Forte Float128 — [H-02] Sqrt silently stops the entire call frame on packed 0

Source & credit. Reproduction of a public audit finding curated by AuditVault — the original finding: 55704-h-02-sqrt-function-silently-reverts-the-entire-control-flow. Standalone Foundry PoC and full write-up: 55704-h-02-sqrt-function-silently-reverts-the-entire-control-flow_exp in the evm-hack-registry mirror.

Vulnerability classes: integer-bounds · logic/wrong-condition · impact/dos/selective · misassumption/math-is-safe

Reproduction: self-contained Foundry PoC with only forge-std — no fork, no RPC. Full trace: output.txt.

Key info#


Impact	HIGH — `sqrt(0)` terminates the whole call frame with empty success; post-sqrt logic never runs
Protocol	Forte Float128 solidity library
Vulnerable code	`Float128.sqrt` zero guard — Yul `stop()`
Bug class	Wrong control-flow primitive (`stop` vs `return 0`)
Finding	Code4rena 2025-04-forte-float128-solidity-library · #55704 · 0xpetern
Report	code4rena.com/reports/2025-04-forte-float128-solidity-library
Source	AuditVault
Compiler	`^0.8.24`

TL;DR#

Mathematically, sqrt(0) = 0.
Float128 uses assembly stop() on packed zero — equivalent to return(0,0) for the entire call frame.
Because sqrt is internal pure, it is inlined: any protocol function that calls sqrt(0) then settles/transfers silently skips the rest while still reporting success to its caller.

The vulnerable code#

SOLIDITY

if iszero(a) {
    stop() // @> VULN: ends the whole call frame
}

Fix:

DIFF

  if iszero(a) {
-     stop()
+     r := 0
+     leave
  }

Root cause#

stop() is not a function-local return. It aborts the current message call with empty returndata. Inlined into a protocol external function, that kills settlement code that follows sqrt.

Preconditions#

A call path invokes sqrt with packedFloat 0 (e.g. empty balance, zero rate).

Attack walkthrough#

Non-zero control path: guard falls through, settlement flag set.
Zero path: stop() succeeds the subcall with empty returndata.
Settlement body never runs (settled stays false) — silent partial execution.

Diagrams#

sequenceDiagram participant P as Protocol.computeAndSettle participant S as Float128.sqrt inlined P->>S: a = 0 Note over S: stop equals return 0,0 S-->>P: empty success Note over P: settlement body never runs

Impact#

Financial protocols that call sqrt mid-function can leave state half-updated or skip payouts without a revert — hard-to-debug silent failures.

Taxonomy (AuditVault)#

[[integer-bounds]] · [[logic/wrong-condition]] · [[impact/dos/selective]] · [[misassumption/math-is-safe]]

Sources#

AuditVault finding #55704
Code4rena report
Reduced from code-423n4/2025-04-forte @4d6694f / src/Float128.sol ~L712

Sources & further analysis#

Reproductions & code

Standalone PoC + full trace: 55704-h-02-sqrt-function-silently-reverts-the-entire-control-flow_exp (evm-hack-registry mirror).
AuditVault finding: 55704-h-02-sqrt-function-silently-reverts-the-entire-control-flow.

Alerts & third-party analyses

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