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block_buffer: panic corrupts inline buffer position

Moderate severity GitHub Reviewed Published Jun 11, 2026 in RustCrypto/utils • Updated Aug 19, 2026

Package

cargo block_buffer (Rust)

Affected versions

< 0.12.1

Patched versions

0.12.1

Description

Summary

A caught panic may leave the cursor position of EagerBuffer or ReadBuffer in a corrupted state; this in turn allows out-of-bounds reads/writes.

Details & PoC

The following two tests fail miri:

#[cfg(miri)]
#[test]
fn eager_digest_blocks_panic_corrupts_inline_position() {
    // `EagerBuffer` stores its cursor in the last byte of the internal block.
    // When `digest_blocks` completes a previously partial block, it overwrites
    // that byte with input data before invoking the caller-provided `compress`
    // callback. If the callback panics, safe code can catch the panic and keep
    // using the buffer while its cursor byte no longer satisfies the internal
    // `pos < block_size` invariant. Under Miri this `get_pos` call reaches the
    // `unreachable_unchecked` used for the assumed-valid cursor.
    let mut buf = EagerBuffer::<U4>::new(&[1, 2]);

    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
        buf.digest_blocks(&[3, 0xff], |_| panic!("simulated compression failure"));
    }));

    let _ = buf.get_pos();
}

#[cfg(miri)]
#[test]
fn read_buffer_generator_panic_corrupts_inline_position() {
    // `ReadBuffer` stores its cursor in `buffer[0]`, but `write_block` gives
    // `gen_block` mutable access to the whole internal block before restoring
    // `buffer[0]` to a valid cursor. If `gen_block` writes an arbitrary first
    // byte and panics, safe code can catch the panic and later observe an
    // invalid cursor. Under Miri this `get_pos` call reaches the
    // `unreachable_unchecked` used for the assumed-valid cursor.
    let mut buf = ReadBuffer::<U4>::default();

    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {
        buf.write_block(
            1,
            |block| {
                block[0] = 0xff;
                panic!("simulated block generation failure");
            },
            |_| {},
        );
    }));

    let _ = buf.get_pos();
}

They fail on an unreachable_unchecked!() under the invariant for the pos to always be within bounds of the block.

Impact

While the byte that overwrites pos may come from untrusted input and is therefore attacker-controlled, this still relies on the surrounding code catching the panic and carrying on, which should be uncommon in practice.

For this to be exploitable, the attacker also needs a way to trigger a panic here; I have not investigated how feasible that is.

Credits

The issue was discovered by GPT-5.5

References

@newpavlov newpavlov published to RustCrypto/utils Jun 11, 2026
Published to the GitHub Advisory Database Aug 19, 2026
Reviewed Aug 19, 2026
Last updated Aug 19, 2026

Severity

Moderate

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 Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality Low
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:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N

EPSS score

Weaknesses

Improper Restriction of Operations within the Bounds of a Memory Buffer

The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data. Learn more on MITRE.

CVE ID

No known CVE

GHSA ID

GHSA-qwgh-2vcv-g2f7

Source code

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