{"id":"CVE-2026-107215","aliases":["GHSA-x2q3-8cjh-766f","GO-2026-6689"],"url":"https://o3.security/vulnerability/CVE-2026-107215","summary":"Excelize: extractPart allocates attacker-controlled, unbounded and negative-sized buffers from CFB directory entries: remote panic / OOM DoS","details":"### Summary\n\n`extractPart` allocates directly from the mscfb directory-entry size with no clamping (`crypt.go:198`, same missing bound at `crypt.go:204`):\n\n```go\nbuf := make([]byte, entry.Size)\n```\n\n`entry.Size` is the raw `streamSize` field from the attacker-controlled CFB directory entry (mscfb v1.0.8 `fixFile`, `file.go:109-120`: full uint64 → int64 for major version 4, low uint32 for v3). mscfb only detects short sector chains later, inside `File.Read` (`file.go:485-489` \"emergency brake\") — i.e. **after** the allocation. The zip path already enforces `UnzipSizeLimit`, and the KDF path bounds work with `maxSpinCount`; this path consults no limit at all.\n\n### Details\n\nTwo consequences, both execution-confirmed against pristine master (`ecd99d761fe0`, 2026-09-08):\n\n1. **Hard crash**: a version-4 CFB declaring `streamSize = 0xFFFFFFFFFFFFFFFF` yields `entry.Size == -1` → `panic: runtime error: makeslice: len out of range`, which escapes `Decrypt` (`openReaderAt` converts errors, not panics, `excelize.go:210-214`) and kills the process.\n2. **Memory exhaustion**: any CFB (v3 suffices) declaring `streamSize` up to `0xFFFFFFFF` forces up to 4 GiB of zeroed allocation from a ~12 KB file — an amplification factor of ~350,000×, trivially repeated per request in memory-limited deployments.\n\nCall path: `OpenFile`/`OpenReader`/`OpenBytes` → `openReaderAt` (`excelize.go:208-215`, OLE magic sniff) → `Decrypt` (`crypt.go:145-150`) → `extractPart` (`crypt.go:198`).\n\n### PoC\n\nA standalone program (public API only) was provided to the maintainer by email (`2-extractpart-oom`): it builds a valid-enough v4 (4096-byte sector) CFB — header (signature, sector shift `0x0C`, `DIFAT[0]=1`), one directory sector with Root Entry (typeID 5, child=1) and a stream entry `EncryptionInfo` (objectType=2, startingSector=endOfChain, `streamSize=0xFFFFFFFFFFFFFFFF`). `mscfb.New` succeeds, `doc.Next()` returns the entry, and `extractPart` calls `make([]byte, -1)` before any chain validation. On master `ecd99d761fe0` the program prints `panic: runtime error: makeslice: len out of range`; with the proposed patch it prints BLOCKED. An `-oom` flag demonstrates the 4 GiB allocation variant with `streamSize = 0xFFFFFFFF`.\n\n### Impact\n\nAn attacker uploads any file starting with the OLE signature `D0 CF 11 E0 A1 B1 1A E1` (~12 KB is enough) to a service that calls `excelize.OpenFile`/`OpenReader`/`OpenBytes` on it (or calls `Decrypt` directly): remote, unauthenticated process crash (hard panic) or forced 4 GiB allocation per request (OOM) with a ~350,000× amplification factor.\n\n### Proposed fix\n\nBound the declared stream size before allocating in `extractPart`: reject negative sizes and sizes above a sane policy cap (1 GiB is far above any real encrypted-workbook part) with `ErrWorkbookFileFormat`, so malformed CFBs are rejected as errors instead of panicking or OOMing. A complete patch has been provided to the maintainer; verified that the patch blocks the PoC while legitimate `Encrypt()` output still opens.","published":"2026-10-07T17:48:03.233Z","modified":"2026-10-21T02:30:48.300224153Z","cvss":{"score":7.5,"severity":"HIGH","vector":"CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"},"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Go","name":"github.com/xuri/excelize/v2","fixedVersion":"2.11.1-0.20260912113515-5f636f9dcde5"}],"fix":{"url":"https://github.com/qax-os/excelize/commit/5f636f9dcde55911f14290060b017f0cc88cd694","label":"qax-os/excelize@5f636f9"},"references":[{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/107xxx/CVE-2026-107215.json"},{"type":"ADVISORY","url":"https://github.com/qax-os/excelize/security/advisories/GHSA-x2q3-8cjh-766f"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-107215"},{"type":"FIX","url":"https://github.com/qax-os/excelize/commit/5f636f9dcde55911f14290060b017f0cc88cd694"},{"type":"FIX","url":"https://github.com/qax-os/excelize/pull/2396"},{"type":"PACKAGE","url":"https://github.com/qax-os/excelize"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T02:30:48.300224153Z"}}