| Internet-Draft | privacy | August 2024 |
| Salz, et al. | Expires 27 February 2025 | [Page] |
Redirecting HTTP requests to HTTPS, a common pattern for human-facing web resources, can be an anti-pattern for authenticated API traffic. This document discusses the pitfalls and makes deployment recommendations for authenticated HTTP APIs. It does not specify a protocol.¶
This note is to be removed before publishing as an RFC.¶
The latest revision of this draft can be found at https://richsalz.github.io/draft-rsalz-httpapi-privacy/draft-rsalz-httpapi-privacy.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-rsalz-httpapi-privacy/.¶
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It is a common pattern for HTTP servers to prefer serving resources over HTTPS. Because HTTPS uses TLS, clients receive authentication of the server and confidentiality of the resource bodies supplied by the server.¶
In order to implement this preference, HTTP servers often listen for unencrypted requests and respond with a 3XX status code directing the client to the equivalent resource over an encrypted connection. For unauthenticated web browsing, this is a reasonable user experience bridge. Users often type bare hostnames (not URIs) into a user agent; if the user agent defaults to an unencrypted connection, the server can correct this default and require the use of encryption. This pattern is so well established that many HTTP server and intermediary implementations have a prominently displayed option to enable it automatically.¶
When client authentication is used, more care must be taken. The client's initial request may include a Bearer token or other credential; once the request has been sent on the network, any passive attacker who can see the traffic can acquire this credential and use it.¶
If the server performs a redirect in this situation, it does not mitigate exposure of the credential. Further, because the request will ultimately succeed if the client follows the redirect, an application developer or user who accidentally configures an unencrypted API endpoint will not necessarily notice the misconfiguration.¶
This document describes actions API servers and clients should take in order to safeguard credentials. These recommendations are not directed at resources where no authentication is used.¶
Although this document is not an IETF Standards Track publication, it adopts the conventions for normative language to provide clarity of instructions to the implementer. 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.¶
Various mechanisms exist to inform clients that unencrypted requests to a server are never appropriate:¶
HTTP Strict Transport Security (HSTS) [RFC6797] informs clients who make a successful connection over HTTPS that secure connections are a requirement in the future.¶
HTTPS DNS records [RFC9460] inform clients at connection time to use only secure connections to the indicated server.¶
Neither mechanism is foolproof. An attacker with control of the network or the DNS server could block resolution of HTTPS records on a client connecting to a new server, while HSTS requires a successful prior connection to the server and relies on the client to implement persistent storage of the HSTS directive.¶
Used together, however, both approaches make clients less likely to send any requests over an insecure channel. Servers with authenticated endpoints SHOULD employ both mechanisms.¶
If an API request succeeds despite having an unencrypted endpoint configured, the developer or user is less likely to notice the misconfiguration. Where possible, it is advantageous for such a misconfiguration to fail immediately so that the error can be noticed and corrected.¶
Servers MAY induce such an early failure by not accepting unencrypted connections, e.g. on port 80. This makes it impossible for a client to send a credential over an insecure channel to the authentic server, as no such channel can be opened.¶
However, this mitigation is limited against active network attackers, who can impersonate the server and accept the client's insecure connection attempt.¶
Whenever possible, credentials should include an indicator to clients that the credential is restricted to secure contexts. For example, Cookie-based authentication SHOULD include the Secure attribute described in Section 4.1.2.5 of [RFC6265]. Bearer tokens MAY use the format described in [RFC8959] to indicate the expected usage to the client.¶
Some deployments may not find it feasible to completely block unencrypted connections, whether because the hostname is shared with unauthenticated endpoints or for infrastructure reasons. Therefore, servers need a response for when a valid credential has been received over an insecure channel.¶
Because a difference in behavior would enable attackers to guess and check possible credentials, a server MUST NOT return a different client response between a valid or invalid credential presented over an insecure connection. Differences in behavior MUST only be visible on subsequent use of the credential over a secure channel.¶
When a request is received over an unencrypted channel, the presented credential is potentially compromised. Servers SHOULD revoke such credentials immediately. When the credential is next used over a secure channel, a server MAY return an error that indicates why the credential was revoked.¶
The following recommendations increase the success rate of the server recommendations above.¶
Clients SHOULD support and query for HTTPS records [RFC9460] when establishing a connection and SHOULD respect HSTS headers [RFC6797] received from a server. This includes implementing persistent storage of HSTS indications received from the server.¶
Clients MUST NOT send a Cookie with the Secure attribute [RFC6265] over an insecure channel. Clients MUST NOT send an Authorization header containing a token whose value begins with "secret-token:" over an insecure channel.¶
When authentication is used, clients SHOULD require an explicit indication from the user or caller that an insecure context is expected. Without such an indication, attempts to send credentials should fail without producing any network traffic.¶
This entire document is about security of HTTP API interactions.¶
The behavior recommended in Section 2.4 creates the potential for a denial of service attack where an attacker guesses many possible credentials over an unencrypted connection in hopes of discovering and revoking a valid one. Servers implementing this mitigation MUST also guard against such attacks, such as by limiting the number of requests before closing the connection and rate-limiting the establishment of insecure connections.¶
This document has no IANA actions.¶
We are grateful to Joachim Viide for his [BLOG] blog posting that brought up the issue.¶