CVE-2026-75804

Issue summary: OpenSSL QUIC stack does not enforce connection

level flow control for streams. Remote peers may send more bytes

as long as they fit within the stream flow control limits.

Impact summary: A malicious remote peer may exploit the lack of connection

flow control for streams to make the QUIC stack receive ~100MB of memory

instead of 768 KiB (default flow control window size).

CWE: CWE-770: Allocation of Resources Without Limits or Throttling

Description: The local QUIC stack advertises two flow control limits

to its remote peer: stream flow control limit and connection flow

control limit. The remote peer must follow both limits when transmitting

stream data.

Whenever the local QUIC stack receives a stream frame, it validates

that the size of the received stream frame stays within flow control limits.

If either limit is exceeded (stream level or connection level), then

the QUIC stack must close the connection with a flow control error.

The vulnerable OpenSSL QUIC stack enforces the stream-level but not

the connection-level limit. To exploit the issue, three conditions must be met:

- the remote peer opens several streams

- each stream must stay within the stream-level flow control limit

- there must be no zero-offset byte sent on any of the streams

(to prevent the vulnerable QUIC stack from consuming data).

By meeting the conditions above, the remote peer may make the local stack

allocate 2 x MAX_STREAMS x (stream flow control limit) bytes

of memory. MAX_STREAMS defaults to 100, and the limit applies to both

bidirectional and unidirectional streams, making it 200 in total. The default

flow control window for a stream is 512kB. The remote peer may

force the vulnerable QUIC stack to allocate 100MB of heap per connection.

FIPS impact: no

The FIPS module is not affected as the QUIC implementation is outside of

the OpenSSL FIPS module boundary.