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How the CVE Works (Technical Details)
The vulnerability exists in the `openssl` crate versions 0.10.39 through 0.10.77, specifically within the `MdCtxRef::digest_final()` method. This method is a safe Rust wrapper around the OpenSSL C function EVP_DigestFinal(), which finalizes a hash digest operation.
1. Unsafe C API Assumption: The underlying C function `EVP_DigestFinal()` unconditionally writes `EVP_MD_CTX_size(ctx)` bytes of the final digest to the provided output buffer (out). This size is determined by the chosen hash algorithm (e.g., 32 bytes for SHA-256) and is fixed at compile time.
2. Missing Length Validation in Rust Wrapper: The `MdCtxRef::digest_final()` method in Rust accepted a mutable byte slice (&mut
</code>) as the output buffer. Critically, it did not verify that the length of this slice (<code>out.len()</code>) was at least as large as the digest size required by the context (<code>EVP_MD_CTX_size(ctx)</code>). 3. Reachable from Safe Rust: Because `digest_final()` is a safe function (not marked <code>unsafe</code>), developers could call it with an undersized buffer using only safe Rust code, bypassing Rust's typical memory safety guarantees. 4. Consequences: If a developer provided a buffer smaller than the required digest size, the Rust wrapper would pass its pointer to <code>EVP_DigestFinal()</code>, which would then write past the end of the allocated buffer. This out-of-bounds write overwrites adjacent memory, which is typically located on the call stack. This leads to stack corruption, resulting in immediate program crashes or, more critically, potentially exploitable undefined behavior. 5. Fix Method: The patch adds a check before the unsafe C call. It compares the required digest size (<code>self.size()</code>) against the provided buffer's length (<code>len</code>). If the buffer is too small, it returns an `Err` immediately, preventing the call to the unsafe C function. <h2 style="color: blue;">DailyCVE Form</h2> [bash] Platform: Rust (crates.io) Version: 0.10.39 - 0.10.77 Vulnerability: Buffer Overflow Severity: High Date: 2026-04-22 Prediction: 2026-04-22
What Undercode Say
The following security analysis and remediation steps are standard practice:
1. Check for vulnerable version in your Cargo.lock grep -A 2 "name = \"openssl\"" Cargo.lock | grep "version =" 2. Immediately upgrade to the patched version cargo update -p openssl --precise 0.10.78 3. Verify the fix is applied cargo tree | grep openssl 4. Run your test suite to ensure compatibility cargo test
Vulnerability Analysis in Rust Code
The vulnerable code pattern before the fix:
// VULNERABLE (pre-0.10.78)
impl MdCtxRef {
pub fn digest_final(&mut self, out: &mut [bash]) -> Result<(), ErrorStack> {
let mut len = u32::try_from(out.len()).unwrap_or(u32::MAX);
unsafe {
// ❌ No check; EVP_DigestFinal writes self.size() bytes, leading to write-overflow
cvt(ffi::EVP_DigestFinal(self.as_ptr(), out.as_mut_ptr(), &mut len))?;
}
Ok(())
}
}
// Example triggering code (safe Rust)
let mut ctx = MdCtx::new()?;
ctx.digest_init(Md::sha256())?;
ctx.digest_update(b"Some Crypto Text")?;
let mut tiny_buffer = [0u8; 16]; // Only 16 bytes, but SHA-256 needs 32!
ctx.digest_final(&mut tiny_buffer)?; // 💥 Stack corruption!
Patched Code (openssl 0.10.78)
// PATCHED (0.10.78+)
impl MdCtxRef {
pub fn digest_final(&mut self, out: &mut [bash]) -> Result<(), ErrorStack> {
let mut len = u32::try_from(out.len()).unwrap_or(u32::MAX);
// ✅ Added length validation
if self.size() > len as usize {
return Err(ErrorStack::get());
}
unsafe {
cvt(ffi::EVP_DigestFinal(self.as_ptr(), out.as_mut_ptr(), &mut len))?;
}
Ok(())
}
}
Exploit
- Type: Stack Buffer Overflow; Out-of-bounds write.
- Prerequisites: An application must call `MdCtxRef::digest_final()` on a context with a final digest size larger than the supplied output buffer.
- Attacker Control: An attacker may not control the overflow content directly, as it is the hash digest output. However, they could influence the data being hashed, potentially crafting specific overflow patterns.
- Outcome: The overflow corrupts adjacent stack memory. This typically results in an immediate crash. In more sophisticated scenarios, an attacker could overwrite a saved return address to gain control of the program's execution flow.
- Example Attack String: While the exact payload is architecture-specific, a minimal triggering condition is:
- Call `MdCtxRef::digest_init(Md::sha512())` (64-byte output)
- Call `digest_final(&mut [0u8; 32])` (only 32-byte buffer)
Protection from this CVE
- Immediate Action: Update the `openssl` crate to version 0.10.78 or later in your
Cargo.toml:[bash] openssl = "0.10.78"
- Dependency Scanning: Use `cargo audit` to identify vulnerable dependencies:
cargo install cargo-audit cargo audit
- Static Analysis: Integrate linters like `clippy` to catch suspicious patterns.
cargo clippy -- -W clippy::pedantic
- Runtime Detection: Use AddressSanitizer (ASan) during testing to catch out-of-bounds writes in development.
RUSTFLAGS="-Z sanitizer=address" cargo run --target x86_64-unknown-linux-gnu
Impact
- Confidentiality: High. Stack corruption could lead to memory disclosure, exposing sensitive information.
- Integrity: High. An attacker may be able to modify adjacent memory, including function pointers, to alter program behavior.
- Availability: High. The most immediate and common impact is a program crash, leading to denial of service.
- Attack Vector: Network. While triggered by a local function call, the vulnerable code is often used to verify network data (e.g., in TLS sessions), making it remotely exploitable in practice.
- CVSS 4.0 Score: 8.3 (High) –
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N. - Overall Risk: Critical for network services. Applications exposed to untrusted input that perform hashing on that input are at immediate risk of crash or potential remote code execution.
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Sources:
Reported By: github.com
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