A SQL injection technique that exploits comment syntax such as double dash to truncate or reshape a query. In PostgreSQL, a negative number rendered into text can accidentally create a line comment marker during interpolation. The result is altered parsing, which may expose hidden syntax or enable malicious statements.
What Line Comment Injection Is
Line comment injection is a SQL injection technique that exploits comment syntax, such as --, to truncate the rest of a query or alter how the database parses it. The attacker’s goal is usually to change query logic without needing to fully rewrite the statement.
This matters because even a small parsing shift can turn a harmless-looking input into a statement that bypasses intended conditions, exposes hidden syntax, or changes which rows are returned.
How It Works in Practice
The technique depends on the database treating a comment marker as the start of a line comment, then ignoring the remaining SQL text. If user-controlled input is interpolated into a query without proper parameterisation, the injected marker can reshape the final statement.
In PostgreSQL, an apparently safe value can become dangerous when interpolation or formatting changes a negative number into text in a way that creates comment syntax. The issue is not the numeric value itself, but the string rendering and concatenation step that changes how the parser sees the query.
Why It Matters for Query Security
Line comment injection is effective because it often targets the “tail” of a query, where defenders expect filters, guardrails, or closing logic to remain intact. Once that tail is neutralised, the attacker may be able to bypass authentication checks, weaken predicates, or expose syntax that helps refine a broader injection attempt.
It is closely related to other SQL injection variants, but the distinguishing feature is the use of comment syntax as a parsing weapon. That makes it especially relevant when application code concatenates strings, formats values unsafely, or assumes that numeric inputs are inherently non-exploitable.
How to Recognize and Prevent It
The safest defence is to avoid building SQL by string concatenation and to use parameterised queries or prepared statements so user input is never parsed as SQL syntax. This is particularly important anywhere text rendering, templating, or query assembly can change the final byte sequence sent to the database.
OWASP Top 10 places injection in the core application security risk set, and line comment injection is one concrete way that injection can take shape. Review any code path that transforms user input into SQL text, especially where quoting, casting, or locale-specific formatting can alter parsing.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP ASVS, CIS Controls v8 and NIST SP 800-53 Rev 5 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| OWASP ASVS | V4 — API and Web Service | Line comment injection exploits unsafe SQL construction in application and API layers. |
| V15 — Secure Coding and Architecture | The issue arises from insecure query assembly and parser-sensitive string handling. | |
| Recommendation — Use parameterized queries to prevent user input from becoming executable SQL text. Remove string-built SQL paths and enforce safe query construction patterns. | ||
| CIS Controls v8 | CIS-16 — Application Software Security | This injection pattern is a software security weakness that CIS addresses through secure development and testing. |
| Recommendation — Test application inputs for injection paths and remediate unsafe database query construction. | ||
| NIST SP 800-53 Rev 5 | SI-10 — Information Input Validation | The term centers on input being transformed into harmful SQL syntax. |
| Recommendation — Validate and constrain inputs before they reach query construction logic. | ||
Related resources from NHI Mgmt Group
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Reviewed and updated by the NHIMG editorial team on September 29, 2026.
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