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C++: exclude printf implementation internals from uncontrolled format string sinks #21493

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C++: exclude printf implementation internals from uncontrolled format string sinks #21493

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21 changes: 20 additions & 1 deletion cpp/ql/src/Security/CWE/CWE-134/UncontrolledFormatString.ql
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Original file line number Diff line number Diff line change
Expand Up @@ -23,13 +23,32 @@

predicate isSource(FlowSource source, string sourceType) { sourceType = source.getSourceType() }

/**
* Holds if `f` is a printf-like function or a (possibly nested) wrapper
* that forwards a format-string parameter to one.
*
* Functions that *implement* printf-like behaviour (e.g. a custom
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@geoffw0 geoffw0 Mar 17, 2026

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Copilot is right, we prefer American spellings in documentation, including in /** comments that may be extracted to documentation.

* `vsnprintf` variant) internally parse the caller-supplied format string
* and build small, bounded, local format strings such as `"%d"` or `"%ld"`
* for inner `sprintf` calls. Taint that reaches those inner calls via the
* parsed format specifier is not exploitable, so sinks inside such
* functions should be excluded.
*/

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Misspelling Warning

This comment contains the non-US spelling 'behaviour', which should instead be 'behavior'.
Comment on lines +26 to +36
private predicate isPrintfImplementation(Function f) {
f instanceof PrintfLikeFunction
or
exists(PrintfLikeFunction printf | printf.wrapperFunction(f, _, _))
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@geoffw0 geoffw0 Mar 17, 2026

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I'm a bit worried this last line may be too inclusive. A bit of testing should confirm one way or another...

}

module Config implements DataFlow::ConfigSig {
predicate isSource(DataFlow::Node node) { isSource(node, _) }

predicate isSink(DataFlow::Node node) {
exists(PrintfLikeFunction printf |
printf.outermostWrapperFunctionCall([node.asExpr(), node.asIndirectExpr()], _)
)
) and
not isPrintfImplementation(node.asExpr().getEnclosingFunction()) and
not isPrintfImplementation(node.asIndirectExpr().getEnclosingFunction())
Comment on lines +50 to +51
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@geoffw0 geoffw0 Mar 17, 2026

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We can use the [] syntax from line 48 here as well:

Suggested change
not isPrintfImplementation(node.asExpr().getEnclosingFunction()) and
not isPrintfImplementation(node.asIndirectExpr().getEnclosingFunction())
not isPrintfImplementation([node.asExpr(), node.asIndirectExpr()].getEnclosingFunction())

}

private predicate isArithmeticNonCharType(ArithmeticType type) {
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