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Level 10 already validates the file name against a PNG/JPEG extension pattern, but never looks at what is actually inside the file, and a name like shell.php.png still passes it since it ends with .png. Level 11 reads the file's own binary signature (the PNG/JPEG magic bytes) instead of trusting the extension, so a renamed non-image file is rejected even when the name looks fine. Level 12 goes further and discards the client-supplied name entirely. The stored file gets a random name plus the extension the server itself detected from the signature, so a crafted name carrying a path, a double extension or a null byte never reaches the file system at all. Ran both against the built jar rather than trusting the unit tests alone: a real PNG clears level 11, a PHP payload renamed to .png does not, and a JPEG posted to level 12 as ../../etc/passwd.jpeg lands on disk under a random name with no trace of that path anywhere. ./gradlew clean build passes, 452 tests, 0 failures.
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Included review availability: Your plan provides up to 2 included reviews per hour; 0 remain after this review. 📝 WalkthroughWalkthroughLevels 11 and 12 validate PNG/JPEG signatures and use UUID-based server-generated filenames. Level 12 also rejects uploads larger than 512 KiB. Tests cover valid, invalid, oversized, and traversal-shaped filenames. ChangesSecure image upload validation
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: ⚪ Minimal · up to The PR adds server-side image signature validation, safe server-generated filenames, and a 512 KB upload cap without reported unresolved merge-blocking issues; no actionable merge-blocking risk remains after normal checks and review. Sequence Diagram(s)sequenceDiagram
participant UploadRequest
participant UnrestrictedFileUpload
participant FileStorage
UploadRequest->>UnrestrictedFileUpload: submit file content
UnrestrictedFileUpload->>UnrestrictedFileUpload: detect image signature
UnrestrictedFileUpload->>UnrestrictedFileUpload: enforce Level 12 size limit
UnrestrictedFileUpload->>FileStorage: store with UUID-based filename
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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In
`@src/main/java/org/sasanlabs/service/vulnerability/fileupload/UnrestrictedFileUpload.java`:
- Line 640: Update the Level 11 upload filename construction in
UnrestrictedFileUpload to use the extension returned by
detectImageExtension(content) instead of file.getOriginalFilename(). Preserve
the existing random-prefix naming while ensuring stored files use only the
detected image extension, matching the Level 12 behavior.
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src/main/java/org/sasanlabs/service/vulnerability/fileupload/UnrestrictedFileUpload.javasrc/main/resources/i18n/messages.propertiessrc/test/java/org/sasanlabs/service/vulnerability/fileupload/UnrestrictedFileUploadTest.java
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CodeRabbit flagged the real gap: level 11 checked the file's binary signature but still built the stored name from file.getOriginalFilename(). A PNG whose bytes are valid but whose name ends in .html cleared the signature check and landed on disk as .html, serving straight back as HTML. The stored name is now built only from the extension the server itself detected, the same way level 12 already worked, so the client-supplied name plays no part in it at all. That made levels 11 and 12 nearly identical, so level 12 now carries its own distinct control: an upload size cap. It has to sit under 512 KB rather than the 1 MB most people would reach for, because Spring Boot's own multipart default already rejects anything over 1 MB with a 500 before the method ever runs, a limit set above that would be dead code no request could ever reach. Checked against the built jar: a real PNG named ../../shell.html on level 11 is written to disk as a plain .png. On level 12, a small JPEG passes, and a 700 KB one, over our cap but comfortably under Spring's, is rejected by our own check rather than the framework's. ./gradlew clean build passes, 454 tests, 0 failures.
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Inline comments:
In
`@src/main/java/org/sasanlabs/service/vulnerability/fileupload/UnrestrictedFileUpload.java`:
- Around line 646-647: Replace the 32-bit RANDOM.nextInt() stored-name
generation in Level 11 with a UUID-sized collision-resistant value while
preserving the detected extension and existing upload flow. Apply the same
name-generation rule to Level 12 at
src/main/java/org/sasanlabs/service/vulnerability/fileupload/UnrestrictedFileUpload.java
lines 673-674; both sites require direct changes.
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CodeRabbit's second finding: genericFileUploadUtility writes with REPLACE_EXISTING, and RANDOM.nextInt() only spans 2^32 values, so a collision between two accepted uploads silently overwrites one of them. The math checks out, at 100,000 uploads the collision odds are already around 69%. That is a real weakness for a level meant to be the secure reference, even if the same generator is what every unsecure level above already uses. Levels 11 and 12 now build their stored name from UUID.randomUUID() instead, leaving every other level untouched. ./gradlew clean build passes, 454 tests, 0 failures.
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Reviewed this by running it rather than reading it, since the claims are behavioural. Disclosure first: I have #793 open, which also adds a secure level, and I commented on #403 about the same family of issues. Different module, nothing competing, but worth saying. Everything the description claims, holds. I built the branch at
The traversal case is the one I most wanted to break and could not. The stored name never carries anything the client sent, and One thing worth knowing, and I do not think it is a defect. The check is a signature check, which is exactly what the description says it is, so this is a limit rather than a gap between claim and behaviour. Four files passed it that are not decodable images: an eight byte PNG signature with nothing after it, the three byte JPEG marker with nothing after it, a PNG signature followed by PHP source, and the 700 KB file I made by putting the JPEG marker in front of half a megabyte of the letter A. Only the genuine PNG decodes through Nothing about that is exploitable through the name, because the name is yours rather than the caller's. Whether the stored bytes matter at all depends on how the upload is later served, and that is the part I could not test: running from the jar puts uploads in a temp directory, which is #255, so I never got a served file back to inspect its response headers. Someone running from an exploded build could answer that quickly. If you did want to close it, Build is green on my side too: 471 tests, 0 failures. |
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thanks for actually running it and the traversal probe, appreciated. agree it's not a defect given the level is specifically about the signature check, not full decodability, so leaving it as is for now. good catch on #255 too, makes sense that's what blocked you from checking response headers. |
3 participants
Closes #401.
FileUpload currently has one secure level (level 10), which checks that the file name ends in
.pngor.jpeg. That check never looks past the name: a file namedshell.php.pngstill clears it, since it ends with.png.Level 11 reads the file's own bytes and checks them against the actual PNG and JPEG binary signatures. The stored name is built only from the extension the server detects that way, never from anything the client sent, so a renamed non-image file is rejected and a real image can never end up stored under an attacker-chosen extension like
.html.Level 12 carries the same signature check and server-generated name as level 11, plus an upload size cap of 512 KB. That cap has to sit below Spring Boot's own multipart default of 1 MB, or a request over it would never reach the method at all, it would be rejected by the framework with a 500 before any of this code ran.
Both are additive. Nothing in levels 1 to 10 changed.
How I checked it: unit tests cover a real PNG being accepted on level 11, a PHP payload renamed to
.pngbeing rejected, a real PNG uploaded with a name of../../shell.htmlstill ending up stored as.png, level 12 accepting a small JPEG and rejecting one over its own cap, and a non-image upload being rejected on both levels. I also ran the built jar and posted the same cases with curl: the.html-named PNG landed on disk as plain.png, and a 700 KB JPEG, over level 12's cap but under Spring's own 1 MB limit, came back rejected by this code rather than by the framework../gradlew clean buildpasses with 454 tests, 0 failures, on masterc833c66.Update: an earlier revision of level 11 in this PR kept the client-supplied extension for the stored name even after validating the signature, which CodeRabbit correctly flagged as still allowing a signature-valid PNG to be stored as
.html. Fixed in the second commit, which is also where level 12's distinct control (the size cap) was added, since after that fix the two levels would otherwise have ended up doing the same thing.Summary by CodeRabbit
New Features
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