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Add advisory for git2: DiffBinaryFile::data() for empty data triggers UB #2981

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Add advisory for git2: DiffBinaryFile::data() for empty data triggers UB #2981

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16 changes: 16 additions & 0 deletions crates/git2/RUSTSEC-0000-0000.md
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```toml
[advisory]
id = "RUSTSEC-0000-0000"
package = "git2"
date = "2026-06-12"
url = "https://github.com/rust-lang/git2-rs/pull/1279"
informational = "unsound"
keywords = ["git2"]

[versions]
patched = ["> 0.21.0"]
```

# Potential undefined behavior when calling DiffBinaryFile::data()

Git diffs with binary file changes do not always include the raw data of such changes, but sometimes just report *that* a change was made (e.g. with a line `Binary files /dev/null and b/binary.pdf differ` for a new file addition). When examining such a diff, `DiffBinaryFile::data()` caused git2 to pass a null pointer to the unsafe function `slice::from_raw_parts()`. Based on the safety section documentation of function, data must be non-null even for slices of length zero. Thus, the use of a null pointer leads to undefined behavior.