x509: read the signed certificate DER from the x509 object

[yosuke-wolfssl]

Problem

wolfCLU_certSetup() aliased derBuf to inBuf, a pointer into the heap buffer holding the input PEM, then handed it to wolfSSL_i2d_X509(). Per the OpenSSL convention, i2d_* copies into a caller-supplied buffer with no bounds parameter:

pt = derBuf;                              /* points into the input PEM */
derBufSz = wolfSSL_i2d_X509(x509, &pt);   /* unbounded XMEMCPY */

The destination capacity is fixed by the input PEM length, while the amount written is the DER re-encoded after wolfCLU_setExtensions() and wolfSSL_X509_sign(). Nothing relates the two. A CSR PEM is roughly 1.37x its DER, so a plain conversion fits, but every extension added through -extfile/-extensions grows the output while inBufSz stays fixed. The i2d return value was also unchecked, so a negative result could reach wolfSSL_BIO_write() as a length.

Fix (src/x509/clu_cert_setup.c)

No new allocation is needed: the block only ever reads derBuf, and wolfSSL_i2d_X509() is wolfSSL_X509_get_der() plus a copy.

• wolfSSL_X509_get_der() returns the object's own DER, which wolfSSL_X509_sign() refreshes. This function already uses it for -fingerprint.
• const byte *derBuf makes the read-only use compiler-enforced.
• ret == WOLFCLU_SUCCESS now guards both output branches.

Closes f-9852.

Tests

TestX509ReqLargeExtensions signs a CSR carrying a twelve-entry subjectAltName supplied by -extfile, covering both output forms:

Case	Output path exercised
test_extfile_larger_than_input_pem_der_out	wolfSSL_BIO_write()
test_extfile_larger_than_input_pem_pem_out	wc_DerToPem()

Both skip when the build has no cert extensions or alt names.

Verification

• Pre-fix, ASan reports a 1967-byte write into the 904-byte input buffer and both new tests fail; post-fix both pass.
• make check: 23 passed, 2 skipped, 0 failed under ASan + UBSan, no sanitizer reports.
• Clean under the project's -Werror warning set.

[Copilot]

🟢 Approval recommended

The fix removes the unsafe unbounded write by switching to wolfSSL_X509_get_der() and is covered by new targeted tests that exercise both affected output paths under large extension growth.

Pull request overview

This pull request fixes a buffer overflow risk in the x509 -req output path by stopping reuse of the input PEM buffer as the destination for wolfSSL_i2d_X509() (which performs an unbounded copy), and instead reading the signed certificate’s DER directly from the WOLFSSL_X509 object. It also adds regression tests that ensure the signed certificate output can exceed the input CSR PEM size when large extensions are applied via -extfile/-extensions.

Changes:

• Replace wolfSSL_i2d_X509() writing into the input buffer with wolfSSL_X509_get_der() (borrowed DER + size) and add error handling for invalid DER length/pointer.
• Guard both DER and PEM output branches on ret == WOLFCLU_SUCCESS to prevent using an invalid length/pointer after DER retrieval failure.
• Add TestX509ReqLargeExtensions to exercise both DER output (wolfSSL_BIO_write) and PEM output (wc_DerToPem) paths when extensions make the output larger than the input CSR PEM.

File summaries

File	Description
src/x509/clu_cert_setup.c	Stops writing DER into the input PEM buffer; reads DER from the X509 object and adds error/flow guards to prevent bad-length writes.
tests/x509/x509-req-test.py	Adds regression tests that sign a CSR with oversized SAN extensions and verify both DER/PEM output paths handle outputs larger than the input CSR PEM.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Lite

---

💡 Add a code-review agent skill or configure MCP servers for context-aware, tailored reviews. Learn more in the docs.

[wolfSSL-Fenrir-bot]

Fenrir Automated Review — PR #292

Scan targets checked: wolfclu-bugs, wolfclu-src

Fenrir result: Approved ✅

No new issues found in the changed files.

Advisory only — this automated result does not count as a GitHub approval.