The HTTP/2 protocol allows a denial of service (server resource consumption) because request cancellation can reset many streams quickly, as exploited in the wild in August through October 2023.

NIOHTTP1 and projects using it for generating HTTP responses can be subject to a HTTP Response Injection attack. This occurs when a HTTP/1.1 server accepts user generated input from an incoming request and reflects it into a HTTP/1.1 response header in some form. A malicious user can add newlines to their input (usually in encoded form) and "inject" those newlines into the returned HTTP response. This capability allows users to work around security headers and HTTP/1.1 framing headers by injecting entirely false responses or other new headers. The injected false responses may also be treated as the response to subsequent requests, which can lead to XSS, cache poisoning, and a number of other flaws. This issue was resolved by adding validation to the HTTPHeaders type, ensuring that there's no whitespace incorrectly present in the HTTP headers provided by users. As the existing API surface is non-failable, all invalid characters are replaced by linear whitespace.

A program using swift-nio-http2 is vulnerable to a denial of service attack, caused by a network peer sending a specially crafted HTTP/2 frame. This vulnerability is caused by a logical error when parsing a HTTP/2 HEADERS or HTTP/2 PUSH_PROMISE frame where the frame contains padding information without any other data. This logical error caused confusion about the size of the frame, leading to a parsing error. This parsing error immediately crashes the entire process. Sending a HEADERS frame or PUSH_PROMISE frame with HTTP/2 padding information does not require any special permission, so any HTTP/2 connection peer may send such a frame. For clients, this means any server to which they connect may launch this attack. For servers, anyone they allow to connect to them may launch such an attack. The attack is low-effort: it takes very little resources to send an appropriately crafted frame. The impact on availability is high: receiving the frame immediately crashes the server, dropping all…

A program using swift-nio-http2 is vulnerable to a denial of service attack caused by a network peer sending ALTSVC or ORIGIN frames. This attack affects all swift-nio-http2 versions from 1.0.0 to 1.19.1. This vulnerability is caused by a logical error after frame parsing but before frame handling. ORIGIN and ALTSVC frames are not currently supported by swift-nio-http2, and should be ignored. However, one code path that encounters them has a deliberate trap instead. This was left behind from the original development process and was never removed. Sending an ALTSVC or ORIGIN frame does not require any special permission, so any HTTP/2 connection peer may send such a frame. For clients, this means any server to which they connect may launch this attack. For servers, anyone they allow to connect to them may launch such an attack. The attack is low-effort: it takes very little resources to send one of these frames. The impact on availability is high: receiving the frame immediately crashe…

A program using swift-nio-http2 is vulnerable to a denial of service attack, caused by a network peer sending a specially crafted HPACK-encoded header block. This attack affects all swift-nio-http2 versions from 1.0.0 to 1.19.1. There are a number of implementation errors in the parsing of HPACK-encoded header blocks that allow maliciously crafted HPACK header blocks to cause crashes in processes using swift-nio-http2. Each of these crashes is triggered instead of an integer overflow. A malicious HPACK header block could be sent on any of the HPACK-carrying frames in a HTTP/2 connection (HEADERS and PUSH_PROMISE), at any position. Sending a HPACK header block does not require any special permission, so any HTTP/2 connection peer may send one. For clients, this means any server to which they connect may launch this attack. For servers, anyone they allow to connect to them may launch such an attack. The attack is low-effort: it takes very little resources to send an appropriately crafte…

A program using swift-nio-http2 is vulnerable to a denial of service attack, caused by a network peer sending a specially crafted HTTP/2 frame. This attack affects all swift-nio-http2 versions from 1.0.0 to 1.19.1. This vulnerability is caused by a logical error when parsing a HTTP/2 HEADERS frame where the frame contains priority information without any other data. This logical error caused confusion about the size of the frame, leading to a parsing error. This parsing error immediately crashes the entire process. Sending a HEADERS frame with HTTP/2 priority information does not require any special permission, so any HTTP/2 connection peer may send such a frame. For clients, this means any server to which they connect may launch this attack. For servers, anyone they allow to connect to them may launch such an attack. The attack is low-effort: it takes very little resources to send an appropriately crafted frame. The impact on availability is high: receiving the frame immediately cras…

The issue was addressed by signaling that an executable stack is not required. This issue is fixed in SwiftNIO SSL 2.4.1. A SwiftNIO application using TLS may be able to execute arbitrary code.

Some HTTP/2 implementations are vulnerable to a settings flood, potentially leading to a denial of service. The attacker sends a stream of SETTINGS frames to the peer. Since the RFC requires that the peer reply with one acknowledgement per SETTINGS frame, an empty SETTINGS frame is almost equivalent in behavior to a ping. Depending on how efficiently this data is queued, this can consume excess CPU, memory, or both.

Some HTTP/2 implementations are vulnerable to ping floods, potentially leading to a denial of service. The attacker sends continual pings to an HTTP/2 peer, causing the peer to build an internal queue of responses. Depending on how efficiently this data is queued, this can consume excess CPU, memory, or both.

Some HTTP/2 implementations are vulnerable to a header leak, potentially leading to a denial of service. The attacker sends a stream of headers with a 0-length header name and 0-length header value, optionally Huffman encoded into 1-byte or greater headers. Some implementations allocate memory for these headers and keep the allocation alive until the session dies. This can consume excess memory.