fix: seclang scanner mis-parsing escaped quotes right after a macro

[fzipi]

Summary

An action value with an escaped quote immediately after a %{VARIABLE} macro expansion fails to parse, e.g.:

SecAction "id:1,phase:5,log,pass,logdata:'\'Field\' : \'%{TX.foo}\''"

fails with:

Rules error. ... Expecting an action, got: 'Field' : '%{TX.foo}''"

This was found via a real plugin rule in coreruleset/traffic-observation-plugin, which fails to load under nginx (owasp/modsecurity-crs Docker image, which links libmodsecurity v3):

https://github.com/coreruleset/traffic-observation-plugin/actions/runs/35602944525/job/106343176175?pr=7

nginx: [emerg] "modsecurity_rules_file" directive Rules error. File: .../traffic-observation-after.conf.
Line: 107. Column: 877. Expecting an action, got:  'Method' : '%{REQUEST_METHOD}', 'LenFilename' : ...

Root cause

In src/parser/seclang-scanner.ll, the free-text macros that consume action values (FREE_TEXT_QUOTE_MACRO_EXPANSION for single-quoted values, FREE_TEXT_DOUBLE_QUOTE_MACRO_EXPANSION for double-quoted values, and FREE_TEXT_COMMA_DOUBLE_QUOTE_MACRO_EXPANSION for unquoted values like msg:foo) recognize an escaped quote (\' / \") as literal text only through alternatives that bundle in a preceding non-backslash "anchor" character as part of the same token match, e.g. [^\\][\\]['].

When a %{VARIABLE} macro closes, the closing } is consumed by its own rule in EXPECTING_ACTION_PREDICATE_VARIABLE, which switches state without emitting a token for it. That } is therefore not available as the anchor for the very next token. If that next token starts with an escaped quote right after the macro, none of the anchor-based alternatives can match it — the scanner falls back to matching just the leading backslash(es) as ordinary text, then treats the following bare quote as the real terminator, ending the value early and leaving the rest of the action list unparseable. The same gap exists at the very start of a value (no anchor exists before the first character either), and applies equally to quoted and unquoted action values.

Fix

Add three "leading escape" macros — one per affected free-text macro — that match an escaped quote at the very start of a token (one backslash, or any longer odd-length run of backslashes, per the existing pairing convention), optionally followed by more ordinary free text:

FREE_TEXT_QUOTE_MACRO_EXPANSION_LEADING_ESCAPE                [\\]([\\][\\])*[']{FREE_TEXT_QUOTE_MACRO_EXPANSION}?
FREE_TEXT_DOUBLE_QUOTE_MACRO_EXPANSION_LEADING_ESCAPE          [\\]([\\][\\])*["]{FREE_TEXT_DOUBLE_QUOTE_MACRO_EXPANSION}?
FREE_TEXT_COMMA_DOUBLE_QUOTE_MACRO_EXPANSION_LEADING_ESCAPE    [\\]([\\][\\])*["]{FREE_TEXT_COMMA_DOUBLE_QUOTE_MACRO_EXPANSION}?

and wire them into every scanner state that consumes the corresponding free-text macro:

• ACTION_PREDICATE_ENDS_WITH_QUOTE (single-quoted action values, e.g. msg:'...', logdata:'...', tag:'...', ver:'...')
• ACTION_PREDICATE_ENDS_WITH_DOUBLE_QUOTE / NO_OP_INFORMED_ENDS_WITH_QUOTE / EXPECTING_PARAMETER_ENDS_WITH_QUOTE (double-quoted action values and double-quoted operator parameters)
• ACTION_PREDICATE_ENDS_WITH_COMMA_OR_DOUBLE_QUOTE (unquoted action values, e.g. msg:foo — added after Copilot's review flagged this state was still uncovered)

src/parser/seclang-scanner.cc was regenerated from the updated .ll via flex after every change (the large diff there is normal DFA-table churn from regeneration, not hand-edited); verified with a clean-room build that compiles only the checked-in .cc (no flex invoked, matching the default ./configure && make path — parser generation is opt-in).

Scope note: the same anchor-character issue likely also exists in the setvar value tokenizer (FREE_TEXT_EQUALS_QUOTE_MACRO_EXPANSION / FREE_TEXT_EQUALS_MACRO_EXPANSION), but that's a structurally different state machine and wasn't part of the reported/reproduced failure, so it's left out of this PR to keep the change scoped and reviewable.

Test plan

test/test-cases/regression/misc-escaped-quote-after-macro-expansion.json — 5 cases:

• logdata with a single-quote value, escaped quote right after a macro (the exact reported failure)
• action value with a double-quote value, escaped quote right after a macro
• logdata with a single-quote value, a 3-backslash (odd-length) run before the quote right after a macro
• action value with a double-quote value, a 3-backslash run before the quote right after a macro
• msg as an unquoted action value, escaped double quote right after a macro

For each of the three fixes (single-quote, double-quote, odd-backslash-run, unquoted-value), verified the corresponding test fails against the prior state (no fix / the previous commit's fix) with the exact reported/reproduced parser error, and passes with the fix applied.

Full regression suite: 723 passed, 0 failed, 13 skipped (skips are pre-existing, feature-gated). test/unit_tests has 11 pre-existing failures unrelated to this change (geoLookup/rbl network-dependent tests and a phpArgsNames transformation edge case in the secrules-language-tests submodule), unaffected by this fix.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Fixed escaped double quotes at the start of unquoted, comma-terminated action values after macro expansions.
  • Preserved complete logdata and unquoted msg values containing escaped quotes, including quotes preceded by odd-length backslash sequences.

• Tests

  • Added regression coverage for escaped single and double quotes after macro expansions in logdata and unquoted msg values.

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📝 Walkthrough

Walkthrough

The lexer now recognizes leading escaped double quotes in unquoted action-predicate values. Five regression cases check preserved logdata and msg values, including odd-length backslash runs.

Changes

Escaped quote lexer handling

Layer / File(s)	Summary
Lexer quote-token handling src/parser/seclang-scanner.ll	The lexer matches a leading escaped double quote after a macro expansion and emits it as free text in the comma-terminated action-predicate state. The pattern covers odd-length backslash runs.
Regression coverage test/test-cases/regression/misc-escaped-quote-after-macro-expansion.json, test/test-suite.in	Five cases check preserved logdata and unquoted msg values for escaped quotes after macro expansions. The suite input list includes the regression file.

Priority: ⚪ Not assessed

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Merge Risk: ⚪ Minimal · up to 91c29

The escaped-quote fix is present in the compiled lexer and covered by registered regression cases; no actionable merge-blocking risk remains.

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Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: fixing seclang scanner parsing for escaped quotes immediately after macro expansion.
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Inline comments:
In `@src/parser/seclang-scanner.ll`:
- Around line 452-453: Update the leading-escape lexer rules
FREE_TEXT_QUOTE_MACRO_EXPANSION_LEADING_ESCAPE and
FREE_TEXT_DOUBLE_QUOTE_MACRO_EXPANSION_LEADING_ESCAPE to match one backslash
followed by zero or more backslash pairs before the respective quote, while
preserving macro expansion handling. Add regression cases covering odd-length
backslash runs at token start for both single- and double-quote styles.

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Open (2)

• Include regenerated scanner with leading-escape fix · New
• Add leading-escape handling for action values ending in comma or quote · New

What changed in this PR

Fixes escaped-quote parsing after macro expansion and adds regression coverage.

Changes:

• Adds leading escaped-quote scanner rules.
• Adds regression cases for affected values.

File	Summary
test/​test-cases/​regression/​misc-escaped-quote-after-macro-expansion.json	Adds regression coverage; the file is not yet included in the standard test list.
src/​parser/​seclang-scanner.ll	Adds escape handling, but leaves one action state uncovered and does not include the required regenerated scanner implementation.

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[airween]

Hi @fzipi,

thank you for this PR.

Notes/questions:

• could you check which version of flex you used? The generated code shows 2.6.4, which is I think the latest, so that's the expected (just to be sure)
• your new test was not checked in CI (with make check); the reason is simple: all test must be exists in test-suite.in, this one somewhere around here; (you can run cd test; ./test-suite.sh if you are in a clean directory, without your own tests)

Thank you again!

[fzipi]

Hi @airween, thanks for the review!

• flex version: confirmed, flex --version gives 2.6.4 locally, matching the generated header — so that's expected.
• Test registration: you were right, the new test json existed but was never added to test-suite.in, so it never ran in CI. Fixed in 91c2933 — added TESTS+=test/test-cases/regression/misc-escaped-quote-after-macro-expansion.json. Verified locally with make check (all 5 assertions in the new test pass; ran the full suite too, remaining failures are pre-existing and due to optional deps — Lua/GeoIP/LibCURL/LibXML2 — not enabled in my local build, unrelated to this change).

🤖 Generated with Claude Code

[airween]

• flex version: confirmed, flex --version gives 2.6.4 locally, matching the generated header — so that's expected.

thanks - I just asked because I see this line:

extern yy_size_t yyleng;

I re-generated this in local, and got this:

extern int yyleng;

which is the same as in the diff above, on the left side.

It would be nice to figure out, why is this diff there.

Note, I got the same flex version:

$ flex --version
flex 2.6.4

• Test registration: you were right, the new test json existed but was never added to test-suite.in, so it never ran in CI. Fixed in 91c2933 — added TESTS+=test/test-cases/regression/misc-escaped-quote-after-macro-expansion.json. Verified locally with make check (all 5 assertions in the new test pass; ran the full suite too, remaining failures are pre-existing and due to optional deps — Lua/GeoIP/LibCURL/LibXML2 — not enabled in my local build, unrelated to this change).

I see that both in CI, and in the diff. Excellent, thank you!

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
26 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud