Microsoft Kiota: Code Generation Literal Injection in Kiota PHP Generator

Impact

The Kiota PHP code generator is vulnerable to a code generation literal injection attack. The generator embeds string values from OpenAPI fields (e.g. description, default values, and property names) directly into PHP double-quoted string literals without properly escaping the $ character. Since PHP evaluates string interpolation expressions like "${expr}", "$var", and "{$obj->prop}" within double-quoted strings at runtime, an attacker who controls an OpenAPI specification file can inject arbitrary PHP code into generated model and request-builder classes.

Who is impacted

Developers using Kiota to generate PHP API clients from external or untrusted OpenAPI specifications

Teams with CI/CD pipelines configured to automatically regenerate client code from remote specs

Applications that deploy generated PHP code to production servers

Vulnerability details

Affected component: StringExtensions.cs

Root cause: The shared SanitizeDoubleQuote() function in Writers/StringExtensions.cs does not escape the $ character. As a result, any schema-derived string emitted as a PHP double-quoted literal preserves $-prefixed interpolation constructs (${...}, $var, {$...}) verbatim, which PHP evaluates at runtime instead of treating as literal text. This is the same class of code-generation literal-injection flaw previously fixed for the Ruby generator (# interpolation), recurring here as a missed variant for PHP's $ interpolation in the sibling sanitizer helper.

Attack vectors

OpenAPI description and default fields in schema properties

Property wire-name keys embedded in deserializer/serializer methods

Any schema-derived string embedded in PHP double-quoted literals

Severity: Critical when generated code reaches production; High for CI/CD environments with access to production secrets; Medium for public third-party specs; Low for developer-controlled specs.

Patches

#7863

Workarounds

If you cannot upgrade immediately:

1. Audit and sanitize OpenAPI specifications: Review all OpenAPI specification files for any descriptions, default values, or property names containing the $ character. Remove or replace any suspicious strings before code generation.
2. Code review of generated files: Implement mandatory code review of all generated PHP files before merging into any branch. Look for double-quoted strings containing ${, $var, or {$ patterns.
3. Restrict specification sources: Only consume OpenAPI specifications from trusted internal sources. Avoid automatic code generation from external or third-party APIs until this patch is applied.
4. Isolate generated code from production: Do not deploy generated PHP models to production environments unless the specification source has been verified and reviewed.
5. Manual escaping (temporary): If regeneration is not possible, manually inspect and edit generated files to escape any $ characters in double-quoted string literals (replace $ with \$).

Remediation

Upgrade Kiota to 1.29.1, 1.32.4, or later.

Regenerate/refresh existing generated clients as a precaution:

Refreshing generated clients ensures previously generated vulnerable code is replaced with hardened output.