CVE-2026-68553 in Coturn
Summary
by MITRE • 08/20/2026
Coturn is a free open source implementation of TURN and STUN Server. Prior to 4.13.0, an authenticated TURN user can place printf-style format specifiers in the STUN USERNAME or REALM attribute, which passes is_secure_string() validation and is embedded into Redis keys at nine call sites in src/apps/relay/ns_ioalib_engine_impl.c. send_message_to_redis() in src/apps/relay/hiredis_libevent2.c then passes the attacker-controlled key as the format argument to redisAsyncCommand() while supplying only one variadic value, causing hiredis redisvFormatCommand() to read past the va_list. Exploitation can crash the coturn process and terminate active TURN sessions or disclose stack memory into Redis. This issue is fixed in version 4.13.0.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified in Coturn, a widely deployed open-source implementation of the Traversal Using Relays around NAT (TURN) and Session Traversal Utilities for NAT (STUN) protocols, represents a critical security flaw rooted in improper input validation and unsafe string formatting practices. Prior to version 4.13.0, the software failed to adequately sanitize user-supplied data within authenticated sessions. Specifically, an attacker who has successfully authenticated as a TURN user can inject printf-style format specifiers into the STUN USERNAME or REALM attributes. These attributes are processed by the is_secure_string() function, which in earlier versions did not sufficiently detect or neutralize these maliciously crafted strings, allowing them to pass validation checks and proceed further into the application logic.
The technical core of this vulnerability lies in how Coturn handles data persistence via Redis integration. The tainted USERNAME or REALM values are embedded directly into Redis keys at multiple call sites within the source file src/apps/relay/ns_ioalib_engine_impl.c. Subsequently, when the send_message_to_redis() function located in src/apps/relay/hiredis_libevent2.c executes, it passes this attacker-controlled key as the format argument to redisAsyncCommand(). This command is part of the hiredis library, which expects a standard printf-style format string followed by corresponding variadic arguments. However, in this specific invocation, only one variadic value is supplied regardless of the complexity or length of the injected format string.
This mismatch between the number of format specifiers and provided arguments triggers undefined behavior within the redisvFormat() function of hiredis. The library attempts to read past the end of the va_list structure on the stack in search for additional arguments that do not exist. This out-of-bounds memory access constitutes a classic buffer over-read vulnerability, categorized under CWE-134: Use of Externally-Controlled Format String and CWE-787: Out-of-Bounds Read. The immediate operational impact is severe instability within the Coturn process. Exploitation typically results in a segmentation fault or crash, leading to the termination of active TURN sessions and causing denial of service for legitimate users relying on the media relay services.
Beyond availability impacts, this vulnerability poses significant risks regarding confidentiality. Because the out-of-bounds read accesses stack memory adjacent to the va_list, it can inadvertently disclose sensitive information stored in that region of memory into Redis logs or keys. This could potentially leak internal application states, authentication tokens, or other private data depending on what was present on the stack at the time of exploitation. From a threat modeling perspective aligned with MITRE ATT&CK techniques, this flaw facilitates Denial of Service (T1499) and potentially Information Discovery through memory disclosure, allowing an authenticated attacker to degrade service quality or gather intelligence about the underlying system architecture.
The resolution for this issue was implemented in Coturn version 4.13.0 by hardening the input validation mechanisms and ensuring that format strings are properly escaped or handled safely before being passed to variadic functions like redisAsyncCommand(). To mitigate risks associated with this vulnerability, organizations running versions prior to 4.13.0 should immediately upgrade to the patched release. Additionally, administrators should enforce strict authentication policies for TURN servers and consider deploying network-level access controls to limit exposure of Coturn instances to only trusted clients. Regular security audits focusing on input validation in C/C++ applications are recommended to prevent similar format string vulnerabilities from being introduced or exploited in other components of the infrastructure.