data, which is the case for some attacks on HTTPS.¶
See [Kelsey] for general background on compression and encryption and [Alawatugoda] for a specific example of attacks on HTTP cookies.¶
For mitigations, see Section 3.6.¶
Per [Sanso], several Javascript Object Signing and Encryption (JOSE)