Security and String Interpolation

Source: Dev.to

Introduction

I’m back after a long hiatus, this time focusing on job searching and revisiting recent CVEs.
One common source of vulnerabilities is string interpolation – a feature in most programming languages that inserts dynamic values into a string at runtime. Below is a quick refresher, followed by several categories of interpolation‑related bugs, real‑world CVE examples, and mitigation tips.

1. Formatted String Literals (f‑strings)

input_string = "Hello, World"
print(f"Variable expanded to: {input_string}")

Output:

Variable expanded to: Hello, World

The {input_string} placeholder is replaced at runtime with the variable’s value. While convenient, f‑strings (and similar interpolation mechanisms) become dangerous when user‑controlled data is inserted without proper sanitisation.

2. eval‑style Execution

• What it is: eval (or equivalents) evaluates a string as code.
• Risk: No execution context → arbitrary code execution.
• Safer alternatives: language‑specific safe parsers (e.g., Python’s ast.literal_eval).

CVE‑2025‑48868 – An insecure use of eval that allowed remote code execution.

3. Command‑Injection via Shell Interpolation

Running external commands with user‑supplied arguments can be exploited, especially when a shell is involved (/bin/bash, sh -c, python -c, etc.).

• Typical mistake: Using subprocess.Popen(..., shell=True) with unsanitised input.
• Mitigations:
  1. Avoid a shell – pass arguments as a list.
  2. Drop privileges – run the subprocess under an unprivileged user.
  3. Sandbox – use containers, chroot, or restricted execution environments.

CVE‑2026‑40030 – Demonstrated command‑injection in a video‑processing service that invoked ffmpeg.

4. Cross‑Site Scripting (XSS) – Browser Interpolation

When a web app reflects user‑generated content without filtering, an attacker can inject malicious JavaScript.

• Impact: Session hijacking, credential theft, drive‑by malware downloads.
• Defences: Content‑Security‑Policy (CSP), output‑encoding, sanitisation libraries.

CVE‑2024‑29184 – An XSS flaw that bypassed a common sanitisation filter on a forum platform.
Reference: Mozilla Developer Network – Defending against XSS.

5. SQL Injection – Database Query Interpolation

SQL statements built by concatenating strings are vulnerable to injection.

Classic example (from xkcd):

SELECT email FROM Students WHERE name='$name_here';

If $name_here = Robert'); DROP TABLE Students;--, the query becomes:

SELECT email FROM Students WHERE name='Robert'); DROP TABLE Students;--';

• -- starts a comment, discarding the rest of the statement.
• The injected DROP TABLE executes, destroying data.

Mitigations:

• Prepared statements / parameterised queries.
• ORM libraries that abstract raw SQL.

CVE‑2025‑1094 – A SQL‑injection flaw in a CLI tool that built queries from command‑line arguments.

6. Path Traversal – File‑System Interpolation

Improper handling of user‑supplied file paths can allow access to files outside the intended directory.

• Typical payload: ../../../etc/passwd
• Result: Server returns the system password file.

Mitigations:

1. Canonicalise & validate paths (e.g., pathlib.PurePath.relative_to in Python).
2. Enforce a root directory (chroot, container file‑system isolation).
3. Web‑application firewalls (AWS WAF, Cloudflare) with path‑traversal rules.

CVE‑2025‑68428 – Exploited a directory‑traversal bug in a file‑download API.

7. General Recommendations

Closing Thought

When you see a string that originates from user input, ask yourself:

1. What context will this string be used in? (HTML, SQL, shell, file system, etc.)
2. Does the language provide a safe API for that context?
3. Have I applied proper validation, sanitisation, or encoding?

By consistently applying these checks and layering defenses, you dramatically reduce the attack surface for interpolation‑related vulnerabilities. Happy coding—and good luck with the job hunt!