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SwiftNIO: Out-of-bounds write via ByteBuffer index and length UInt32 overflow

High severity GitHub Reviewed Published May 21, 2026 in apple/swift-nio • Updated Jun 12, 2026

Package

swift github.com/apple/swift-nio (Swift)

Affected versions

>= 1.0.0, <= 2.99.0

Patched versions

2.100.0

Description

Summary

A program using swift-nio is vulnerable to a potential out-of-bounds write when attacker-controlled index or length values exceeding UInt32.max are passed to some ByteBuffer methods. This affects all swift-nio versions from 1.0.0 to 2.99.0. It is fixed in 2.100.0 and later releases.

Details

ByteBuffer internally stores indices and capacities as UInt32 values. The internal helper functions _toIndex and _toCapacity, which convert from Int to UInt32, used UInt32(truncatingIfNeeded:). On 64-bit platforms, this silently discards the upper 32 bits of the value rather than trapping on overflow. For example, a value of UInt32.max + 1 (0x100000000) would be truncated to 0.

This truncation can cause safety preconditions to pass when they should fail. Subsequent operations would then use the incorrect truncated value, potentially leading to out-of-bounds memory writes or reads.

The affected ByteBuffer methods that may lead to out-of-bounds writes are:

  • copyBytes(at:to:length:) — a crafted destination index exceeding UInt32.max could copy bytes to an incorrect offset.
  • writeWithUnsafeMutableBytes(minimumWritableBytes:) — a crafted minimumWritableBytes exceeding UInt32.max could provide the caller with a buffer pointer of incorrect length, which can easily be subsequently overflowed.

The affected ByteBuffer methods that have logic errors but do neither expose out-of-bounds reads nor out-of-bounds writes:

  • moveReaderIndex(forwardBy:) / moveWriterIndex(forwardBy:) — a crafted offset exceeding UInt32.max could move indices to incorrect positions, bypassing bounds checks. These indices cannot be out of the bounds of the buffer, so they do not expose access to uninitialized memory or produce wild pointers.
  • The ByteBuffer(takingOwnershipOf:allocator:) initialiser — passing a buffer larger than UInt32.max bytes could create a ByteBuffer with an incorrect capacity.

Outside of these methods, there are still impacts, but they are simply logical bugs. In these cases applications can be forced to read from or write to unexpected parts of the buffer. This does not cause memory-safety issues, but it can cause logical issues or corruption of outbound packets.

Impact

Exploitation requires an attacker to influence the index, offset, or length parameter of the affected ByteBuffer methods with a value exceeding UInt32.max (approximately 4 GiB). This is a high bar for most applications: attacker-controlled length parameters to ByteBuffer are typically used on the read path, and the above methods are typically not used on the read paths. However, applications that calculate buffer positions arithmetically from untrusted input when attempting to do writes, or that process very large payloads, may be at risk of memory safety issues.

Other applications may encounter logical issues due to reading unexpected bytes, or writing to unexpected parts of the buffer.

When the memory-safety issue is exploitable, the consequences are severe. Because truncatingIfNeeded silently produces an incorrect but valid UInt32 value, subsequent operations may write to or read from memory outside the valid buffer region. In optimised (release) builds where preconditions are not checked, this could lead to out-of-bounds memory writes, potentially corrupting adjacent heap memory.

In debug builds, some of these conditions are caught by assertions, but truncatingIfNeeded occurs before the assertion checks the (already-truncated) value, so even assertions may not reliably catch the issue.

Patches

The issue is fixed by replacing UInt32(truncatingIfNeeded:) with UInt32(_:) in the _toIndex and _toCapacity helper functions. The UInt32(_:) initialiser traps on overflow in both debug and release builds, converting a potential silent memory corruption into a deterministic crash.

One call site in getSlice(at:length:) retains truncatingIfNeeded because prior bounds checks against the non-truncated Int values mathematically guarantee the values fit within UInt32.

Workarounds

Applications can mitigate this issue by validating that all index and length values passed to ByteBuffer methods do not exceed UInt32.max (4,294,967,295). In practice, most applications are not affected because buffer indices are derived from protocol parsing rather than raw untrusted input.

References

@Lukasa Lukasa published to apple/swift-nio May 21, 2026
Published to the GitHub Advisory Database Jun 12, 2026
Reviewed Jun 12, 2026
Last updated Jun 12, 2026

Severity

High

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 None
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
Integrity High
Availability Low
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:N/UI:N/VC:L/VI:H/VA:L/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Out-of-bounds Write

The product writes data past the end, or before the beginning, of the intended buffer. Learn more on MITRE.

CVE ID

CVE-2026-43671

GHSA ID

GHSA-r3rc-9hpw-54v9

Source code

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