StackVec::extend used the lower and upper bounds from an Iterator's size_hint to determine how many items to push into the stack based vector. If the size_hint implementation returned a lower bound that was larger than the upper bound, StackVec would write out of bounds and overwrite memory on the stack. As mentioned by the size_hint documentation, size_hint is mainly for optimization and incorrect implementations should not lead to memory safety issues.

References

• https://nvd.nist.gov/vuln/detail/CVE-2021-29939
• Alexhuszagh/rust-stackvector#2
• https://rustsec.org/advisories/RUSTSEC-2021-0048.html