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📝 Walkthrough
Merge Risk: ⚪ Minimal · up to The multipart error-reporting changes appear ready to merge after normal checks. Architecture SummaryArchitecture risk: 🔵 Low · up to The change affects 3 systems. Changed systems: Architecture concerns Review details
🚥 Pre-merge checks | ✅ 4 | ❌ 1
✨ Finishing Touches
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Copilot review overview
🟡 Changes recommended
Multipart completion overwrites the original header error with an inaccurate missing-boundary message.
Review effort: Balanced
Findings: 1
What changed in this PR
Corrects multipart header error handling and aligns request-body diagnostics with REQBODY_ERROR.
Changes:
- Stops parsing immediately after invalid multipart headers and sets explicit validation flags.
- Updates strict multipart expectations for invalid
filename*values. - Replaces deprecated processor-error references in configuration and regression tests.
| File | Description |
|---|---|
src/request_body_processor/multipart.cc |
Corrects return values and multipart flags. |
modsecurity.conf-recommended |
Logs REQBODY_ERROR in strict validation. |
test/test-cases/regression/request-body-parser-multipart.json |
Updates multipart parser rules. |
test/test-cases/regression/variable-MULTIPART_STRICT_ERROR.json |
Updates strict-error expectations and diagnostics. |
test/test-cases/regression/variable-MULTIPART_INVALID_HEADER_FOLDING.json |
Uses the cross-version request-body error variable. |
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@hnakamur could you take a review on this? |
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Actionable comments posted: 2
- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @src/request_body_processor/multipart.cc:
- Line 809: Update multipart_complete so it sets “Multipart: Final boundary
missing” only when the error is empty, preserving any earlier error set during
Multipart::process.
- Around line 994-997: In the duplicate-header branch of the multipart parser,
assign the duplicate-header message to error before returning -1 so the parse
reports the specific failure instead of the generic final-boundary error.
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📒 Files selected for processing (5)
modsecurity.conf-recommendedsrc/request_body_processor/multipart.cctest/test-cases/regression/request-body-parser-multipart.jsontest/test-cases/regression/variable-MULTIPART_INVALID_HEADER_FOLDING.jsontest/test-cases/regression/variable-MULTIPART_STRICT_ERROR.json
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Note: v2 counterpart is #3651. |
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Copilot review overview
🟡 Changes recommended
One invalid Content-Disposition path still omits the invalid-part flag, and missing-name flag behavior lacks a direct regression assertion.
Review effort: Balanced
Findings: 1
Open (2)
Resolved since last review (1)
fzipi
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The fix looks correct to me. Because the only caller checks process_part_header() < 0, the old return false paths were treated as success. With -1/1 the function now matches v2's multipart_process_part_header(), including the duplicate-header branch, which now sets the flag and the error and returns -1 the same way v2 does. I also checked the completion-overwrite concern: Transaction::processRequestBody() passes the same error string to process() and multipart_complete(), so the error->empty() guard does keep the first, more specific error. The later commits address the filename = "a.txt" and missing-name findings, and both tests now assert MULTIPART_INVALID_PART.
Two optional nits. process_part_data() has the same int-returning-bool shape: return false after "unknown part type" (line 738) and return true at the end (line 754), and its caller at line 1803 also checks < 0. The branch can't run today, because m_type is only ever MULTIPART_FORMDATA or MULTIPART_FILE, but since this PR is about exactly this mistake, changing those to -1/1 here would close it out. Second, the "error message - duplicate part header" test only matches REQBODY_ERROR_MSG. Chaining MULTIPART_DUPLICATE_PART_HEADER "@eq 1" would cover the flag, as the counterpart to the %ZZ case where DH now correctly goes to 0.
One note from the CRS side, not a change request. Parsing now stops at the first invalid part header, as in v2, so later parts never reach ARGS, FILES or MULTIPART_PART_HEADERS. CRS has no REQBODY_ERROR rule of its own and relies on rule 200002 from modsecurity.conf-recommended. A deployment that has dropped 200002 won't have anything after a malformed part header inspected by CRS. I haven't measured how this compares to before; the old parser also mostly stopped a little later, at "data contains boundary". It might be worth a line in the release notes next to the PE → RE change in rule 200003's message.
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Upon reviewing the AI suggestions of the last release, I found a few changes worth making. Suggestions:
what
1.
Multipart::process_part_header()returned the wrong values (commit e9f6663)Note, this is not between the suggestions, but found during the investigation.
The function is declared as
int, and its only caller checks the result with< 0:However, all eleven error paths returned
false(i.e.0) and the success path returnedtrue(i.e.1). The error paths now return-1and the success path returns1, the same as the v2 counterpart (multipart_process_part_header()inapache2/msc_multipart.c).2. Parser flags for invalid part headers
MULTIPART_INVALID_PARTis now set when the part has noContent-Dispositionheader, when theContent-Dispositionheader cannot be parsed (any negative result ofparse_content_disposition()), and when thenameparameter is missing.MULTIPART_INVALID_QUOTINGis now also set whenfilename*has no valid charset before the first'(error-16, e.g. a quotedfilename*="UTF-8''...").3. Test expectations
In
variable-MULTIPART_STRICT_ERROR.json, two cases were updated:filename*with an invalid percent-encoding (%ZZ):DH 1→DH 0filename*:DH 1, IQ 0→DH 0, IQ 14.
REQBODY_PROCESSOR_ERROR→REQBODY_ERROR(commit 8082be1)modsecurity.conf-recommended: the strict multipart rule (id:200003) now logsRE %{REQBODY_ERROR}instead ofPE %{REQBODY_PROCESSOR_ERROR}.request-body-parser-multipart.json,variable-MULTIPART_STRICT_ERROR.json,variable-MULTIPART_INVALID_HEADER_FOLDING.json): rules, macros and expectations useREQBODY_ERROR.variable-REQBODY_PROCESSOR_ERROR.jsonis intentionally unchanged, since it tests the variable itself, which still exists.For reason, please see the section below.
why
Wrong return value type: parsing continued after an invalid part header. Because the error paths returned
0, the caller treated them as success. The parser stayed in header state after an invalid header, read the following body lines as part headers, and parsed the sameContent-Dispositionheader again at the next empty line. For the%ZZcase, the debug log showed:The consequences:
MULTIPART_DUPLICATE_PART_HEADERbecame1for requests that contain no duplicate header at all. The second parse of the same header foundnamealready set by the first attempt.MULTIPART_INVALID_PARTwas set only as a side effect of the cascade ("data contains boundary"), not because of the invalid header itself.The request body error was still set, so such requests were rejected by
REQBODY_ERROR. The impact was incorrect flag values and misleading logs, not a bypass. The behavior has been like this since the v3 multipart parser was introduced. v2 has always returned-1.Explicit flags. Once parsing stops at the first invalid header, the cascade no longer sets
MULTIPART_INVALID_PART. An invalid, missing or namelessContent-Dispositionmakes the part invalid, so the flag is now set where the error is detected. Afilename*without a valid charset is a quoting problem, so it setsMULTIPART_INVALID_QUOTING.REQBODY_ERRORinstead ofREQBODY_PROCESSOR_ERROR. v2 removedREQBODY_PROCESSOR_ERRORin 2.9.14 (#3578), and its recommended configuration usesRE %{REQBODY_ERROR}. In v3,REQBODY_ERRORis a superset: it is also set whenSecRequestBodyNoFilesLimitis exceeded, which leavesREQBODY_PROCESSOR_ERRORunset. Rules and tests should use the variable that is present in both versions.REQBODY_PROCESSOR_ERRORitself is kept in 3.0.x for compatibility, because existing rules referencing it would otherwise fail to load.Behaviour changes to note:
msgof rule200003inmodsecurity.conf-recommendedcontainsREinstead ofPE. Anyone parsing that log line may need to adjust.references
variable-MULTIPART_STRICT_ERROR.jsonthat surfaced theDH/IPexpectationsREQBODY_PROCESSOR_ERRORin v2multipart_process_part_header()inapache2/msc_multipart.c(returns-1on error)Summary by CodeRabbit