Description

FreeSWITCH is a Software Defined Telecom Stack enabling the digital transformation from proprietary telecom switches to a software implementation that runs on any commodity hardware. Prior to 1.11.2, rtmp_rtp2rtmpH264() built an AVC decoder configuration record in a fixed stack buffer by copying wire-derived SPS and PPS values without checking that their sizes and framing fit. An established H.264 call bridged toward an RTMP endpoint could supply oversized SPS and PPS parameter sets and overrun the function's stack frame. The flaw affects only deployments that build and load mod_rtmp and bridge RTP media into an RTMP leg; the module is disabled in the stock build and configuration. The corruption can abort FreeSWITCH and terminate every active call, and builds without stack protection receive a limited stack-write primitive, although code execution has not been demonstrated. This issue is fixed in version 1.11.2.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.9",
  "pubDate": "2026-11-05T20:17:44.470Z",
  "pubdate": "2026-11-05T20:17:44.470Z",
  "executiveSummary": "A stack-based buffer overflow vulnerability exists in FreeSWITCH versions prior to 1.11.2, specifically within the mod_rtmp module.\nThe vulnerability occurs in the rtmp_rtp2rtmpH264 function, which fails to perform bounds checking when processing Sequence Parameter Set (SPS) and Picture Parameter Set (PPS) values extracted from H.264 streams.\nBy supplying maliciously crafted, oversized H.264 parameter sets during a bridged call to an RTMP endpoint, a remote attacker can trigger a memory corruption event.\nThe primary impact of this flaw is a denial-of-service (DoS) condition, resulting in the termination of the FreeSWITCH process and the immediate dropping of all active calls.\nWhile remote code execution has not been explicitly demonstrated, systems compiled without stack protection mechanisms may be susceptible to a limited stack-write primitive.\nThe vulnerability is restricted to deployments that explicitly load mod_rtmp and configure media bridging between RTP and RTMP legs; the module is disabled by default in standard configurations.\nGiven the nature of telecom software, the risk is elevated for deployments where untrusted endpoints are permitted to bridge media, potentially allowing for service disruption by unauthenticated or compromised remote participants.",
  "technicalDetails": "The vulnerability is located in the rtmp_rtp2rtmpH264 function within the FreeSWITCH mod_rtmp component. This function is responsible for constructing an AVC (H.264) decoder configuration record during the bridging of RTP media to an RTMP endpoint. The root cause of the vulnerability is an insecure memory copy operation involving wire-derived SPS and PPS data structures into a fixed-size stack buffer.\nDuring the processing of the H.264 bitstream, the function extracts SPS and PPS parameter sets from the incoming RTP payload. The implementation lacks sufficient input validation to verify that the length of these parameter sets fits within the allocated destination stack frame. When a remote endpoint sends an oversized H.264 packet containing malicious or malformed parameter sets, the copy operation overflows the stack buffer, overwriting adjacent memory space on the stack.\nThe attack flow requires an established H.264 call that is bridged toward an RTMP destination. An attacker providing the malicious RTP traffic can trigger the overflow at the point where the decoder configuration record is generated. Because the data is copied directly from the network-derived stream to the stack without sanitization or size-checking logic, an attacker can influence the stack's integrity.\nThe impact of this memory corruption is typically a crash of the FreeSWITCH process, as the overwritten stack frame likely corrupts function return addresses or local variables, leading to an immediate segmentation fault or exception. For builds lacking modern security mitigations such as stack canaries (stack protection), the vulnerability provides an attacker with a limited stack-write primitive. While exploitability for arbitrary code execution remains theoretical and unproven in current research, the inability to guarantee the safety of the memory write makes this a high-severity stability concern for critical telecom infrastructure.\nThe vulnerability is specific to deployments where mod_rtmp is active and used for media bridging. Since the module is not enabled in the default build or configuration, the attack surface is limited to environments where administrators have explicitly configured support for RTMP media legs. No specific authentication or elevated privileges are required beyond the ability to establish or influence an H.264 call that reaches the vulnerable bridging logic, making it accessible from any network segment capable of injecting RTP traffic into the media pipeline."
}