Advisory [CVE-2026-56386] Integer underflow and out-of-bounds read in the HTTP/2 server DATA frame parser in Apache bRPC allows an unauthenticated remote attacker to cause denial of service by sending a malformed DATA frame with the PADDED flag set and a zero-length payload

Integer underflow and out-of-bounds read in the HTTP/2 server DATA frame parser in Apache bRPC allows an unauthenticated remote attacker to cause denial of service by sending a malformed DATA frame with the PADDED flag set and a zero-length payload.

Root Cause: The HTTP/2 DATA frame handler computed the fragment size from the attacker-controlled payload length and, when the PADDED flag was present, decremented the fragment size before ensuring that the frame contained the mandatory pad length byte.

A zero-length padded DATA frame could therefore underflow the unsigned fragment size and read the pad length byte past the end of the frame buffer.

Affected Scenarios: Apache bRPC servers that accept HTTP/2 traffic are affected when a remote client can send crafted HTTP/2 frames to the server.

HTTP/2 may be reached through cleartext h2c detection on an HTTP port or through TLS ALPN negotiation.

How to Fix: (Choose one of the following options)

1. Upgrade Apache bRPC to version 1.18.0, which fixes this issue.

2. Apply this patch:

Note: The fix validates that a padded DATA frame has enough payload bytes for the pad length field before decrementing the fragment size or reading the pad length byte.

Malformed zero-length padded DATA frames are rejected with HTTP/2 FRAME_SIZE_ERROR.