A stack buffer overflow was found in GStreamer's DTLS plugin (gst-plugins-bad). In openssl_verify_callback(), the peer certificate Subject DN is printed using X509_NAME_print_ex() into a memory BIO, then read via BIO_read() into a fixed gchar buf[2048] on the stack. The number of bytes read comes directly from the X509_NAME_print_ex() return value without bounds checking against the buffer size.

A remote DTLS peer can send a certificate with an oversized Subject DN (many long OU fields) so that the formatted output exceeds 2048 bytes, causing a stack buffer overflow during the DTLS handshake.

File: subprojects/gst-plugins-bad/ext/dtls/gstdtlsconnection.c
Function: openssl_verify_callback() (lines 1136-1144)

Exploitability analysis:
- The overflow content is TEXT ONLY (X509_NAME_print_ex with XN_FLAG_MULTILINE escapes all binary/control characters to printable ASCII sequences like \XX)
- Stack canary (-fstack-protector-strong) terminates the process before return address corruption is exploitable
- Arbitrary byte injection is impossible: null bytes become \00 (3 printable chars), high-bit bytes become \XX
- RCE is NOT achievable. Maximum impact is denial of service (crash)

Attack vector: Unauthenticated remote DTLS-SRTP/WebRTC peer triggers this during handshake by presenting a crafted certificate. No user interaction required.

Affected: GStreamer gst-plugins-bad (reproduced on 1.28.3)
Fixed: Planned for GStreamer 1.28.5
Fix MR: https://gitlab.freedesktop.org/gstreamer/gstreamer-security/-/merge_requests/99
Upstream issue: https://gitlab.freedesktop.org/gstreamer/gstreamer/-/work_items/5172 (confidential)
Advisory: GST-SA-2026-0062

Reporter: Clouditera Security; Z.ai Security; NSFOCUS
PSIRT Ticket: PSIRTSUPT-19090