Haproxy - CraftedSignal Threat Feedhttps://feed.craftedsignal.io/tags/haproxy/Trending threats, MITRE ATT&CK coverage, and detection metadata. Fed continuously.Hugoenhello@craftedsignal.iohello@craftedsignal.ioThu, 18 Jun 2026 17:24:14 +0000CVE-2026-55204: HAProxy Null Pointer Dereference Leads to Denial of Servicehttps://feed.craftedsignal.io/briefs/2026-06-cve-2026-55204-haproxy-dos/Thu, 18 Jun 2026 17:24:14 +0000hello@craftedsignal.iohttps://feed.craftedsignal.io/briefs/2026-06-cve-2026-55204-haproxy-dos/An unauthenticated attacker can exploit CVE-2026-55204, a null pointer dereference vulnerability in HAProxy through version 3.4.0, by triggering excessive HPACK dynamic table insertions under memory pressure, causing HAProxy worker processes to crash and resulting in a denial of service.HAProxy through version 3.4.0 is affected by CVE-2026-55204, a null pointer dereference vulnerability residing in the hpack_dht_insert() function within src/hpack-tbl.c. This flaw occurs because the function fails to validate the return value of hpack_dht_defrag() when the memory pool is exhausted. An unauthenticated attacker can exploit this by sending specially crafted HTTP/2 requests that trigger excessive HPACK dynamic table insertions. By intentionally inducing memory pressure, the attacker forces hpack_dht_defrag() to return a NULL pointer, which hpack_dht_insert() then attempts to dereference. This action crashes HAProxy worker processes, leading to a denial of service for all services fronted by the vulnerable HAProxy instance. The vulnerability was fixed in commit 9a6d1fe.

Attack Chain

  1. Attacker crafts malicious HTTP/2 request: An unauthenticated attacker sends specifically designed HTTP/2 requests targeting a vulnerable HAProxy instance.
  2. Request triggers HPACK dynamic table insertions: The crafted request's headers are designed to cause numerous HPACK dynamic table insertions within the HAProxy worker process.
  3. Memory pressure induced: These excessive insertions consume memory, leading to memory pressure on the targeted HAProxy worker process.
  4. hpack_dht_defrag() returns NULL: Under severe memory exhaustion, the hpack_dht_defrag() function, called by hpack_dht_insert(), fails to allocate memory and returns a NULL pointer.
  5. Null pointer dereference occurs: The hpack_dht_insert() function proceeds without validating the NULL return value, attempting to dereference this invalid pointer.
  6. HAProxy worker process crashes: This dereference results in a critical error, causing the targeted HAProxy worker process to unexpectedly terminate.
  7. Denial of Service: Repeated exploitation of this vulnerability leads to cascading crashes of HAProxy worker processes, rendering the HAProxy instance unable to process legitimate requests and causing a denial of service for all services it fronts.

Impact

Successful exploitation of CVE-2026-55204 results in a denial of service for services load-balanced or proxied by the vulnerable HAProxy instance. This can lead to severe business disruption, including website or application unavailability, financial losses due to interrupted services, and reputational damage. While no specific victim counts are detailed, any organization utilizing affected HAProxy versions as a critical infrastructure component is at risk. The impact is primarily on system availability, with no direct impact on confidentiality or integrity unless other systems rely on HAProxy's functionality in a critical security path.

Recommendation

  • Immediately patch HAProxy installations by updating to a version containing the fix for CVE-2026-55204, specifically referencing commit 9a6d1fe or later versions.
  • Deploy the Sigma rule "Detects HAProxy Process Crashes (CVE-2026-55204 Impact)" to monitor for unexpected haproxy process terminations.
  • Implement the Sigma rule "Detects High Rate of HAProxy 5xx Errors" to identify unusual spikes in server-side HTTP errors, which may indicate a denial-of-service condition or ongoing exploitation.
  • Configure HAProxy to limit HTTP/2 header sizes and HPACK dynamic table sizes to reduce the attack surface for memory exhaustion attacks, if applicable to your configuration.
]]>mediumadvisorydenial-of-servicevulnerabilityHAProxyCVE-2026-55204CVE-2026-55203 HAProxy Integer Overflow in FastCGI Handlinghttps://feed.craftedsignal.io/briefs/2026-06-cve-2026-55203-haproxy-integer-overflow/Thu, 18 Jun 2026 17:23:08 +0000hello@craftedsignal.iohttps://feed.craftedsignal.io/briefs/2026-06-cve-2026-55203-haproxy-integer-overflow/An integer overflow vulnerability (CVE-2026-55203) in HAProxy through version 3.4.0 allows malicious FastCGI backends to desynchronize the FCGI framing parser, leading to request routing errors, response smuggling, or memory safety issues.CVE-2026-55203 impacts HAProxy versions up to and including 3.4.0, stemming from an integer overflow within the fcgi_conn structure's drl field. This vulnerability is triggered when HAProxy receives a FastCGI record from a backend where contentLength is precisely 65535 and paddingLength is 1 or more. Under these specific conditions, the drl field wraps to 0, causing HAProxy to misinterpret subsequent data as new FastCGI record headers. This desynchronization of the FCGI framing parser enables malicious FastCGI backends to manipulate HAProxy's internal state, potentially resulting in request routing errors, response smuggling, or various memory safety issues. Organizations utilizing HAProxy as a reverse proxy for FastCGI applications are particularly susceptible, making immediate patching crucial.

Attack Chain

  1. An attacker establishes or compromises a FastCGI backend service configured to communicate with a vulnerable HAProxy instance.
  2. The malicious FastCGI backend constructs and sends a specially crafted FastCGI record to HAProxy.
  3. The crafted FastCGI record includes a contentLength value of 65535 and a paddingLength of 1 or more.
  4. HAProxy receives and attempts to process this record, triggering an integer overflow in the fcgi_conn structure's drl field, causing the field to wrap to 0.
  5. Due to the drl field's incorrect value, HAProxy misinterprets the subsequent data stream from the backend as new FastCGI record headers.
  6. This misinterpretation desynchronizes HAProxy's internal FastCGI framing parser, leading to incorrect consumption of subsequent records.
  7. The desynchronization allows the attacker to control HAProxy's processing, potentially leading to request routing errors (e.g., client request routed to wrong backend), response smuggling (e.g., appending arbitrary content to legitimate responses), or various memory safety issues (e.g., crashes, arbitrary code execution).
  8. The ultimate objective is achieved, ranging from data manipulation, unauthorized access, to denial of service or remote code execution depending on the specific memory safety issue exploited.

Impact

The successful exploitation of CVE-2026-55203 can lead to severe consequences, with a CVSS v3.1 Base Score of 7.5. Primary impacts include the desynchronization of HAProxy's FastCGI parser, enabling attackers to cause request routing errors, potentially redirecting user traffic to unintended services or malicious content. More critically, it can facilitate response smuggling, where attackers can inject arbitrary data or even entire unauthorized responses into a legitimate client's connection. Furthermore, the underlying integer overflow can lead to various memory safety issues, potentially resulting in HAProxy crashes, denial-of-service, information disclosure, or even remote code execution, undermining the stability and security of the proxy layer.

Recommendation

  • Patch CVE-2026-55203 immediately by updating HAProxy to a version beyond 3.4.0 (e.g., 3.4.1 or later containing commit 5985276).
  • Deploy the Sigma rule "CVE-2026-55203 - Detect HAProxy FCGI Parsing Errors" to your SIEM to identify internal errors indicative of attempted exploitation.
  • Deploy the Sigma rule "CVE-2026-55203 - Detect High Volume of HAProxy 5xx Errors" to monitor for unusual spikes in server-side errors that could signal instability or routing issues caused by exploitation.
  • Enable comprehensive logging for HAProxy and its FastCGI backends, including detailed error messages, to facilitate investigation.
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