In the unique reclaim path of BytesMut::reserve, the condition

if v_capacity >= new_cap + offset

uses an unchecked addition. When new_cap + offset overflows usize in release builds, this condition may incorrectly pass, causing self.cap to be set to a value that exceeds the actual allocated capacity. Subsequent APIs such as spare_capacity_mut() then trust this corrupted cap value and may create out-of-bounds slices, leading to UB.

This behavior is observable in release builds (integer overflow wraps), whereas debug builds panic due to overflow checks.

PoC

use bytes::*;

fn main() {
    let mut a = BytesMut::from(&b"hello world"[..]);
    let mut b = a.split_off(5);

    // Ensure b becomes the unique owner of the backing storage
    drop(a);

    // Trigger overflow in new_cap + offset inside reserve
    b.reserve(usize::MAX - 6);

    // This call relies on the corrupted cap and may cause UB & HBO
    b.put_u8(b'h');
}

Workarounds

Users of BytesMut::reserve are only affected if integer overflow checks are configured to wrap. When integer overflow is configured to panic, this issue does not apply.

Impact

When user-provided input is provided to any type that parses with the RFC 2822 format, a denial of service attack via stack exhaustion is possible. The attack relies on formally deprecated and rarely-used features that are part of the RFC 2822 format used in a malicious manner. Ordinary, non-malicious input will never encounter this scenario.

Patches

A limit to the depth of recursion was added in v0.3.47. From this version, an error will be returned rather than exhausting the stack.

Workarounds

Limiting the length of user input is the simplest way to avoid stack exhaustion, as the amount of the stack consumed would be at most a factor of the length of the input.

If a certificate had more than one distributionPoint, then only the first distributionPoint would be considered against each CRL's IssuingDistributionPoint distributionPoint, and then the certificate's subsequent distributionPoints would be ignored.

The impact was that correctly provided CRLs would not be consulted to check revocation. With UnknownStatusPolicy::Deny (the default) this would lead to incorrect but safe Error::UnknownRevocationStatus. With UnknownStatusPolicy::Allow this would lead to inappropriate acceptance of revoked certificates.

This vulnerability is thought to be of limited impact. This is because both the certificate and CRL are signed -- an attacker would need to compromise a trusted issuing authority to trigger this bug. An attacker with such capabilities could likely bypass revocation checking through other more impactful means (such as publishing a valid, empty CRL.)

More likely, this bug would be latent in normal use, and an attacker could leverage faulty revocation checking to continue using a revoked credential.

This vulnerability is identified as GHSA-pwjx-qhcg-rvj4. Thank you to @1seal for the report.