CVE-2026-88975

Http4s is a Scala interface for HTTP services. Prior to 0.23.37 and 1.0.0-M48, Ember’s HTTP/2 read loop parses a frame’s 24-bit declared length but waits to buffer the entire payload before comparing it with SETTINGS_MAX_FRAME_SIZE. An unauthenticated peer can declare a payload near 16 MiB on a connection where Ember advertised 16 KiB and either complete or slowly stream it, causing up to 1024-fold memory amplification per connection before processFrame can reject the frame. The shared H2Connection.readLoop affects withHttp2 servers and clients, while HTTP/2-disabled configurations are unaffected, and the patch rejects oversized frames before buffering their payloads. This issue is fixed in versions 0.23.37 and 1.0.0-M48.

Http4s is a Scala interface for HTTP services. Prior to 0.23.37 and 1.0.0-M48, Ember’s HTTP/2 read loop parses a frame’s 24-bit declared length but waits to buffer the entire payload before comparing it with SETTINGS_MAX_FRAME_SIZE. An unauthenticated peer can declare a payload near 16 MiB on a connection where Ember advertised 16 KiB and either complete or slowly stream it, causing up to 1024-fold memory amplification per connection before processFrame can reject the frame. The shared H2Connection.readLoop affects withHttp2 servers and clients, while HTTP/2-disabled configurations are unaffected, and the patch rejects oversized frames before buffering their payloads. This issue is fixed in versions 0.23.37 and 1.0.0-M48.

Summary

An unauthenticated peer can make Ember's HTTP/2 read loop hold 16 MiB of a single frame in memory on a connection where Ember advertised a 16 KiB limit. The declared length is readable from the frame's first 9 bytes, but it is not compared against SETTINGS_MAX_FRAME_SIZE until the whole payload has been read into a contiguous buffer. That is 1024x amplification per connection, bounded by the protocol's 24-bit length field rather than by any setting, and at the default maxConnections of 1024 it puts about 16 GiB of heap in reach.

Details

H2Frame.RawFrame.fromByteVector (ember-core/.../h2/H2Frame.scala:66-69) reads the length, then refuses to produce a frame until 9 + length bytes are present:

val length = (bv(2) & 0xff) | ((bv(1) & 0xff) << 8) | ((bv(0) & 0xff) << 16)
if (bv.length >= 9 + length) {

readNextFrame in H2Connection.readLoop (H2Connection.scala:246-266) reads that None as "need more" and concatenates further socket reads into one accumulator until it is satisfied.

The only check of an inbound frame against the connection's own SETTINGS_MAX_FRAME_SIZE is in processFrame at H2Connection.scala:534, which cannot run until the frame is assembled. So the value that condemns the frame sits in the accumulator from byte 9 onward and is never consulted. A peer that declares 16777215 and dribbles the payload without finishing gets the same buffering with no reaction at all, since an incomplete frame never reaches processFrame.

RFC 9113 4.2 requires treating an oversized frame as a connection error and explicitly permits responding without reading the rest of the payload.

Impact

Uncontrolled resource consumption leading to unauthenticated remote denial of service by memory exhaustion.

• ember-server built .withHttp2: any peer able to open an HTTP/2 connection can trigger it. No authentication and no valid request, since the frame is rejected before it is associated with a stream, and the incomplete variant is never rejected.
• ember-client built .withHttp2 against a hostile origin: readLoop is shared, so the client side is symmetric. Reasoned from the shared code path, not reproduced.
• Not affected: HTTP/2 off, which is the default on both builders.

The overshoot is not configurable. It comes from the protocol's length field, not from Ember's limit, so tuning SETTINGS_MAX_FRAME_SIZE down does not reduce exposure, and withIdleTimeout does not apply because the read loop is making steady progress. Workarounds: leave HTTP/2 off, or terminate it at a proxy that enforces frame size and speak HTTP/1.1 to Ember.

Http4s is a Scala interface for HTTP services. Prior to 0.23.37 and 1.0.0-M48, Ember’s HTTP/2 read loop parses a frame’s 24-bit declared length but waits to buffer the entire payload before comparing it with SETTINGS_MAX_FRAME_SIZE. An unauthenticated peer can declare a payload near 16 MiB on a connection where Ember advertised 16 KiB and either complete or slowly stream it, causing up to 1024-fold memory amplification per connection before processFrame can reject the frame. The shared H2Connection.readLoop affects withHttp2 servers and clients, while HTTP/2-disabled configurations are unaffected, and the patch rejects oversized frames before buffering their payloads. This issue is fixed in versions 0.23.37 and 1.0.0-M48.

Http4s is a Scala interface for HTTP services. Prior to 0.23.37 and 1.0.0-M48, Ember’s HTTP/2 read loop parses a frame’s 24-bit declared length but waits to buffer the entire payload before comparing it with SETTINGS_MAX_FRAME_SIZE. An unauthenticated peer can declare a payload near 16 MiB on a connection where Ember advertised 16 KiB and either complete or slowly stream it, causing up to 1024-fold memory amplification per connection before processFrame can reject the frame. The shared H2Connection.readLoop affects withHttp2 servers and clients, while HTTP/2-disabled configurations are unaffected, and the patch rejects oversized frames before buffering their payloads. This issue is fixed in versions 0.23.37 and 1.0.0-M48.

Summary

An unauthenticated peer can make Ember's HTTP/2 read loop hold 16 MiB of a single frame in memory on a connection where Ember advertised a 16 KiB limit. The declared length is readable from the frame's first 9 bytes, but it is not compared against SETTINGS_MAX_FRAME_SIZE until the whole payload has been read into a contiguous buffer. That is 1024x amplification per connection, bounded by the protocol's 24-bit length field rather than by any setting, and at the default maxConnections of 1024 it puts about 16 GiB of heap in reach.

Details

H2Frame.RawFrame.fromByteVector (ember-core/.../h2/H2Frame.scala:66-69) reads the length, then refuses to produce a frame until 9 + length bytes are present:

val length = (bv(2) & 0xff) | ((bv(1) & 0xff) << 8) | ((bv(0) & 0xff) << 16)
if (bv.length >= 9 + length) {

readNextFrame in H2Connection.readLoop (H2Connection.scala:246-266) reads that None as "need more" and concatenates further socket reads into one accumulator until it is satisfied.

The only check of an inbound frame against the connection's own SETTINGS_MAX_FRAME_SIZE is in processFrame at H2Connection.scala:534, which cannot run until the frame is assembled. So the value that condemns the frame sits in the accumulator from byte 9 onward and is never consulted. A peer that declares 16777215 and dribbles the payload without finishing gets the same buffering with no reaction at all, since an incomplete frame never reaches processFrame.

RFC 9113 4.2 requires treating an oversized frame as a connection error and explicitly permits responding without reading the rest of the payload.

Impact

Uncontrolled resource consumption leading to unauthenticated remote denial of service by memory exhaustion.

• ember-server built .withHttp2: any peer able to open an HTTP/2 connection can trigger it. No authentication and no valid request, since the frame is rejected before it is associated with a stream, and the incomplete variant is never rejected.
• ember-client built .withHttp2 against a hostile origin: readLoop is shared, so the client side is symmetric. Reasoned from the shared code path, not reproduced.
• Not affected: HTTP/2 off, which is the default on both builders.

The overshoot is not configurable. It comes from the protocol's length field, not from Ember's limit, so tuning SETTINGS_MAX_FRAME_SIZE down does not reduce exposure, and withIdleTimeout does not apply because the read loop is making steady progress. Workarounds: leave HTTP/2 off, or terminate it at a proxy that enforces frame size and speak HTTP/1.1 to Ember.