How to get rid of prominent Specular highlight below the mesh surface

I am using modern OpenGL to render a scene having obj(wavefront) meshes. I have 3 point lights, a directional light and a spot-light.

And it suffers from the problem of unwanted specular highlight in the bottom.

My vertex Shader :

#version 330 core

layout (location = 0) in vec3 pos;          
layout (location = 1) in vec3 normal;   
layout (location = 2) in vec2 texCoord;

uniform mat4 model;         // model matrix
uniform mat4 view;          // view matrix
uniform mat4 projection;    // projection matrix

out vec3 FragPos;
out vec3 Normal;
out vec2 TexCoord;

void main()
{
    FragPos = vec3(model * vec4(pos, 1.0f));            // vertex position in world space
    Normal = mat3(transpose(inverse(model))) * normal;  // normal direction in world space

    TexCoord = texCoord;

    gl_Position = projection * view *  model * vec4(pos, 1.0f);
}

And here is the fragment shader :

#version 330 core

struct Material 
{
    vec3 ambient;
    sampler2D diffuseMap;
    vec3 specular;
    float shininess;
};

struct DirectionalLight
{
    vec3 direction;
    vec3 ambient;
    vec3 diffuse;
    vec3 specular;
};

struct PointLight
{
    vec3 position;
    vec3 ambient;
    vec3 diffuse;
    vec3 specular;

    float constant;
    float linear;
    float exponent;
};

struct SpotLight
{
    vec3 position;
    vec3 direction;
    float cosInnerCone;
    float cosOuterCone;
    vec3 ambient;
    vec3 diffuse;
    vec3 specular;
    int on;

    float constant;
    float linear;
    float exponent;
};


in vec2 TexCoord;
in vec3 FragPos;
in vec3 Normal;

#define MAX_POINT_LIGHTS 3

uniform DirectionalLight sunLight;
uniform PointLight pointLights[MAX_POINT_LIGHTS];
uniform SpotLight spotLight;
uniform Material material;
uniform vec3 viewPos;

out vec4 frag_color;

vec3 calcDirectionalLightColor(DirectionalLight light, vec3 normal, vec3 viewDir);
vec3 calcPointLightColor(PointLight light, vec3 normal, vec3 fragPos, vec3 viewDir);
vec3 calcSpotLightColor(SpotLight light, vec3 normal, vec3 fragPos, vec3 viewDir);

void main()
{ 
    vec3 normal = normalize(Normal);  
    vec3 viewDir = normalize(viewPos - FragPos);

    vec3 ambient = spotLight.ambient * material.ambient * vec3(texture(material.diffuseMap, TexCoord)) ;
    vec3 outColor = vec3(0.0f); 

    outColor += calcDirectionalLightColor(sunLight, normal, viewDir);

   for(int i = 0; i < MAX_POINT_LIGHTS; i++)
        outColor += calcPointLightColor(pointLights[i], normal, FragPos, viewDir);  

    // If the light isn't on then just return 0 for diffuse and specular colors
    if (spotLight.on == 1)
        outColor += calcSpotLightColor(spotLight, normal, FragPos, viewDir);

    frag_color = vec4(ambient + outColor, 1.0f);
}

vec3 calcDirectionalLightColor(DirectionalLight light, vec3 normal, vec3 viewDir)
{
    vec3 lightDir = normalize(-light.direction);  // negate => Must be a direction from fragment towards the light

    // Diffuse ------------------------------------------------------------------------- --------
    float NdotL = max(dot(normal, lightDir), 0.0);
    vec3 diffuse = light.diffuse * NdotL * vec3(texture(material.diffuseMap, TexCoord));

     // Specular - Blinn-Phong ------------------------------------------------------------------
    vec3 halfDir = normalize(lightDir + viewDir);
    float NDotH = max(dot(normal, halfDir), 0.0f);
    vec3 specular = light.specular * material.specular * pow(NDotH, material.shininess);

    return (diffuse + specular);
}

vec3 calcPointLightColor(PointLight light, vec3 normal, vec3 fragPos, vec3 viewDir)
{
    vec3 lightDir = normalize(light.position - fragPos);

    // Diffuse ----------------------------------------------------------------------------------
    float NdotL = max(dot(normal, lightDir), 0.0);
    vec3 diffuse = light.diffuse * NdotL * vec3(texture(material.diffuseMap, TexCoord));

     // Specular - Blinn-Phong ------------------------------------------------------------------
    vec3 halfDir = normalize(lightDir + viewDir);
    float NDotH = max(dot(normal, halfDir), 0.0f);
    vec3 specular = light.specular * material.specular * pow(NDotH, material.shininess);

    // Attenuation using Kc, Kl, Kq -------------------------------------------------------------
    float d = length(light.position - FragPos);
    float attenuation = 1.0f / (light.constant + light.linear * d + light.exponent * (d * d));

    diffuse *= attenuation;
    specular *= attenuation;

    return (diffuse + specular);
}

vec3 calcSpotLightColor(SpotLight light, vec3 normal, vec3 fragPos, vec3 viewDir)
{
    vec3 lightDir = normalize(light.position - fragPos);
    vec3 spotDir  = normalize(light.direction);

    float cosDir = dot(-lightDir, spotDir);  // angle between the lights direction vector and spotlights direction vector
    float spotIntensity = smoothstep(light.cosOuterCone, light.cosInnerCone, cosDir);

    // Diffuse ----------------------------------------------------------------------------------
    float NdotL = max(dot(normal, lightDir), 0.0);
    vec3 diffuse = spotLight.diffuse * NdotL * vec3(texture(material.diffuseMap, TexCoord));

     // Specular - Blinn-Phong ------------------------------------------------------------------
    vec3 halfDir = normalize(lightDir + viewDir);
    float NDotH = max(dot(normal, halfDir), 0.0f);
    vec3 specular = light.specular * material.specular * pow(NDotH, material.shininess);

    // Attenuation using Kc, Kl, Kq -------------------------------------------------------------
    float d = length(light.position - FragPos);
    float attenuation = 1.0f / (light.constant + light.linear * d + light.exponent * (d * d));

    diffuse *= attenuation * spotIntensity;
    specular *= attenuation * spotIntensity;

    return (diffuse + specular);
}

Mesh_file.obj Here is the object file for the mesh :

The bottom of the mesh is expected to show only ambient light but a sharp specular highlight is seen. I am using Bling-Phong illumination with Phong shading for rendering.

I will share more info if needed to debug the issue but I feel only sharing the shader will suffice.