CVE-2026-55203
Description
A flaw was found in HAProxy. A malicious FastCGI (Fast Common Gateway Interface) backend can exploit an integer overflow vulnerability in the fcgi_conn structure's drl field. This occurs when specific contentLength and paddingLength values cause the drl field to wrap to zero, leading to incorrect record consumption. This desynchronizes the FCGI framing parser, potentially resulting in response smuggling, request routing errors, or memory safety issues.
Statement
Conditions for Exploitation: Successful exploitation requires a specific configuration where HAProxy is actively utilizing a FastCGI backend. Crucially, an attacker must already have control over this backend server to supply the maliciously crafted responses.
This requirement significantly restricts the attack vector, as it relies on an unlikely configuration or an already compromised backend, rather than a typical remote attack initiated by a client against a default frontend server.
Impact Limitations: Although the vulnerability can cause the FastCGI framing parser to desynchronize—potentially leading to response smuggling, request routing errors, or memory safety issues—the threat is contained entirely to environments that process traffic from untrusted or compromised backend servers.
Mitigation
To mitigate this issue, restrict access to HAProxy instances that utilize FastCGI to trusted FastCGI backends only. If FastCGI is not required, consider disabling its use in HAProxy configurations to eliminate the attack vector. Ensure that all FastCGI backends are secured and not susceptible to compromise. If the HAProxy service is reloaded or restarted after configuration changes, ensure proper validation of the new configuration.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 7.5 | 9.1 | 7.5 |
| Attack Vector | Network | Network | Network |
| Attack Complexity | High | Low | High |
| Privileges Required | None | None | None |
| User Interaction | None | None | None |
| Scope | Changed | Unchanged | Changed |
| Confidentiality | Low | None | Low |
| Integrity Impact | High | High | High |
| Availability Impact | None | High | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N
NVD: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:H
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:L/I:H/A:N
Understanding the Weakness (CWE)
Availability
Technical Impact: DoS: Crash, Exit, or Restart; DoS: Resource Consumption (Memory); DoS: Instability
This weakness can generally lead to undefined behavior and therefore crashes. When the calculated result is used for resource allocation, this weakness can cause too many (or too few) resources to be allocated, possibly enabling crashes if the product requests more resources than can be provided.
Integrity
Technical Impact: Modify Memory
If the value in question is important to data (as opposed to flow), simple data corruption has occurred. Also, if the overflow/wraparound results in other conditions such as buffer overflows, further memory corruption may occur.
Confidentiality,Availability,Access Control
Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism
This weakness can sometimes trigger buffer overflows, which can be used to execute arbitrary code. This is usually outside the scope of the product's implicit security policy.
Availability,Other
Technical Impact: Alter Execution Logic; DoS: Crash, Exit, or Restart; DoS: Resource Consumption (CPU)
If the overflow/wraparound occurs in a loop index variable, this could cause the loop to terminate at the wrong time - too early, too late, or not at all (i.e., infinite loops). With too many iterations, some loops could consume too many resources such as memory, file handles, etc., possibly leading to a crash or other DoS.
Access Control
Technical Impact: Bypass Protection Mechanism
If integer values are used in security-critical decisions, such as calculating quotas or allocation limits, integer overflows can be used to cause an incorrect security decision.
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