GHSA-X2Q3-8CJH-766F
Vulnerability from github – Published: 2026-10-08 16:31 – Updated: 2026-10-08 16:31Summary
extractPart allocates directly from the mscfb directory-entry size with no clamping (crypt.go:198, same missing bound at crypt.go:204):
buf := make([]byte, entry.Size)
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.
Details
Two consequences, both execution-confirmed against pristine master (ecd99d761fe0, 2026-09-08):
- Hard crash: a version-4 CFB declaring
streamSize = 0xFFFFFFFFFFFFFFFFyieldsentry.Size == -1→panic: runtime error: makeslice: len out of range, which escapesDecrypt(openReaderAtconverts errors, not panics,excelize.go:210-214) and kills the process. - Memory exhaustion: any CFB (v3 suffices) declaring
streamSizeup to0xFFFFFFFFforces 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.
Call path: OpenFile/OpenReader/OpenBytes → openReaderAt (excelize.go:208-215, OLE magic sniff) → Decrypt (crypt.go:145-150) → extractPart (crypt.go:198).
PoC
A 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.
Impact
An 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.
Proposed fix
Bound 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.
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/xuri/excelize/v2"
},
"ranges": [
{
"events": [
{
"introduced": "2.3.1"
},
{
"fixed": "2.11.1-0.20260912113515-5f636f9dcde5"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-107215"
],
"database_specific": {
"cwe_ids": [
"CWE-789"
],
"github_reviewed": true,
"github_reviewed_at": "2026-10-08T16:31:26Z",
"nvd_published_at": "2026-10-07T18:17:19Z",
"severity": "HIGH"
},
"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 \u2192 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\") \u2014 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` \u2192 `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 \u2014 an amplification factor of ~350,000\u00d7, trivially repeated per request in memory-limited deployments.\n\nCall path: `OpenFile`/`OpenReader`/`OpenBytes` \u2192 `openReaderAt` (`excelize.go:208-215`, OLE magic sniff) \u2192 `Decrypt` (`crypt.go:145-150`) \u2192 `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 \u2014 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\u00d7 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.",
"id": "GHSA-x2q3-8cjh-766f",
"modified": "2026-10-08T16:31:26Z",
"published": "2026-10-08T16:31:26Z",
"references": [
{
"type": "WEB",
"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": "WEB",
"url": "https://github.com/qax-os/excelize/pull/2396"
},
{
"type": "WEB",
"url": "https://github.com/qax-os/excelize/commit/5f636f9dcde55911f14290060b017f0cc88cd694"
},
{
"type": "PACKAGE",
"url": "https://github.com/qax-os/excelize"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
],
"summary": "Excelize: extractPart allocates attacker-controlled, unbounded and negative-sized buffers from CFB directory entries: remote panic / OOM DoS"
}
Sightings
| Author | Source | Type | Date | Other |
|---|
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- Not confirmed: The user expressed doubt about the validity of the vulnerability.
- Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.
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