EXIM: CVE-2020-28025: Heap out-of-bounds read in pdkim_finish_bodyhash()

Mon, 26 May 2025 00:00:00 +0000

EXIM: Analysis 4.92.1-R

This is just as re-analysis to better understand, the reporter and author of there vulnerabileties is company Qualys official link: https://www.qualys.com/2021/05/04/21nails/21nails.txt

CVE-2020-28025: Heap out-of-bounds read in pdkim_finish_bodyhash()

Callers of pdkim_finish_bodyhash()

main() → receive_msg() → dkim_exim_verify_finish()

dkim_exim_verify_finish() be called to verify DKIM (DomainKeys Identified Mail) signature, then calls pdkim_feed_finish() , which calls pdkim_finish_bodyhash()

DKIM (DomainKeys Identified Mail) looks like this.

Code of the function pdkim_finish_bodyhash()

...
 /* VERIFICATION --------------------------------------------------------- */
 /* Be careful that the header sig included a bodyash */

 if ( sig->bodyhash.data
 && memcmp(b->bh.data, sig->bodyhash.data, b->bh.len) == 0)
 {
 DEBUG(D_acl) debug_printf("PDKIM [%s] Body hash compared OK\n", sig->domain);
 }
 else
 {
 DEBUG(D_acl)
 {
	debug_printf("PDKIM [%s] Body hash signature from headers: ", sig->domain);
	pdkim_hexprint(sig->bodyhash.data, sig->bodyhash.len);
	debug_printf("PDKIM [%s] Body hash did NOT verify\n", sig->domain);
	}
 sig->verify_status = PDKIM_VERIFY_FAIL;
 sig->verify_ext_status = PDKIM_VERIFY_FAIL_BODY;
 }
...

Unfortunately, at line 826, sig->bodyhash.data is attacker-controlled (through a “DKIM-Signature:” mail header) and memcmp() is called without checking first that sig->bodyhash.len is equal to b->bh.len: memcmp() can read sig->bodyhash.data out-of-bounds. If the acl_smtp_dkim is set (it is unset by default), an unauthenticated remote attacker may transform this vulnerability into an information disclosure; we have not fully explored this possibility.

Exim on Arzedlab 🪵

EXIM: CVE-2020-28025: Heap out-of-bounds read in pdkim_finish_bodyhash()

EXIM: Analysis 4.92.1-R

CVE-2020-28025: Heap out-of-bounds read in pdkim_finish_bodyhash()