{"id":"CVE-2025-47285","aliases":["EUVD-2025-15445","GHSA-qhr6-mgqr-mchm","PYSEC-2026-2029"],"status":"published","title":"Vyper's `concat()` builtin may elide side-effects for zero-length arguments","description":"Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, `concat()` may skip evaluation of side effects when the length of an argument is zero. This is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero. In practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`. The fix is available in pull request 4644 and expected to be part of the 0.4.2 release. As a workaround, don't have side effects in expressions which construct zero-length bytestrings.","severity":{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P","level":"low","assessor":"cna","from":"cve"},"epss":{"day":"2026-11-09","score":0.00464,"percentile":0.40458,"model":"v2026.06.15"},"facts":{"in_kev":false,"known_exploited":false,"public_exploit":false,"fix_available":false},"cwes":["CWE-691"],"published":"2025-05-15T17:24:03Z","modified":"2026-10-28T02:30:33Z","affected":[{"source":"ghsa","ecosystem":"PyPI","package":"vyper","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"0.4.2rc1"}]}]},{"source":"osv","ecosystem":"PyPI","package":"vyper","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"0.4.2rc1"}]}],"versions":["0.1.0b1","0.1.0b10","0.1.0b11","0.1.0b12","0.1.0b13","0.1.0b14","0.1.0b15","0.1.0b16","0.1.0b17","0.1.0b2","0.1.0b3","0.1.0b4","0.1.0b5","0.1.0b6","0.1.0b7","0.1.0b8","0.1.0b9","0.2.1","0.2.10","0.2.11","0.2.12","0.2.13","0.2.14","0.2.15","0.2.16","0.2.2","0.2.3","0.2.4","0.2.5","0.2.6","0.2.7","0.2.8","0.2.9","0.3.0","0.3.1","0.3.10","0.3.10rc1","0.3.10rc2","0.3.10rc3","0.3.10rc4","0.3.10rc5","0.3.2","0.3.3","0.3.4","0.3.5","0.3.6","0.3.7","0.3.8","0.3.9","0.4.0","0.4.0b1","0.4.0b2","0.4.0b3","0.4.0b4","0.4.0b5","0.4.0b6","0.4.0rc1","0.4.0rc2","0.4.0rc3","0.4.0rc4","0.4.0rc5","0.4.0rc6","0.4.1","0.4.1b1","0.4.1b2","0.4.1b3","0.4.1b4","0.4.1rc1","0.4.1rc2","0.4.1rc3","0.4.2rc1"]},{"source":"osv","ecosystem":"PyPI","package":"vyper","ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"last_affected":"0.4.2rc1"}]}],"versions":["0.1.0b1","0.1.0b10","0.1.0b11","0.1.0b12","0.1.0b13","0.1.0b14","0.1.0b15","0.1.0b16","0.1.0b17","0.1.0b2","0.1.0b3","0.1.0b4","0.1.0b5","0.1.0b6","0.1.0b7","0.1.0b8","0.1.0b9","0.2.1","0.2.10","0.2.11","0.2.12","0.2.13","0.2.14","0.2.15","0.2.16","0.2.2","0.2.3","0.2.4","0.2.5","0.2.6","0.2.7","0.2.8","0.2.9","0.3.0","0.3.1","0.3.10","0.3.10rc1","0.3.10rc2","0.3.10rc3","0.3.10rc4","0.3.10rc5","0.3.2","0.3.3","0.3.4","0.3.5","0.3.6","0.3.7","0.3.8","0.3.9","0.4.0","0.4.0b1","0.4.0b2","0.4.0b3","0.4.0b4","0.4.0b5","0.4.0b6","0.4.0rc1","0.4.0rc2","0.4.0rc3","0.4.0rc4","0.4.0rc5","0.4.0rc6","0.4.1","0.4.1b1","0.4.1b2","0.4.1b3","0.4.1b4","0.4.1rc1","0.4.1rc2","0.4.1rc3","0.4.2rc1"]}],"products":[{"vendor":"vyperlang","product":"vyper"}],"references":[{"url":"https://github.com/vyperlang/vyper/security/advisories/GHSA-qhr6-mgqr-mchm","tags":["x_refsource_CONFIRM","advisory","web"]},{"url":"https://github.com/vyperlang/vyper/pull/4644","tags":["x_refsource_MISC","fix","web"]},{"url":"https://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562","tags":["x_refsource_MISC","web"]},{"url":"https://nvd.nist.gov/vuln/detail/CVE-2025-47285","tags":["advisory"]},{"url":"https://github.com/advisories/GHSA-qhr6-mgqr-mchm","tags":["advisory"]},{"url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2025/47xxx/CVE-2025-47285.json","tags":["advisory"]},{"url":"https://github.com/vyperlang/vyper","tags":["package"]},{"url":"https://pypi.org/project/vyper","tags":["package"]}],"sources":{"cve":[{"id":"CVE-2025-47285","title":"Vyper's `concat()` builtin may elide side-effects for zero-length arguments","description":"Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, `concat()` may skip evaluation of side effects when the length of an argument is zero. This is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero. In practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`. The fix is available in pull request 4644 and expected to be part of the 0.4.2 release. As a workaround, don't have side effects in expressions which construct zero-length bytestrings.","status":"published","published":"2025-05-15T17:24:03.442Z","modified":"2025-05-15T18:37:33.747Z","cvss":[{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P","level":"low","assessor":"cna"}],"cwes":["CWE-691"],"updated_at":"2026-11-02T14:32:25.982948Z"}],"euvd":[{"id":"EUVD-2025-15445","description":"Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, `concat()` may skip evaluation of side effects when the length of an argument is zero. This is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero. In practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`. The fix is available in pull request 4644 and expected to be part of the 0.4.2 release. As a workaround, don't have side effects in expressions which construct zero-length bytestrings.","status":"published","published":"2025-05-15T17:24:03Z","modified":"2025-05-15T18:37:33Z","cvss":[{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P","level":"low"}],"updated_at":"2026-11-02T20:32:46.094311Z"}],"ghsa":[{"id":"GHSA-qhr6-mgqr-mchm","title":"Vyper's `concat()` builtin may elide side-effects for zero-length arguments","description":"### Impact\n`concat()` may skip evaluation of side effects when the length of an argument is zero. this is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero:\nhttps://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562\n\nin practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`.\n\nthe following example demonstrates how the issue would look in user code\n```vyper\ncounter: public(uint256)\n\n@external\ndef test() -\u003e Bytes[256]:\n    a: Bytes[256] = concat(b\"\" if self.sideeffect() else b\"\", b\"aaaa\")\n    return a\n\ndef sideeffect() -\u003e bool:\n    self.counter += 1\n    return True\n```\n\nthe severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.\n\n### Patches\n\nfix is tracked in https://github.com/vyperlang/vyper/pull/4644\n\n### Workarounds\ndon't have side effects in expressions which construct zero-length bytestrings.\n\n### References\n_Are there any links users can visit to find out more?_","status":"published","published":"2025-05-16T14:10:25Z","modified":"2025-05-16T14:10:26Z","cvss":[{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P","level":"low"}],"cwes":["CWE-691"],"updated_at":"2026-11-02T14:38:43.998596Z"}],"nvd":[{"id":"CVE-2025-47285","description":"Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, `concat()` may skip evaluation of side effects when the length of an argument is zero. This is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero. In practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`. The fix is available in pull request 4644 and expected to be part of the 0.4.2 release. As a workaround, don't have side effects in expressions which construct zero-length bytestrings.","status":"published","published":"2025-05-15T18:15:38.177Z","modified":"2026-06-17T09:27:40.103Z","cvss":[{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X","level":"low","assessor":"secondary"}],"cwes":["CWE-691"],"updated_at":"2026-11-02T14:11:08.321805Z"}],"osv":[{"id":"CVE-2025-47285","title":"Vyper's `concat()` builtin may elide side-effects for zero-length arguments","description":"Vyper is the Pythonic Programming Language for the Ethereum Virtual Machine. In versions up to and including 0.4.2rc1, `concat()` may skip evaluation of side effects when the length of an argument is zero. This is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero. In practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`. The fix is available in pull request 4644 and expected to be part of the 0.4.2 release. As a workaround, don't have side effects in expressions which construct zero-length bytestrings.","status":"published","published":"2025-05-15T17:24:03.442Z","modified":"2026-10-28T02:30:33.608459208Z","cvss":[{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P","level":"low"}],"cwes":["CWE-691"],"updated_at":"2026-11-02T15:44:49.707645Z"},{"id":"GHSA-qhr6-mgqr-mchm","title":"Vyper's `concat()` builtin may elide side-effects for zero-length arguments","description":"### Impact\n`concat()` may skip evaluation of side effects when the length of an argument is zero. this is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero:\nhttps://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562\n\nin practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`.\n\nthe following example demonstrates how the issue would look in user code\n```vyper\ncounter: public(uint256)\n\n@external\ndef test() -\u003e Bytes[256]:\n    a: Bytes[256] = concat(b\"\" if self.sideeffect() else b\"\", b\"aaaa\")\n    return a\n\ndef sideeffect() -\u003e bool:\n    self.counter += 1\n    return True\n```\n\nthe severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.\n\n### Patches\n\nfix is tracked in https://github.com/vyperlang/vyper/pull/4644\n\n### Workarounds\ndon't have side effects in expressions which construct zero-length bytestrings.\n\n### References\n_Are there any links users can visit to find out more?_","status":"published","published":"2025-05-16T14:10:25Z","modified":"2026-07-07T17:56:14.307189784Z","cvss":[{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P","level":"low"}],"cwes":["CWE-691"],"updated_at":"2026-11-02T15:46:54.760124Z"},{"id":"PYSEC-2026-2029","title":"Vyper's `concat()` builtin may elide side-effects for zero-length arguments","description":"### Impact\n`concat()` may skip evaluation of side effects when the length of an argument is zero. this is due to a fastpath in the implementation which skips evaluation of argument expressions when their length is zero:\nhttps://github.com/vyperlang/vyper/blob/68b68c4b30c5ef2f312b4674676170b8a6eaa316/vyper/builtins/functions.py#L560-L562\n\nin practice, it would be very unusual in user code to construct zero-length bytestrings using an expression with side-effects, since zero-length bytestrings are typically constructed with the empty literal `b\"\"`; the only way to construct an empty bytestring which has side effects would be with the ternary operator introduced in v0.3.8, e.g. `b\"\" if self.do_some_side_effect() else b\"\"`.\n\nthe following example demonstrates how the issue would look in user code\n```vyper\ncounter: public(uint256)\n\n@external\ndef test() -\u003e Bytes[256]:\n    a: Bytes[256] = concat(b\"\" if self.sideeffect() else b\"\", b\"aaaa\")\n    return a\n\ndef sideeffect() -\u003e bool:\n    self.counter += 1\n    return True\n```\n\nthe severity assigned is low, since, as mentioned, this would be a very unusual pattern in user-code.\n\n### Patches\n\nfix is tracked in https://github.com/vyperlang/vyper/pull/4644\n\n### Workarounds\ndon't have side effects in expressions which construct zero-length bytestrings.\n\n### References\n_Are there any links users can visit to find out more?_","status":"published","published":"2026-07-07T16:02:52.484672Z","modified":"2026-07-07T17:47:20.266877349Z","cvss":[{"version":"4.0","score":2.9,"vector":"CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P","level":"low"}],"updated_at":"2026-11-02T15:52:55.537808Z"}]}}