ep_etherpad-lite: Import/export uses Math.random() for temp file paths; predictable paths on shared /tmp enable symlink-based file overwrite

Description

src/node/handler/ImportHandler.ts and src/node/handler/ExportHandler.ts both compute their temporary working-file paths as:

const randNum = Math.floor(Math.random() * 0xFFFFFFFF);
const srcFile = `${os.tmpdir()}/etherpad_export_${randNum}.html`;
const destFile = `${os.tmpdir()}/etherpad_export_${randNum}.${type}`;

Two flaws compound:

1. Math.random() is not crypto-secure. It yields at most ~32 bits of entropy and is predictable across calls within the same Node process (V8 shares PRNG state between consecutive Math.random() invocations). An attacker on the same host who observes any earlier temp-file name from logs or other side channels can predict subsequent names.

2. The paths land in os.tmpdir(). On a typical Linux system this is /tmp — a shared world-writable directory. An unprivileged local attacker can pre-create a symbolic link at a predicted path pointing at any file the Etherpad process can write:

   ln -s /etc/etherpad/SESSIONKEY.txt /tmp/etherpad_export_<predicted>.html

   When ExportHandler calls fs.writeFile(srcFile, html) (or ImportHandler calls fs.rename(srcFile, destFile) / soffice writes its converted output to the path), the open syscall follows the symlink and either reads from or overwrites the linked target. For deployments where the Etherpad process runs as a privileged user (notably some Docker base images that run as root, snap confinement edge cases, or hand-rolled systemd units), this becomes arbitrary file overwrite.

The Import path is more impactful in practice: the file content the attacker can land in the symlink target is partially attacker-controlled (the post-soffice/post-mammoth conversion output of the uploaded document).

Severity rationale

• AV:L — requires local access to the host that runs Etherpad. Multi-tenant hosts (shared dev boxes, k8s shared-node setups, single-server CI workers) are the realistic threat surface.
• AC:H — attacker needs to predict the temp filename, which requires observing prior names or shared PRNG state.
• PR:L — any unprivileged local account.
• UI:N — no user interaction.
• S:C — scope change from local user to whatever the Etherpad process can write.
• C:L / I:L / A:N — bounded by what the Etherpad process can already touch; confidentiality is via secondary read-paths (the LibreOffice conversion error log can echo bytes from the symlinked file).

Single-tenant deployments where only the Etherpad operator has shell access on the host are not exposed.

Affected versions

• All ep_etherpad-lite versions through v3.0.0 (inclusive). The Math.floor(Math.random() * 0xFFFFFFFF) pattern is present in src/node/handler/ImportHandler.ts and src/node/handler/ExportHandler.ts for as far back as the repository history extends — older than the 2024-03-16 snapshot at commit 107598b where it first appears in the current file paths, and predating the v1.x era.

Patched versions

• ep_etherpad-lite >= 3.1.0 — the fix is on develop HEAD as commit 8c6104c. Update this field with the actual tagged release version when it ships.

Proof of concept (sketch)

# Pre-condition: attacker has shell on the same host as Etherpad,
# has read access to /tmp, and Etherpad's process can write to the
# chosen target (e.g. runs as root inside a Docker container).

# 1. Observe a temp filename from logs (or guess via prior exports).
TARGET=/etc/etherpad/SESSIONKEY.txt
GUESS=/tmp/etherpad_export_$(./predict-next-random)$EXT  # implementation-specific

# 2. Place the symlink before Etherpad creates the file.
ln -s "$TARGET" "$GUESS"

# 3. Trigger an export from the Etherpad UI (or via the API).
# Etherpad calls fs.writeFile(srcFile, ...) which open()s the symlink
# target with O_WRONLY|O_TRUNC and clobbers $TARGET with HTML content.

In practice the exact reproduction depends on the host's PRNG state predictability and the deployment's process-owner privileges. A reliable exploit chain needs setpriv-style host access or a sibling tenant.

Workarounds

• Run Etherpad in a container with a private /tmp (Docker: --tmpfs /tmp:rw,noexec,nosuid,nodev,size=64m, systemd: PrivateTmp=true).
• Ensure the Etherpad process does NOT run as root and has write access only to its own data directory.
• Set TMPDIR to an Etherpad-private directory in the environment.

Fix

Patched in 8c6104c (PR #7784):

-const randNum = Math.floor(Math.random() * 0xFFFFFFFF);
+const randNum = crypto.randomBytes(16).toString('hex');

128 bits of CSPRNG entropy. Predicting the next filename is no longer feasible.

A bigger follow-up — per-request mkdtemp subdirectories with O_EXCL/O_NOFOLLOW semantics — is deferred to a later release; the immediate window (predictable 32-bit collisions across processes) is what the patch closes.

Resources

• Patched in: https://github.com/ether/etherpad/pull/7784 (squash commit 8c6104c).
• Math.random() is not appropriate for security-sensitive contexts. See the MDN guidance for Math.random() and the Node.js recommendation to use crypto.randomBytes for unpredictable values.

Credits

Reported during an internal security audit by Claude (via @JohnMcLear).