FA-6966 / Proxy forwarding / Open access

Trusted proxy walk stops at first untrusted hop · case 01

Trusted proxy walk stops at first untrusted hop.

Verified by executionVariant 1 · 4 checks per implementationDownload source bundle ↓JSON ↗

ROOT CAUSE

The faulty implementation uses `chain[0] if chain else None` for the trusted-proxy-walk-stops-at-first-untrusted-hop decision.

THE FAILURE

The faulty implementation uses `chain[0] if chain else None` for the trusted-proxy-walk-stops-at-first-untrusted-hop decision.

Unsuccessful approach: The attempted repair uses `next((h for h in chain if h not in trusted),None)` and still violates a separate fixture.

Case contract

Bounded offline decision model: trusted proxy walk stops at first untrusted hop. Inputs are already validated protocol fields; the explicit fixtures define the supported policy, not a complete protocol implementation.

Why this case matters

This deterministic proxy forwarding model isolates the trusted-proxy-walk-stops-at-first-untrusted-hop policy at a network component boundary.

1 / The failure

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(chain, trusted):
    return chain[0] if chain else None
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(['spoof', 'client', 'proxy'], ['proxy']), 'client')", "check('fixture 2', solve(['proxy'], ['proxy']), None)", "check('fixture 3', solve([], []), None)", "check('fixture 4', solve(['a', 'b'], []), 'b')"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`proxy`	`None`	Failed
fixture 3	`None`	`None`	Passed
fixture 4	`a`	`b`	Failed
fixture 1	`spoof`	`client`	Failed

SHA-256 / 7fa953d580b4c5b1e058e3e5ddbae7fb7762bfbf60cea8e1e275291ac75c23ba

2 / The unsuccessful fix

Exit 1

"""Failure Map reference implementation. Python standard library only."""
import json
import ipaddress
import re
from urllib.parse import urlsplit, urljoin
N = 1
observations = []
def solve(chain, trusted):
    return next((h for h in chain if h not in trusted),None)
def check(label, actual, expected):
    observations.append({"check": label, "actual": actual, "expected": expected, "passed": actual == expected})
_fixtures = ["check('fixture 1', solve(['spoof', 'client', 'proxy'], ['proxy']), 'client')", "check('fixture 2', solve(['proxy'], ['proxy']), None)", "check('fixture 3', solve([], []), None)", "check('fixture 4', solve(['a', 'b'], []), 'b')"]
for _line in _fixtures[N % len(_fixtures):] + _fixtures[:N % len(_fixtures)]:
    exec(_line)
print(json.dumps({"observations": observations, "passed": all(x["passed"] for x in observations)}, ensure_ascii=False))
raise SystemExit(0 if all(x["passed"] for x in observations) else 1)

Boundary fixture	Actual	Expected	Outcome
fixture 2	`None`	`None`	Passed
fixture 3	`None`	`None`	Passed
fixture 4	`a`	`b`	Failed
fixture 1	`spoof`	`client`	Failed

SHA-256 / d98f8e3265835f4a29bc274cd5875cc7b972f18acc567a34783718fcdbb07b00

HELD IN THE MEMBER ARCHIVE

The verified repair and its recorded checks are member-only.

This mechanism has 4 recorded checks per implementation. The open-access tier publishes the failure and the unsuccessful fix; the repaired source that passes every check, and the observations that prove it, are available to members.

Every case sharing this mechanism uses the same contract and the same repair, so this one record is held back for all of them.

Member access is invitation-based. Sign in with your invited account to inspect the repair.

Verification & scope

Bounded deterministic policy model over validated inputs; not a complete protocol stack or an interoperability claim. The five variants rotate the same explicit fixture set. This reproducer isolates one failure mechanism. Results cover the supplied fixtures. Variants within a family share a test contract and should remain grouped when constructing evaluation splits. Related mechanisms with a shared evaluation_group must also remain together; these controlled models are not independent production incidents.

Observations recorded using Python 3.12.14 at 2026-09-29T14:38:07.134885+00:00.

Case digest / 9a299c174e740f14b19010a29258e1b6c21215904d21deda6ac07dc8de97683d