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Low

Vyper's sqrt doesn't define rounding behavior

  • Disclosed: 21.02.2025
  • Updated: 11.10.2026 12:19

Vyper's sqrt() builtin uses the babylonian method to calculate square roots of decimals. Unfortunately, improper handling of the oscillating final states may lead to sqrt incorrectly returning rounded up results.

the fix is tracked in https://github.com/vyperlang/vyper/pull/4486

Vulnerability Details

Vyper injects the following code to handle calculation of decimal sqrt. x is the input provided by user.

python
assert x >= 0.0
z: decimal = 0.0

if x == 0.0:
    z = 0.0
else:
    z = x / 2.0 + 0.5
    y: decimal = x

    for i: uint256 in range(256):
        if z == y:
            break
        y = z
        z = (x / z + z) / 2.0

Notably, the terminal condition of the algorithm is either z_cur == z_prev, or the algorithm runs for 256 rounds.

However, for certain inputs, z might actually oscillate between N and N + epsilon, where N ** 2 <= x < (N + epsilon) ** 2. This means that the current behavior does not define whether it will round up or down to the nearest epsilon.

The example snippet here returns 0.9999999999, the rounded up result for sqrt(0.9999999998). This is due to the oscillation ending in N + epsilon instead of N.

vyper
@external
def test():
    d: decimal = 0.9999999998
    r: decimal = sqrt(d)    #this will be 0.9999999999

Note that sqrt() diverges from isqrt() here -- isqrt() consistently rounds down, so it is not subject to the same issue.

Impact Details

Since sqrt() can be used for determining boundary conditions, rounding down is preferred. However, since sqrt() is used very rarely in the wild, this advisory has been assigned an impact of low.

Affected software

Click a technology to see all vulnerabilities affecting it.

PlatformPackageVulnerable versionsFixed in
Python (PyPI) vyper < 0.4.1 0.4.1

References

CVSS score -
CVSS vector
CVSS:4.0/AV:N/AC:H/AT:P/PR:L/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
Weakness (CWE)
CWE-682
Platform
Python (PyPI)
Package
vyper
Disclosed
21.02.2025
Updated
11.10.2026 12:19

Sources and licenses

OSV / GitHub Advisory Database - PYSEC-2025-29 - PyPA Advisory Database; CC BY 4.0

OSV / GitHub Advisory Database - GHSA-2p94-8669-xg86 - CC BY 4.0

Vulnerability history: vyper

All published vulnerabilities of this package (Python (PyPI)), newest first. Shows whether this is a one-off slip or a package that needs watching.

All (47) in the archive
Total entries
47
First → latest
03.2020 10.2026
Without a fix
5
Exploited in attacks
0

Severity breakdown

  • Critical 3
  • High 14
  • Medium 20
  • Low 10

Disclosures per year

Severity:
  1. Vyper's sqrt doesn't define rounding behavior this entry
    Low CVE-2025-26622 < 0.4.1 0.4.1
  2. Vyper negative array index bounds checks
    Critical · 9.8 CVE-2024-24563 < 0.4.0 0.4.0
  3. Vyper's bounds check on built-in `slice()` function can be overflowed
    Critical · 9.8 CVE-2024-24561 < 0.4.0 0.4.0
  4. concat built-in can corrupt memory in vyper
    High · 7.3 CVE-2024-22419 >= 0.3.0, < 0.4.0 0.4.0
  5. incorrect storage layout for contracts containing large arrays
    High · 7.5 CVE-2023-46247 < 0.3.8 0.3.8
  6. Vyper vulnerable to memory corruption in certain builtins utilizing `msize`
    High · 8.1 CVE-2023-42443 >= 0.3.4, < 0.3.10 0.3.10
  7. Vyper has incorrectly allocated named re-entrancy locks
    Critical · 8.7 CVE-2023-39363 >= 0.2.15, < 0.3.1 0.3.1
  8. Vyper vulnerable to incorrect ordering of arguments for kwargs passed to internal calls
    High · 7.5 CVE-2023-32059 < 0.3.8 0.3.8
  9. Vyper vulnerable to integer overflow in loop
    High · 7.5 CVE-2023-32058 < 0.3.8 0.3.8
  10. Vyper vulnerable to OOB DynArray access when array is on both LHS and RHS of an assignment
    High · 7.5 CVE-2023-31146 < 0.3.8 0.3.8
  11. vyper vulnerable to storage allocator overflow
    High · 7.5 CVE-2023-30837 < 0.3.8 0.3.8
  12. Incorrect success value returned in vyper
    High · 7.5 CVE-2023-30629 >= 0.3.1, < 0.3.8 0.3.8
  13. Multiple evaluation of contract address in call in vyper
    High · 7.5 CVE-2022-29255 < 0.3.4 0.3.4
  14. Vyper is a pythonic Smart Contract Language for the ethereum virtual machine. In affected versions, the return of `<iface>.returns_int128()` is not validated to fall within the bounds of `int128`. Thi
    High · 8.8 CVE-2022-24845 < 0.3.2 0.3.2
  15. Buffer Overflow in vyper
    High · 7.1 CVE-2022-24788 < 0.3.2 0.3.2
  16. Incorrect Comparison in Vyper
    High · 7.5 CVE-2022-24787 < 0.3.2 0.3.2
  17. Memory corruption when returning a literal struct with a private call inside of it
    High · 7.5 CVE-2021-41121 < 0.3.0 0.3.0
  18. Storage corruption due to variables overwritten by re-entrancy locks
    High ghsa-7f92-rr6w-cq64 >= 0.2.13, < 0.2.15 0.2.15

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