GHSA-QWGH-2VCV-G2F7

Vulnerability from github – Published: 2026-08-19 19:15 – Updated: 2026-08-19 19:15
VLAI
Summary
block_buffer: panic corrupts inline buffer position
Details

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

Severity
6.3 (Medium)
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
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{
  "affected": [
    {
      "package": {
        "ecosystem": "crates.io",
        "name": "block_buffer"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "0.12.1"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [],
  "database_specific": {
    "cwe_ids": [
      "CWE-119"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-08-19T19:15:34Z",
    "nvd_published_at": null,
    "severity": "MODERATE"
  },
  "details": "### Summary\n\nA caught panic may leave the cursor position of `EagerBuffer` or `ReadBuffer` in a corrupted state; this in turn allows out-of-bounds reads/writes.\n\n### Details \u0026 PoC\n\nThe following two tests fail miri:\n\n```rust\n#[cfg(miri)]\n#[test]\nfn eager_digest_blocks_panic_corrupts_inline_position() {\n    // `EagerBuffer` stores its cursor in the last byte of the internal block.\n    // When `digest_blocks` completes a previously partial block, it overwrites\n    // that byte with input data before invoking the caller-provided `compress`\n    // callback. If the callback panics, safe code can catch the panic and keep\n    // using the buffer while its cursor byte no longer satisfies the internal\n    // `pos \u003c block_size` invariant. Under Miri this `get_pos` call reaches the\n    // `unreachable_unchecked` used for the assumed-valid cursor.\n    let mut buf = EagerBuffer::\u003cU4\u003e::new(\u0026[1, 2]);\n\n    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {\n        buf.digest_blocks(\u0026[3, 0xff], |_| panic!(\"simulated compression failure\"));\n    }));\n\n    let _ = buf.get_pos();\n}\n\n#[cfg(miri)]\n#[test]\nfn read_buffer_generator_panic_corrupts_inline_position() {\n    // `ReadBuffer` stores its cursor in `buffer[0]`, but `write_block` gives\n    // `gen_block` mutable access to the whole internal block before restoring\n    // `buffer[0]` to a valid cursor. If `gen_block` writes an arbitrary first\n    // byte and panics, safe code can catch the panic and later observe an\n    // invalid cursor. Under Miri this `get_pos` call reaches the\n    // `unreachable_unchecked` used for the assumed-valid cursor.\n    let mut buf = ReadBuffer::\u003cU4\u003e::default();\n\n    let _ = std::panic::catch_unwind(std::panic::AssertUnwindSafe(|| {\n        buf.write_block(\n            1,\n            |block| {\n                block[0] = 0xff;\n                panic!(\"simulated block generation failure\");\n            },\n            |_| {},\n        );\n    }));\n\n    let _ = buf.get_pos();\n}\n```\n\nThey fail on an `unreachable_unchecked!()` under the invariant for the `pos` to always be within bounds of the block.\n\n### Impact\n\nWhile 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.\n\nFor this to be exploitable, the attacker also needs a way to trigger a panic here; I have not investigated how feasible that is.\n\n### Credits\n\nThe issue was discovered by GPT-5.5",
  "id": "GHSA-qwgh-2vcv-g2f7",
  "modified": "2026-08-19T19:15:34Z",
  "published": "2026-08-19T19:15:34Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/RustCrypto/utils/security/advisories/GHSA-qwgh-2vcv-g2f7"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/RustCrypto/utils"
    },
    {
      "type": "WEB",
      "url": "https://github.com/RustCrypto/utils/releases/tag/block-buffer-v0.12.1"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "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",
      "type": "CVSS_V4"
    }
  ],
  "summary": "block_buffer: panic corrupts inline buffer position"
}



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