C++: Model secure versions of scanf as flow sources#21856
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Pull request overview
Adds modelling of the secure scanf_s/fscanf_s/sscanf_s/... variants as local/remote flow sources. Because every string-buffer vararg in these functions is followed by a buffer-size argument, the existing "all varargs are sources" approach would incorrectly mark size integers as sources. To address this, the PR extends RemoteFlowSourceFunction::hasRemoteFlowSource and LocalFlowSourceFunction::hasLocalFlowSource with an additional Call column so a model can express that the set of source arguments depends on the specific call, and updates ScanfFunctionCall.getOutputArgument to skip size arguments for S variants.
Changes:
- Recognise the
scanf_s/fscanf_s/sscanf_s/swscanf_s/_*_s_lfamily inScanf.qlland add anisSVariantpredicate plus a size-argument detector sogetOutputArgumentskips buffer-size varargs. - Add new 3-argument
hasLocalFlowSource(Call, FunctionOutput, string)/hasRemoteFlowSource(Call, FunctionOutput, string)predicates with default bodies delegating to/from the existing 2-argument versions, and migrateScanfModel/FscanfModelplus consumers (CleartextTransmission.ql,ExternalAPIsSpecific.qll,FlowSources.qll) to the new API. - Extend scanf library tests and the source/sink dataflow tests, and add a change-note documenting the new models and API.
Show a summary per file
| File | Description |
|---|---|
| cpp/ql/lib/semmle/code/cpp/commons/Scanf.qll | Adds S-variant detection, scanf_s/fscanf_s/sscanf_s/swscanf_s/_*_s_l recognition, and updates getOutputArgument to skip size arguments for S variants |
| cpp/ql/lib/semmle/code/cpp/models/implementations/Scanf.qll | Switches ScanfModel/FscanfModel to the new call-aware hasLocalFlowSource/hasRemoteFlowSource and factors logic into a shared hasFlowSource helper |
| cpp/ql/lib/semmle/code/cpp/models/interfaces/FlowSource.qll | Introduces 3-arg hasLocalFlowSource/hasRemoteFlowSource predicates with a Call column and default bodies bridging the old API |
| cpp/ql/lib/semmle/code/cpp/security/FlowSources.qll | Uses the new call-aware predicates in RemoteModelSource and LocalModelSource |
| cpp/ql/src/Security/CWE/CWE-311/CleartextTransmission.ql | Uses the new call-aware hasRemoteFlowSource overload |
| cpp/ql/src/Security/CWE/CWE-020/ExternalAPIsSpecific.qll | Simplifies the remote-source predicate using the new call-aware API |
| cpp/ql/test/library-tests/scanf/test.c | Adds a scanf_s call and a second integer variable to exercise S-variant handling |
| cpp/ql/test/library-tests/scanf/scanfFunctionCallOutput.ql / .expected | New test query and expected output verifying getOutputArgument results |
| cpp/ql/test/library-tests/scanf/scanfFunctionCall.expected / scanfFormatLiteral.expected | Updated baselines reflecting scanf_s modelling |
| cpp/ql/test/library-tests/dataflow/source-sink-tests/sources-and-sinks.cpp | Adds inline expectations for scanf_s/fscanf_s as local/remote sources |
| cpp/ql/lib/change-notes/2026-05-15-secure-scanf.md | Change-note describing the new models and the additional Call column on the flow-source predicates |
Copilot's findings
- Files reviewed: 13/13 changed files
- Comments generated: 2
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Does what it says on the tin.
There are a couple of small details that need to be fixed in order to model these as flow sources: since every string-like output buffer is succeeded by a buffer size argument we cannot blindly mark all variadic argument as an output buffer.
To fix this I've added a new version of
hasLocalFlowSourceandhasRemoteFlowSourcewith aCallcolumn. Using these new predicates thescanf_svariants can specify which output arguments of that call are sources of flow instead of marking all of them.Commit-by-commit review recommended.